Abiotic Stress In Horticulture
2021104307
Salome Njeri Ndombi
Masters in Tea Science, College of Horticulture
Nanjing Agricultural University
Hort7001: Advances in Horticulture
Professor Shang
14th December 2021
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Abiotic Stress In Horticulture
Thesis
Write a thesis about abiotic stress in Horticulture. At least 40 references and 8000 words
Send it to my mail before the end of this semester on shangguanlf@njau.edu.cn
Abiotic stress in Horticulture
Abstract
The natural habitat for plants is composed of a complex set of abiotic stresses and biotic stresses
which often challenge plants during their lifespan. Abiotic stress is defined as environmental
conditions that reduce growth and yield below optimum levels. These changing environmental
conditions are unfavourable or stressful for the growth and development of plants. Plant
responses to these stresses are equally complex. In this paper, the research progress of abiotic
stresses in horticulture is summarized from the physiological level to the molecular level. Up to
date insights attained from the inclusion of omics datasets are highlighted. Gaps in our
knowledge are identified hence providing additional focus areas for horticultural crops
improvement research in the future.
Introduction
Plants are sessile organisms which incessantly endure varied climate conditions that are usually
unfavorable as well as stressful for growth and development. These stressful environmental
factors can either be biotic or abiotic. Among these adverse environmental changes are abiotic
stresses that include salinity, heavy metal exposure, oxidative stress, extreme temperatures,
drought and high light are significant factors that can threaten plant productivity and food
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security (Zhu 2016).While biotic stress include pathogen infection and herbivore attack. Abiotic
stresses affect the geographical distribution of plants in nature, limit plant productivity in
agriculture, and threaten food security. The adverse effects of these abiotic stresses are worsened
by climate change, which has been predicted to result in an increased frequency of extreme
weather (Fedoroff et al., 2010). Equally, plant responses to abiotic stresses are dynamic and
complex (Cramer, 2010; Skirycz & Inzé, 2010); they are both elastic (reversible) and plastic
(irreversible). Therefore, understanding how plants sense stress signals and adapt to adverse
environments is fundamental. Besides, improving plant stress resistance is critical for
agricultural productivity and also for the environment sustainability because crops with poor
stress resistance consume too much water and fertilizers and thus greatly burdening the
environment.
Effects of abiotic stress on horticultural crops
Plants are complex organisms that experience developmental changes, cell differentiation and
interactions with the environment. Thus it is easy to see that there are an infinite number of
permutations to this complexity. There is an additional complexity within the cell with multiple
organelles, interactions between nuclear, plastidial and mitochondrial genomes, and between
cellular territories that behave like symplastically isolated domains that are able to exchange
transcription factors controlling gene expression and developmental stages across the
plasmodesmata. A typical plant cell has more than 30,000 genes and an unknown number of
proteins, which can have more than 200 known post-translational modifications (PTMs). The
molecular responses of plant cells to their environment are extremely complex(Cramer et al.,
2011). While it is difficult to get accurate estimates of the effects of abiotic stress on crop
production, it is evident that abiotic stresses continue to have a significant impact on plants based
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on the percentage of land area affected and the number of scientific publications directed at
various abiotic stresses. There are inherent physical, morphological and molecular limitations to
the plant’s ability to respond to stress. Even though proteomics analyses especially the study of
post-translational modifications are lagging behind, transcriptomics studies are well advanced.
The integration of multiple omics studies has revealed new areas of interactions and regulation.
Time series experiments have revealed the kinetics of stress responses, identifying multiple
response phases involving core sets of genes and condition-dependent changes. The early down
regulation of energy metabolism and protein synthesis is one consistent trend in response to
abiotic stress. This may indicate a conservation of energy by the plant and may reflect a shift
from plant growth to protective mechanisms. In many cases, ABA signaling mediates the plant
responses to abiotic stress. Co-expression analyses are useful in that they have revealed key
regulatory hubs that can be manipulated to produce different phenotypes. To get a
comprehensive understanding of plant responses to abiotic stress, more extensive mapping of
these responses at the organ, tissue and cellular level are required. Such network analyses need to
be extended to the proteomics and enzyme activities levels. Models need to be constructed and
linked to phenotypic traits. The linkage of key regulatory hubs to phenotypic traits will allow for
more rapid progress in the genetic manipulation and production of crop plants(Cramer et al.,
2011). Current progress is exemplified by the identification and validation of several key genes
that improved stress tolerance of crops in the field. It is expected that progress in the plant
sciences and systems biology will continue to accelerate in the near future.
Mechanisms of abiotic stress tolerance of plants
Stress sensing
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It is evident that plant cells are capable of sensing various environmental signals since plants
exhibit specific changes in gene expression, metabolism, and physiology in response to different
environmental stress conditions(Zhu, 2016). Plant abiotic stress can therefore be sensed in
various cellular compartments to initiate molecular responses at multiple levels. Cell signaling in
response to salt, drought and the stress hormone ABA largely depends on the SnRK family of
protein kinases in plants. SnRKs are related to the yeast SNF1 and mammalian AMPK, which
are vital sensors of cellular energy status (Hardie et al., 2016). In plants, abiotic stresses reduce
the energy supply by inhibiting photosynthesis and energy-releasing catabolic reactions. Thus,
SNF1/AMPK-related kinases proliferated and diversified during evolution to mediate the
signaling of various abiotic stresses. SnRK1s are SNF1/AMPK orthologs that function in
regulating metabolism in plants. All SnRK2s participate in osmotic stress and ABA signaling,
whereas SnRK3s are key regulators of ion homeostasis required to cope with salt and nutrient
stress in soil. Many of these stress-signaling pathways also involve the calcium-dependent
protein kinase CPKs, which share homology to SnRKs in their kinase domains (Hrabak et al.,
2003). In addition, virtually all of the stress pathways also involve MAPKs, which is a conserved
feature of stress signaling in organisms from fungi to plants and metazoans. Other conserved
features include the widespread use of calcium, ROS, NO, and lipid molecules as second
messengers, although the generation and signal transduction of the second messengers are
different in plants. Even though identifying stress sensors is still an important goal for abiotic
stress the research involved in plants is also challenging. Efficient gene-editing technologies and
chemical genetic approaches will enable control of gene redundancy problems that prevent the
genetic identification of stress sensors. The expanding acknowledgement of the importance of
various cell organelles in stress sensing and responses and the dispersed stress-sensing model
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will also enable researchers to understand stress sensing and stress resistance, although the
integration of signals from perturbed organelles is still poorly understood(Zhu, 2016). Similarly,
it is vital to understand the crosstalk between stress signaling pathways and hormonal as well as
growth and developmental signaling pathways because plant stress responses must be
coordinated with growth and development. More attention should also be directed toward plant
responses to simultaneous, multiple abiotic stresses and to the crosstalk between abiotic and
biotic stress signaling because much of the abiotic stress research thus far has been carried out on
sterile plants grown in culture media in the laboratory; in nature, however, plants co-exist with
insects and microorganisms. The root and shoot microbiomes presumably include many
beneficial bacteria and fungi that help plants resist stress. Understanding how bacteria and fungi
boost plant stress resistance should increase our ability to use these beneficial organisms and
should also increase our understanding of stress resistance in plants (Zhu, 2016).
Epigenetic regulation in plant abiotic stress responses
Adverse environments threaten agricultural productivity therefore increasing plant stress
resistance is critical for agriculture. The core stress signaling pathways have been gradually
unraveled during the past decade (Zhu 2016). Recently, in addition to the elucidation of the
signal transduction mechanisms underlying abiotic stress responses, increased numbers of
studies have shown important participation of epigenetic mechanisms in the response of plants to
abiotic stresses (Sahu et al. 2013; Kim et al. 2015). A good example of epigenetic regulation in
plant response to the environment is the extensive involvements of epigenetic marks in
vernalization, a process where plants recall a prolonged low temperature exposure in the winter
in order to flower in the spring (Zhao et al. 2018; Luo and He 2020). Epigenetic mechanisms
participate in the regulation of stress‐responsive genes at the transcriptional and
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posttranscriptional levels by altering the chromatin status of the genes. Stress treatments can
cause changes in the chromatin modifications (Kim et al. 2015; Lamke and Baurle 2017; Luo
and He 2020). Furthermore, epigenetic mechanisms play vital roles in the formation of stress
memory, which may be inherited by the offspring of the stress‐treated plants (Friedrich et al.
2019). Therefore, deciphering the epigenetic codes of plant stress responses could be of great
significance for breeding stress‐tolerant crops. Plants make a variety of changes to adapt to their
environment since they are exposed to continuously changing conditions in nature. As
summarized in figure 1 below, countless efforts have been made to explore the epigenetic
mechanisms involved in plant abiotic stress responses (Popova et al. 2013; Kim et al. 2015;
Lamke and Baurle 2017).
Figure 1. A summary of the cross‐talks between epigenetic mechanisms and abiotic stress
responses (Chang et al., 2020)
It is clear that epigenetic mechanisms are widely involved in the plant abiotic stress response. As
summarized above, a large number of components involved in the abiotic stress response have
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been shown to be under epigenetic regulation, and the levels of epigenetic marks are activated or
repressed after abiotic stress treatments. In addition to DNA methylation, histone modifications,
chromatin remodeling complex and histone variants, some long non‐coding RNAs (lncRNAs)
may also participate in various stress responses (Zhao et al. 2018). Besides, small RNA‐mediated
RNA silencing is also another regulatory mechanism of abiotic stress response. It has been
shown that Arabidopsis ARGONAUTE 1 (AGO1) could associate with SWI/SNF chromatin
remodeling complex and small RNAs to bind to a number of stress‐responsive genes and
regulate their expression (Liu et al. 2018a), suggesting a cross‐talk of different epigenetic
mechanisms in response to abiotic stresses. The dynamic changes in epigenetic marks on
stress‐responsive genes make their chromatin status accessible or inaccessible, which in turn
regulates the expression of stress‐responsive genes at the transcriptional or posttranscriptional
level. However, the roles of epigenetic mechanisms in plant response to abiotic stresses are yet to
be fully and clearly understood. Also, how stresses regulate the epigenetic machineries to cause
chromatin changes and consequent transcriptional reprogramming is poorly understood.
Furthermore, it is unclear how epigenetic changes may be inherited by the offspring as a stress
memory mechanism. Certainly, deciphering the epigenetic codes of plant abiotic stress responses
deserves more attention in future studies. With the rapid advancement of high‐throughput
sequencing and various chromatin profiling technologies, the epigenomes of increasing numbers
of crop plants are being determined, which will greatly increase the number of studies on the
epigenetic mechanisms of stress adaptation in model plants as well as in crops.
Melatonin Mediates Enhancement of Stress Tolerance in Plants
Melatonin (N-acetyl-5-methoxytryptamine) is a pleiotropic molecule that has amphiphilic
properties in plants. It is a multifunctional signaling molecule that exists ubiquitously in different
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parts of plants and is responsible for stimulating several physiological responses to adverse
environmental conditions (Debnath et al., 2019). The biosynthesis of melatonin occurs in plants
by themselves, and its accumulation fluctuates sharply by modulating its biosynthesis and
metabolic pathways under stress conditions. It has been established that melatonin, with its
precursors and derivatives, is involved in improving physiological processes, for example,
spreading the plant’s normal growth thus acting as a powerful growth regulator as well as
shielding emergent tissues from injury and stress signals from environmental hazards (Arnao &
Hernández-Ruiz, 2015; Debnath et al., 2018; Erland et al., 2015). Melatonin also acts as a biostimulator, and antioxidant, which delays leaf senescence, lessens photosynthesis inhibition, and
improves redox homeostasis and the antioxidant system through a direct scavenging of reactive
oxygen species (ROS) and reactive nitrogen species (RNS) under abiotic and biotic stress
conditions. In addition, recent reviews have described the significant characteristics of melatonin
in plant behavioral responses against environmental stress (Erland et al., 2018; Manchester et al.,
2015). Furthermore, exogenous melatonin boosts the growth, photosynthetic, and antioxidant
activities in plants, confirming their tolerances against drought, unfavorable temperatures,
salinity, heavy metals, acid rain, and pathogens. The physiological and molecular activities of
melatonin in plants indicate that melatonin is an essential molecule in the stimulation of field
crops, especially where biotic and abiotic stress is a limiting factor for crop production. However,
in future research, the role of endogenous melatonin and the uses of exogenous melatonin against
viruses, nematodes, or insects call for detailed investigations. Information on the genes and core
pathways that are precisely regulated by melatonin is also required. To conclude, there is
enormous research potential for bettering our understanding of the impact that melatonin has in
basic life functions across plant kingdoms, and the creation of new approaches to advance
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progress in plant cultivation and industrial agriculture. This would support emerging new
approaches to adopt strategies in overcoming the effect of hazardous environments on crops and
may have potential implications in expanding crop cultivation against harsh conditions. Thus,
farming communities and consumers will benefit from elucidating food safety concerns.
The CBL–CIPK Pathway in Plant Response to Stress Signals
Calcium functions as a ubiquitous secondary messenger in response to numerous stresses and
developmental processes in plants. The major Ca2+ sensors, calcineurin B-like proteins (CBLs),
interact with CBL-interacting protein kinases (CIPKs) to form a CBL–CIPK signaling network,
which functions as a key component in the regulation of multiple stimuli or signals that responds
to plant hormones and various stresses in plants (Ma et al., 2020). In order to understand CBLs,
calcineurin, the Ser/Thr phosphatase that is activated by Ca2+ and CaM, is important to consider
(Kudla et al., 1999). In recent years, the CBL–CIPK network has been extensively researched to
establish a firm foundation enabling research progress, especially in the model plant Arabidopsis
and the staple food crops wheat and rice. CBL and CIPK family members have been discovered
widely in different plant species; though it remains unclear which members play essential roles
in stress responses. The relationships among CBLs, CIPKs, and other target proteins in responses
to diverse forms of stress have been gradually clarified (Figure 2).
Figure 2;. A model of CBL-CIPK pathway during stimuli. CBLs combine with the Ca2+
increased by stimuli, and activate the CIPKs in response to relevant stresses. The solid lines
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represent the classical Ca2+/CBL-CIPK signaling. The atypical function of CBLs and the target
protein PP2C are shown with dotted lines
Even though regulatory functions of CBLs and CIPKs are well-known, there are still many
questions to be answered (Batistic & Kudla, 2004). What the mechanism of binding affinity
between Ca2+ signals and CBLs are, how CBLs influence CIPKs and their target proteins, what
the functions of CBLs with other proteins are, beyond the classically linked CIPKs, how the
multiple CBL–CIPK networks function under the same stress in vivo, what mechanisms remain
undiscovered between the CBL–CIPK network and other signaling pathways, can CBLs or
CIPKs play functions in other localizations despite normal membrane system conditions, the
potential functions enabled by alternative splicing in CBLs and intron-rich CIPKs (Ma et al.,
2020). Through biotechnological approaches, such as biosensors, phosphorylation assay,
crystallography, the calcium-mediated CBL–CIPK network is worth exploring for in-depth
insights into plant responses to stress signaling.
Cold stress
Plants have developed a remarkable ability to adapt to harsh environmental conditions, and
thrive in habitats characterized by abiotic stresses such as extreme temperatures. Cold stress
responses in plants are exceptionally sophisticated events that alter the biochemical composition
of cells for protection from damage caused by low temperatures. Moreover, cold stress has a
great impact on plant morphologies, causing growth repression and reduced yields. Complex
signalling cascades are utilised to induce changes in cold-responsive gene expression that enable
plants to withstand chilling or even freezing temperatures. These cascades are governed by the
activity of plant hormones, and recent research has provided a better understanding of how cold
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stress responses are integrated with developmental pathways that modulate growth and initiate
other events that increase cold tolerance (Eremina et al., 2016). Species-specific differences in
temperature tolerance have evolved in plants that occupy different geographic zones. There is a
significant management challenge for agriculture and horticulture since crops are regularly
cultivated in geographical regions where temperature preferences of the plant are not fully met
during the growing season. Moreover, extreme short-term weather events, such as late frost
during spring, impact yields, particularly of fruit crops and spring cereals (Frederiks et al., 2015).
Consequently, chilling and freezing stress constitute some of the most severe abiotic factors that
reduce crop productivity (Ceccon, 2008; Cramer et al., 2011). Reliable and high-quality crop
yields are critical for food security therefore an understanding of the molecular modes that
encourage cold stress resistance is essential to further optimise horticultural and agricultural crop
breeding and production.
Hormones act as central regulators of cold stress responses in plants. Even though, knowledge of
their regulatory activities remains limited and seemingly contradicting results have occasionally
been published (Eremina et al., 2016). Many reasons may account for this fact. First, changes in
hormone levels at the cellular levels are usually not determined, although it is known that local
hormone concentrations control adaptive growth and development. Second, the experimental
setups are decisive for determining cold stress tolerance of plants. However, no standardised
procedures exist as yet. Third, hormone mutants often have strong morphological defects, which
likely impact on the outcome of chilling or freezing tolerance assays due to secondary effects
that are not related to cold signalling but rather to decreased biomass, altered membrane
morphologies and compositions or defective developmental phase transitions (such as flowering
time control). Furthermore, dwarfism as in GA, BR, auxin and SA mutants may reduce cold
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damage simply because there is less vegetative tissue exposed. In such cases, it will be necessary
to additionally include weak alleles that are less impaired in growth and complement results of
qualitative assessments such as survival rates with those of quantitative evaluations such as
electrolyte leakage following freezing treatments (Thalhammer et al., 2014). Techniques
required for the study of plant hormone action under those highly challenging circumstances
have already been established by developmental biologists. In the future, it will be important that
these systems are more readily adopted and optimised for the study of cold stress responses,
which would further facilitate the rapid progress been made in the field in recent years.
Protein S-nitrosylation in plant abiotic stresses
Roles of nitric oxide in heavy metal stress in plants: Cross-talk with phytohormones and
protein S-nitrosylation
Heavy metal (HM) stress is another major hazard, which significantly affects plant growth and
development. Exposure to cadmium (Cd), arsenic (As), lead (Pb) and copper (Cu) are great
threats to plants from heavy metal (HM). HM at low concentrations may enter soils and
groundwater and bioaccumulate in food webs, posing threats to agriculture and human health
(Gall et al., 2015; Luo et al., 2012). Acute and chronic HM exposure induces a serious reactive
oxygen species (ROS) and reactive nitrogen species (RNS) burst in plant cell, and thereby
triggering an oxidative/nitrosative stress, which potentially leads to HM toxicity, growth
inhibition, accelerated senescence, nutrient uptake inhibition or cell death (Gallego and
Benavides, 2019). In order to confront HM stress, plants directly or indirectly regulate the levels
of endogenous nitric oxide (NO), a redox-related signaling molecule which is a common feature
underpinning the multiple modes of stress tolerance in plants and is also involved in wide range
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of plant growth and development (Wei et al., 2020). Evidence suggests that Nitric oxide (NO), a
small ubiquitous signaling molecule, functions as a physiological mediator in major biological
processes in plant growth and development including seed germination, root development,
stomatal closure, stress responses and photomorphogenesis (Corpas and Palma, 2018; Yu et al.,
2014). Moreover, there is also compelling experimental evidence that NO usually mediates
signaling processes through interactions with different biomolecules like phytohormones to
regulate HM tolerance. Apart from phytohormones, NO partly operates through posttranslational
modification of proteins, notably via S-nitrosylation in response to HM stress. The roles of Snitrosylation as a regulator of plant responses to HM stress and S-nitrosylated candidates have
also been established and detected lately. Once generated, NO may directly regulate protein
function via a variety of distinct mechanisms, including tyrosine (Tyr) nitration, metal
nitrosylation and S-nitrosylation (aka S-nitrosation) (Astier et al. 2011; Sehrawat et al. 2013a;
Sehrawat and Deswal 2014a; Wang et al. 2015a; Fancy et al. 2017). NO can also exert its
biological activity through protein S-nitrosylation/ denitrosylation. NO-mediated protection in
plants under drought, salt, high temperatures, ozone, and HM stresses has been described (Fancy
et al., 2017; BegaraMorales et al., 2019). The crosstalk between NO and ROS scavenging
enzymes also enhances stress tolerance in plants (Arora et al., 2016). Recently, S-nitrosylation
has emerged as the main mechanism in response to abiotic stress (Fancy et al., 2017; BegaraMorales et al., 2019; Yu et al., 2014). The interconnection between NO and phytohormones in
response to HM stress is derived predominantly from the change of phytohormones
concentration and regulation of related genes and biosynthetic enzymes. In this regard, further
work is needed to reveal the complicated interplay between NO and plant hormones, which is
involved in PTMs, targets proteins, protein kinases, cytoskeletal proteins to help understand the
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roles of NO in HM stress conditions. Besides, though a number of reports have indicated that
exogenous plant hormones might alleviate HM stress, such as IAA, GA, JA and ABA, very little
is known about the relationship between S-nitrosylation and plant hormones in response to HM
toxicity (Wei et al., 2020). Identification and functional analysis of S-nitrosylation may
contribute to demonstrations of molecular mechanisms based on NO and will give insights of
PTM mechanisms(Zhang & Liao, 2019). As a result, a deeper understanding of the plants stress
adaptation mechanisms will provide a new opportunity to the development of crop plants with a
better ability to confront HM stimulus, ultimately leading to increased yields.
Copper stress
Copper (Cu) is a heavy metal element widely existing in the universe, and is one of the essential
micronutrients for horticultural plants. However, when the concentration of Cu in soil exceeds a
certain threshold, it will hinder the growth and development of plants, and even cause plant death
in serious cases. In recent years, the environmental Cu pollution has become increasingly serious,
mainly from the discharge of industrial "three wastes", and the wide application of Cu-containing
feed additives and fungicides (such as Bordeaux liquid), the copper content in soil has generally
increased. The copper content of plants growing in copper-contaminated soil increased greatly,
which had a significant impact on plant growth and development.
Effects of copper stress on Horticultural plants
Copper usually promotes seed germination at low concentration and inhibits it at high
concentration. It was found that high concentration of copper ion inhibited seed germination
mainly by affecting enzyme activity and osmosis. For example, excessive copper was found to
inhibit seed germination in both rice and cucumber. The main reason may be that the activities of
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some enzymes such as amylase and protease in seeds were inhibited. This could not meet the
material and energy required for seed growth and development, thus inhibiting seed growth.
When a heavy metal stress occurs, plant root growth is inhibited. It has been proved that the
accumulation of Cu ions in root cells may affect root development by changing the proliferation
rate of root meristem cells or regulating plant hormones such as auxin (IAA) and cytokinin
(CTK). Huang et al. found that the biomass of white pomelo did not change significantly at the
concentration of copper from 0.5 μmol/L to 300μmol/L, but decreased significantly when the
concentration reached 400μmol/L. In addition, repeated application of Bordeaux liquid can slow
down the growth of citrus and pineapple trees and cause partial defoliation. The reduction of
plant biomass will directly affect its yield. Studies on crops such as citrus and grapes have found
that copper stress can significantly reduce crop quality and yield.
The right amount of Cu ions can promote the photosynthesis of plants, but high concentration
can make inactivation of chlorophyll, change in chloroplast ultrastructure, destroy the structure
and function of thylakoid, and also can decrease the ribulose - 1, 5 - bishosphate
carboxylase/oxygenase (RuBisCo) efficiency, are activated to restrain activity of photosynthetic
efficiency and PSII electron transfer, eventually inhibiting photosynthesis. This is because Cu is
a component of plasticyanin in plant chloroplasts which is involved in electron transfer in
photosynthesis and is also an activator of some enzymes in chlorophyll formation.
The production and clearance of reactive oxygen species (ROS) in plants are in a dynamic
balance under normal circumstances. Oxidative stress occurs when the concentration of ROS
accumulated under heavy metal stress exceeds the threshold of plant defense mechanism. Copper
stress can induce a large number of ROS production in cells, resulting in membrane lipid
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peroxidation, decreased plasma membrane selective permeability, extravasation of cell contents,
increased malondialdehyde (MDA) content, and damage to photosynthetic organelles, thus
affecting the normal operation of various physiological metabolic processes such as plant
material exchange and photosynthesis.
Low degree of excess copper stress (Low-ECS) in ‘Shine Muscat’ grapevine
To gain insights into the multi-omics responses of ‘SM’ grapevine to a low degree of excess
copper stress (Low-ECS), a study by (Chen et al., 2021) was carried out. The study also
provides genetic and agronomic information that will guide better vinery management and
breeding copper-resistant grape cultivars. Grapevine has a relatively higher copper tolerance than
other fruit crops. However, there are no reports regarding the tolerance mechanisms of the ‘Shine
Muscat’ (‘SM’) grape to a Low ECS. Based on the physiological indicators and multi-omics
(transcriptome, proteome, metabolome, and microRNAome) data, 8 h (h) after copper treatment
was the most severe stress time point. Nonetheless, copper stress was alleviated 64 h after
treatment. Cu ion transportation, photosynthesis pathway, antioxidant system, hormone
metabolism, and autophagy were the primary response systems in ‘SM’ grapevine under LowECS. Numerous genes and proteins, such as HMA5, ABC transporters, PMM, GME, DHAR,
MDHAR, ARGs, and ARPs, played essential roles in the ‘SM’ grapevine’s response to LowECS. (Chen et al., 2021)
Copper (Cu) is an essential microelement for all living organisms. It plays a part in various
physiological processes that include the photosynthetic and respiratory electron transport chains,
C and N metabolism ratio, hormone sensing, cell wall metabolism, oxidative stress protection,
and biogenesis of the cofactor molybdenum (Mendel, 2013; Pilon et al., 2006; Puig et al., 2007;
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Yruela, 2009). Even though Cu deficiency in plants causes chlorosis of young leaves, pale green
leaves, curled leaf edges, stunted growth, withered stems, and reduced fruit formation. In
addition, Cu deficiency restricts the photosynthetic transport chain, decreases non-photochemical
quenching, lowers plastoquinone synthesis, and inhibits photosystem II (PSII) activity (Thomas
et al., 2016). Nevertheless, a high copper concentration can hinder root growth, stem elongation,
and seed germination, and promote leaf chlorosis (Adrees et al., 2015; Ali et al., 2015; Cook et
al., 1997; Feigl et al., 2013). At the cellular level, excess Cu can damage the chloroplast and
thylakoid membrane composition, cause oxidative stress, and turn down the photosynthetic
pigment contents and photosynthetic electron transport (Gonzalez-Mendoza et al., 2013;
Maksymiec, 1998; Vassilev et al., 2003). Although the visible symptoms may be slightly
significant at low Cu concentrations, crop yield and biomass reduction may persist (Marschner,
2012; Yruela, 2005). Thus, copper stress is one of the most extreme heavy metal stresses in
plants.
Many excess copper stress tolerance mechanisms in plants have been proposed. Major pathways
that ameliorate excessive copper toxicity in plant cells include reducing Cu absorption,
increasing Cu chelation using chelators and subsequent storage into vacuoles, and Cu induction
in antioxidant defence (Adrees et al., 2015; Choudhary et al., 2012). These processes can be
categorized into four: (I) Cu2+ is reduced into Cu + by FRO4/5 before being absorbed by plants.
Cu+ is then transported into plant cells through the transmembrane using high-affinity
transporters COPTs (Jung et al., 2012; Li et al., 2018; Sancenon ´ et al., 2003; Sanz et al., 2019).
ZIP2 and ZIP4 in Arabidopsis had been reported to transport Cu2+ into the cytoplasm.
Cu2+/Cu+ absorbed by plants can be transferred to the metal-requiring proteins and cell
organelles in the form of Cu-complexes (Lange et al., 2017). (13)
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Excessive Cu is sequestered into the vacuoles or excluded outside the cytoplasm by chelators
such as phytochelatins (PCs), metallothioneins (MTs), and organic acids (OAs) to inhibit the
overgeneration of reactive oxygen species (ROS) (Callahan et al., 2007; Cobbett and
Goldsbrough, 2002; Sharma and Dietz, 2006). (II) Another plant detoxification mechanism
comprised of different antioxidants becomes active to scavenge over ROS production when the
plant machinery fails to control Cu-mediated enhanced ROS generation (Shabbir et al., 2020).
The antioxidants include several enzymes/proteins, such as superoxide dismutase (SOD),
peroxidase (POD), glutathione peroxidase (GPX), dehydroascorbate reductase (DHAR),
monodehydroascorbate reductase (MDHAR), and thioredoxin (Trx). There are also nonenzymatic low molecular mass antioxidants scavengers, such as ascorbate acid (AsA), proline
(Pro), tocopherol, and glutathione (GSH) (Apel and Hirt, 2004; Bartoli et al., 2017; Gill and
Tuteja, 2010; Mittler, 2002; Mittler et al., 2004; Munoz ˜ and Munn´e-Bosch, 2019). (III)
Phytohormones, such as abscisic acid (ABA) and brassinosteroid (BR), also play an essential
role in plant stress tolerance. For example, heavy metals such as Cu, Cd, and Hg have been
demonstrated to induce expressions of ABA synthesis genes, which subsequently increase the
endogenous ABA level (Bücker-Neto et al., 2017; Hollenbach et al., 1997). BRs can help
alleviate multiple abiotic stresses, including salinity, drought, and heavy metal stresses (Nawaz
et al., 2017; Vardhini et al., 2010). (IV) Numerous studies on yeast, mammalian cells, and plants
reveal that autophagy plays a vital role in nutrient recycling and cell homeostasis (Hanaoka et al.,
2002; Ren et al., 2014). For example, Arabidopsis with impaired autophagy function is
hypersensitive to oxidative stress (Avin-Wittenberg, 2019). Recently, numerous studies have
been done to determine the role of autophagy under heavy metal stress conditions, such as Cd
(Calero-Munoz ˜ et al., 2019; Xiao et al., 2020), TiO2 nanoparticles (Shull et al., 2019), Ni
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(P´erez-Martín et al., 2015), and Cu (Shangguan et al., 2018). Bordeaux mixture (CuSO4 5H2O
+ Ca (OH)2), a copper-based fungicide and bactericide, are widely used in grape production.
However, it causes severe copper stress in the vineyards, limiting grape productivity and quality
because of irrational use. The ‘Shine Muscat’ (‘SM’, Vitis labruscana × Vitis vinifera) grape is a
diploid European and American hybrid grown for its rich rose aroma, large green berries, high
sugar content, and strong storage and transportation resistance. Its cultivation area has drastically
risen recently, reflecting its huge market demand (Wu et al., 2019, 2020b). Numerous studies
focusing on ‘SM’ volatile compounds, fruit colour, maturation, and bud dormancy have been
done because of its good quality as the primary grape in Japan and China (Hou et al., 2018;
Khalil-Ur-Rehman et al., 2019; Khalil-Ur-Rehman et al., 2020; Lin et al., 2018; Matsumoto and
Ikoma, 2016; Wu et al., 2020c; Xu et al., 2019). Previous studies report that spraying ‘Summer
Black’ grapevine leaves with 0.4 mM CuSO4 solution stimulate copper responses such as
changes in the expression of autophagy-related genes (ARGs) at different time points
(Shangguan et al., 2018). However, transcriptomic analysis of these changes has not been studied
in depth. Leng et al. (2015) used RNA-seq to analyze the responsive mechanism of ‘Summer
Black’ grapevine leaves sprayed with Cu2+ (0.1 mM). However, the study did not compare
multiple time points and systematic analysis at multi-omics level (Leng et al., 2015). Herein,
multi-time sampling and multi-omics techniques were employed to study the short-term and
long-term grapevine copper-stress response mechanisms. The study was done to systematically
explore the response mechanisms and establish a regulatory network of ‘SM’ grapevine under
the Low-ECS, and discover critical pathways during short- and long-term spans of copper stress.
(13). To obtain more accurate sequencing results, the ‘SM’ genome was first used as a reference
for the multiomics analyses(Chen et al., 2021). An integrated multi-omics analysis by Chen et al.
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(2021) clearly establishes the response network of grapevine under Low-ECS. The excess
Cu2+/Cu+ would be chelated and stored in the vacuole or be discharged outside the cell after
Cu2+/Cu+ absorption and translocation to copper-demanding tissues and proteins,. Moreover,
excessive Cu inhibited chlorophyll synthesis and promoted its degradation. It also adversely
affected photosynthesis by inhibiting the activity of PSI. Additionally, copper stress-induced
intracellular oxidative stress, thereby activated a series of antioxidant mechanisms, including
conventional antioxidants and antioxidant enzymes, and various hormonal changes and
autophagosome formation. Oxidative damage and resistance gradually intensified during the
short-term responsive period. In contrast, the stress degree steadily reduced, and the resistance
response slowly recovered during the long-term responsive period. The results of this study
could guide the construction of regulation network of fruit trees under Low-ECS. In future, a
comprehensive analysis of the plants’ regulatory network under low-, moderate-, and severecopper adversities should be conducted to establish a regulatory network for fruits trees under
different copper supplies. (13)
A missing link that fine-tunes ABA signaling and drought tolerance in Arabidopsis
Phytohormones play a central role in environmental adaptation by inducing many biochemical
and physiological changes to protect against abiotic stresses, including high salinity, dehydration,
and temperature changes (Fernando & Schroeder, 2016; Finkelstein, 2013; Pozo et al., 2015).
Among the plant hormones, abscisic acid (ABA) is one of the most important regulators of plant
growth and development and plays a central role in both biotic and abiotic stress responses(Adie
et al., 2007; Finkelstein, 2013; Lee et al., 2006). Abscisic acid (ABA) specifically regulates plant
adaptation to osmotic stresses, such as drought and high salinity, by controlling the internal water
status in plants. A significant accumulation of ABA occurs in response to conditions of water
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deficit; this is followed by a sophisticated signaling relay, known as the ABA signaling pathway,
which decreases the rate of transpiration through stomatal closure, thereby suppressing
photosynthetic activity. Snf1-related kinases (SnRK2s) are the major components regulating the
ABA signaling pathway. Of these, SnRK2.6 (OST1) and SnRK2.3 are negatively regulated by
HOS15 (High Expression Of Osmotically Responsive15), in an ABA-dependent manner, to
cease the signaling relay(Ali & Yun, 2020). HOS15 is a WD40-repeat protein that regulates
several physiological processes, including plant growth and development, freezing stress
responses, and ABA signaling. HOS15 is one of the most important missing components in the
ABA signaling network. It was of interest to determine the exact position of HOS15 in the
current model of the ABA signaling pathway since it plays a central role in the ABA signaling
network. Under normal conditions, PP2Cs interact with and inhibit OST1 activity through its
dephosphorylation (Phosphatases et al., 2013). In the presence of ABA, ABA receptors inhibits
PP2Cs, thus releasing OST1, which first autophosphorylates and then transphosphorylates the
target TFs (Antoni et al., 2012). ABA also impairs the interaction between HOS15 and
OST1,(Ali et al., 2019) implying that, in the presence of ABA, all inhibitory components stay
away from OST1 and the SnRK2s that phosphorylate and activate the ABA-responsive
components. In contrast, ABA has no clear effect on the interaction of HOS15 with ABI1 and
ABI2 (Ali et al., 2019). This shows that once the ABA pathway is activated, OST1 is released
from the HOS15-ABI1/2 complex and is free to interact with its targets. Toward the end, when
the ABA pathway is about to turn-over, ABI1/ABI2 promotes HOS15 and OST1 interaction,
resulting in a much stronger complex, and finally HOS15 degrades OST1 (Ali et al., 2019). In
summary, HOS15 plays a crucial role in regulating ABA signaling through the degradation of
OST1, thus maintaining a balance between the active and inactive states. Determining the
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manner in which HOS15 is signaled to interact with OST1 and how ABI1/ABI2 stabilizes the
HOS15-OST1 complex should be the research focus in future(Ali & Yun, 2020).
Abiotic stress tolerance in plants via endophytic microbes
Endophytes are micro-organisms including bacteria and fungi that survive within healthy plant
tissues and promote plant growth under stress. The rhizobacteria and mycorrhizae are well
known for plant growth promotion. The symbiotic interactions between a plant and an endophyte
may result in several outcomes as defined by fitness benefits by each of the partners (Lewis
1985). Benefits to host plants can be positive (mutualism), neutral (commensalism and
neutralism) or negative (parasitism, competition and amensalism). Variations in the outside
environment put the plant metabolism out of homeostasis, which creates necessity for the plant
to harbour some advanced genetic and metabolic mechanisms within its cellular system (Gill and
Tuteja 2010). Consequently, the importance of microbes, especially the endophytes, increases
immensely. Endophytic microbes aid in plant health by deterring herbivory and pathogenesis
while also facilitating plant growth through nutrient uptake (modification of root morphology,
alteration of nitrogen accumulation and metabolism), water use efficiency (osmotic adjustment,
stomatal regulation) and curtailing of environmental stresses (Lata et al., 2018). The endophytes,
in return, obtain access to the host plant’s nutrients and dissemination to the next generation.
Symbiotically conferred abiotic stress tolerance involves at least two mechanisms: (i) activation
of host stress response systems soon after exposure to stress, allowing the plants to avoid or
mitigate the impacts of the stress (Redman et al. 1999) and (ii) biosynthesis of antistress
biochemicals by endophytes (Schulz et al. 2002).
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The molecular mechanisms for increasing stress tolerance in plants by endophytes include
induction of plant stress genes as well as biomolecules like reactive oxygen species scavengers.
Studies conducted on Arthrobacter sp. and Bacillus sp. isolated from pepper plant showed
significant reduction in upregulation and even downregulation of some stress-inducible genes
when compared with gene expression in uninoculated plants. Phoma glomerata and Penicillium
sp. significantly increased plant biomass, related growth parameters, assimilation of essential
nutrients, such as potassium, calcium, magnesium, and reduced the sodium toxicity in cucumber
plants under sodium chloride and polyethylene glycol-induced salinity and drought stress when
compared with control plants (Waqas et al. 2012). Research is still ongoing to understand the
endophytic nature of micro-organisms. It is still under the scan that how a pathogenic microbe is
switching over endophytic lifestyle. It is necessary to understand physiological and molecular
changes in endophytes as well as host during their interaction. Microbe-mediated stress tolerance
in plants is an ecofriendly approach for better crop yield. They can increase the crop land and
species diversity. Endophytes may also be a good tool for improvement of quality and yield of
plant products through genetic engineering and tissue culture. Induced drought tolerance was
observed during studies conducted by Hasegawa et al. (2004) on tissue cultured Kalmia latifolia
L. seedlings infected with endophyte Streptomyces padanus AOK-30. Gokhale et al. (2017) have
enlisted several patents granted on different aspects of endophytic fungi. The commercial
application of endophytic microbes for plant growth promotion and natural products is not as
common as PGPR or mycorrhizae due to a range of scientific and technical challenges especially
in formulations (Schonwandt € et al. 2014). Studies need to be carried out on how to overcome
the challenges for commercial formulations of endophytic micro-organisms.
Page 24 of 34
Enhancing the abiotic stress tolerance of plants: From chemical treatment to
biotechnological approaches
To enhance plant stress tolerance, different strategies have been developed to ensure plant
survival and improve production efficiency. Both chemical priming agents and genetic
engineering can enhance regulatory and functional genes and increase stress resistance of plants
(Nguyen et al., 2018). Utilization of chemical compounds as priming agents was developed after
it was found to significantly improve plant tolerance against a range of different abiotic stresses
(Irani and Todd 2018). Priming is a technique in which plants are treated with chemical agents to
protect them against environmental effects. Many types of molecules, such as reactive oxygennitrogen-sulfur species (Wang et al. 2009, Antoniou et al. 2017), hormones (Koo 2017), and
synthetic compounds (Lin et al. 2008a), are potential plant protectants utilized for reducing the
effects of abiotic stresses.
There is an urgent need for new strategies that do not rely on pesticides or single resistance genes,
the exploitation of the capacity of the plant immune system in combination with other strategies
may hold the potential to achieve better protection of crops. The development of molecular
genetics has increasingly identified many genes related to plant stress response. Consequently,
the application of molecular approach based on those genes showed an effective strategy for
enhancing stress tolerance in plant. Engineering genes involved in regulatory and functional
genes protect plant against the stress tolerance through various mechanisms by maintaining the
structure and function of cellular components. Therefore, gene discovery and genetic
manipulation will continuously develop in the future as a promising approach for enhancing
abiotic stress resistance. With increasing knowledge on plant response to abiotic stress and
molecular understanding of gene networks underlying the physiological processes, transgenic
Page 25 of 34
approach targeting a unique gene has become possible. The advent of CRISPR/Cas provides new
sources of genetic variation for plant selections. The generation and use of genome-edited
variants is a great addition to the current breeding toolbox (Shi et al. 2017). Transcriptome
sequences of a number of plants are now available in public databases. This vast information will
assist in identifying various genes governing various important traits and will help in identifying
the target sites for genome editing and genetic transformation. Recent research on chemical
priming and molecular biology has provided further understanding of the mode of action of
specific signaling molecules in plant stress resistance. Utilization of chemical agents as
protectants to improve abiotic stress resistance in plants is highly promising. Meanwhile, due to
dramatic developments in gene transfer techniques, many transgenic plants were reported to
show significantly improved tolerance to different stresses. These achievements are important for
farming in stressed areas and may open up opportunities for the development of crop stress
management in the near future(Nguyen et al., 2018).
Conclusion
Abiotic stresses are the major factors impeding plant growth and development processes thus
affecting crop plants and causing decrease in plant quality and productivity worldwide. A
combination of different abiotic stresses may act synergistically or additively in terms of their
impact on plant growth. To survive, plants have evolved a plethora of complex and dynamic
mechanisms that are effective in to protecting themselves from diverse stresses.
Page 26 of 34
Abbreviations
BRs
Brassinosteroids
GAs Gibberellins
HM Heavy metal
SA
Salicylic acid
Page 27 of 34
Page 28 of 34
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