Analysis of how 4 different Brassica rapa genotypes metabolize gibberellic acid (GA3).
Abstract:
Plant hormones, such as auxin, cytokinin, gibberellic acid, and abscisic acid, regulate plant
growth rates. Growth promoters (auxin, cytokinin, and gibberellic acid) and growth inhibitors
(abscisic acid) endogenously and exogenously impact the growth of plants. The purpose of this
experiment was to analyze how four different genotypes of Brassica rapa synthesize the
hormone gibberellic acid (GA3) for primary growth. Brassica rapa (Wisconsin Fast plants) are
common model species used because of their rapid regeneration rates. The experiment was
conducted utilizing four different genotypes (wild-type, petite, elongate, and rosette) by
separating them into three groups based on treatment. The treatments were water, exogenous
GA3, and cycocel (growth inhibitor). The duration of the experiment was one week, with the
plants receiving treatment once a day (except for the weekend). Each plant’s height was to be
measured in millimeters before treatments were given. The results showed that GA3 treatments
to the wild-type and rosette genotypes prompted noticeable growth (1.2-fold and 1.6-fold
increase); the elongate and petite genotypes had little growth (1.0-fold increase) in response to
GA3. Cycocel treatments to the wild-type and elongate genotypes exhibited reduced growth (0.7fold and 0.8-fold decrease). The results show that the different responses to GA3 were affected
by the presence of signal receptors in the genotypes. Cycocel responses were affected by the
amount of endogenous GA3 in each plant. This experiment allowed us to look into future studies
involving sustainable agriculture practices that use external plant hormone treatments.
Introduction:
Plants rely on signaling molecules, called hormones, to regulate growth, development, and
responses to environmental stimuli. The endogenous and exogenous effects of plant hormones
combine to display the different physiological differences in plants. This manuscript delves into
the multiple roles of plant hormones, focusing on their effects on various plant functions and
responses.
Plant hormones play critical roles in regulating defense mechanisms against biotic and abiotic
stressors, adjusting growth patterns, and influencing developmental processes. Defense responses
are mediated by hormones such as salicylic acid, jasmonates, and ethylene, which all primarily
regulate the plant's reaction to abiotic stressors (Bari & Jones, 2009). Additionally, certain
hormones, such as abscisic acid, inhibit growth to induce seed dormancy (which slows down
seed germination). On the other hand, hormones like auxin, gibberellin, and cytokinin promote
growth by regulating primary and secondary growth processes.
The classification of plant hormones into growth promoters, inhibitors, and stress hormones
allows scientists to understand each of their individual functions. The main growth promoters in
plants are auxin, gibberellin, and cytokinin. Auxin triggers cell elongation—a form of primary
growth—in plants. Phototropism, the ability for a plant to grow towards a light source, is fully
functional in plants due to auxin's influence. The Acid Growth Hypothesis explains that an
increase in auxin stimulates the growth of the cell wall, allowing for the entire plant to grow.
Gibberellin activates stem elongation and breaks seed dormancy, which permits germination to
occur. Germination happens when the growth potential of embryos increases and hydrolytic
enzymes are induced (Gupta & Chakrabarty, 2013). When there is more gibberellin than abscisic
acid in a plant, stem growth will happen. Cytokinin promotes cell division and differentiation in
the roots; apical dominance is inhibited by cytokinin to permit the lateral growth of the axillary
buds.
In contrast, growth inhibitors like abscisic acid (ABA) function to reduce the growth of a plant,
mostly with the germination of the seeds. ABA is involved with regulating seed germination
under severe environmental conditions. The interaction of ABA with gibberellic acid (GA3)
determines the timing of germination (ABA:GA3). Moreover, ABA plays a significant role in
plant water conservation by promoting the closure of stomata, thus reducing water loss through
transpiration (Leung & Giraudat, 1998). Another inhibitor, ethylene, serves as a "stress"
hormone, as it is also denoted as a plant's "fight or flight" response. Ethylene is involved in
various physiological processes, including leaf abscission, fruit ripening, and the triple response,
where plants alter their growth patterns in response to mechanical stress or environmental cues.
To investigate the effects of plant hormones in a controlled setting, Brassica rapa, commonly
known as Wisconsin Fast Plants, serves as an ideal model species. These plants offer several
advantages, including short generation times and phenotypic diversity, making them ideal for
experimental studies. Previous research utilizing Brassica rapa has explored various aspects of
plant physiology. For example, the effects of ozone exposure on stomatal conductance and net
assimilation during stages of vegetative growth have been conducted (Black et al. 2007).
Another study has been conducted on the different Brassica rapa genotype responses to
competition (Kelly & Terrana, 2004).
The primary objective of this study is to investigate how GA 3 metabolism differs among four
distinct Brassica rapa genotypes: wild-type, petite, elongate, and rosette. Through a controlled
setting involving the different treatments of water, exogenous GA3, and cycocel, we are looking
at the genetic variations in Brassica rapa and their impacts on plant hormone metabolism and
response pathways.
Methods:
Our experimental design consisted of three treatments: water, exogenous GA 3, and cycocel. The
water treatment was the control group, which aimed to determine the normal growth of each
genotype. Exogenous GA3 was utilized to measure the effects of an added growth promoter
hormone. Cycocel, a plant growth regulator, was added to inhibit/regulate the growth of the
different genotypes. The Brassica rapa genotypes utilized in this experiment were wild-type
(control), petite (dwarf), elongate (giant), and rosette (dwarf).
Plants were treated once a day, excluding the weekend, for one week, beginning on Thursday
and ending on Thursday. Experimental units were defined by measuring the height of the plants
in millimeters (mm). Each treatment consisted of two plants of each genotype, with two wildtype plants receiving the same treatment in separate wells.
Fig. 1. The experiment setup is shown as a self-watering planter. In the top left corner, the plants
are treated with water; in the top right corner, the plants are treated with exogenous GA3. In the
bottom right, the plants are treated with cycocel. The order of the genotypes from left to right are
wild-type, petite, elongate, and rosette. Toothpicks were used as shown to hold the plants that
were drooping upright (essentially a manmade form of gravitropism). Each well contains one
plant (shorter shoots should have been removed before the start of the treatments).
Results:
The effect sizes from different treatments on the growth of four Brassica rapa genotypes were
evaluated. The wild-type plants exhibited a 1.2-fold increase in growth with GA3 treatment but a
0.77-fold decrease with cycocel treatment. Similarly, petite and elongate genotypes showed a
1.0-fold increase in growth with GA3 treatment and a 0.94-fold and 0.8-fold decrease with
cycocel treatment. The rosette genotype displayed a significant response to GA 3 treatment, with
a 1.6-fold increase in growth compared to the control, while experiencing a 0.9-fold decrease in
growth with cycocel treatment. These findings, as depicted in Figure 2, illustrate the different
responses of Brassica rapa genotypes to the various treatments.
Fig 2: The mean height for four different genotypes of Brassica rapa is illustrated. From left to
right the genotypes are wild-type, petite, elongate, and rosette. Three treatment types are shown
as water, GA3, and cycocel. The error bars represent the standard deviation.
When comparing the genotype responses, it is evident that the rosette genotype exhibited a more
significant response to GA3 treatment, displaying a 1.6-fold increase in growth compared to the
1.2-fold increase observed in the wild-type. Both the petite and elongate genotypes showed
minimal growth enhancement, with a 1.0-fold increase when treated with GA3. Additionally, the
wild-type genotype demonstrated a significant decrease in growth (0.77-fold) when treated with
cycocel compared to the petite genotype (0.94-fold decrease), highlighting distinctive
sensitivities to growth inhibition among genotypes.
Discussion:
Our objective was to compare and contrast how gibberellic acid (GA3) metabolizes among
different genotypes of Brassica rapa. The data obtained from our study suggests that while the
wild-type and rosette genotypes effectively metabolized GA3, the petite and elongate genotypes
exhibited limited capacity for GA3 metabolism.
The wild-type genotype displayed a normal response to both GA3 and cycocel treatments. This
genotype utilizes GA3 as the primary hormone involved with plant growth. The wild-type
genotype demonstrated responsiveness to both endogenous and exogenous GA 3, making it a
perfect control group genotype.
In contrast, the petite genotype exhibited minimal response to both GA3 and cycocel treatments.
This lack of response to GA3 can be attributed to the inhibition of GA3 synthesis by DELLA
proteins, leading to blocked GA3 receptors and limited growth (Achard & Genschik, 2009).
Additionally, the petite genotype lacks GA3 signal receptors, making it insensitive to the cycocel
treatments.
The elongate genotype displayed no significant reaction to exogenous GA 3 treatment but
experienced a substantial reduction in growth with the cycocel treatment. This observation
suggests that the elongate genotype already has an excess of endogenous GA3, making the
application of exogenous GA3 ineffective (Shan et al. 2009). Furthermore, the excessive amount
of GA3 receptors in the elongate genotype makes it highly susceptible to the inhibitory effects of
cycocel.
The rosette genotype responded to both GA3 and cycocel treatments. The deficiency of
endogenous GA3 in the rosette genotype leads to inhibited germination and reduced stem growth.
However, the application of exogenous GA3 induces mass growth and flowering, indicating that
the rosette genotype relies on external sources of GA3 for growth promotion (Rood et al. 1989).
Furthermore, while the rosette genotype is not entirely dependent on GA 3 for growth, the cycocel
treatments reduce the growth promoted by the applied endogenous GA3. These explanations
show the variances in the hormone signaling pathways in this genotype.
Conclusion:
Our study demonstrates the effects of plant hormones on growth regulation, based on their
specific signal pathways. The modification of plant growth by hormones such as gibberellic acid
(GA3) is linked to signal pathways that respond to both growth promoters and inhibitors.
We found that DELLA proteins are majorly involved with the regulation of GA3 signal
receptors, as they restrict growth and enable regular increases in stem elongation. While the
wild-type and petite genotypes both possess DELLA proteins, the petite receptors connected to
these enzymes are blocked, which inhibits GA3 production only in the petite genotype. The
elongate and rosette genotypes lack regulatory growth receptors, leading to varying responses to
exogenous GA3 that are completely dependent on the levels of endogenous GA3 production.
For future research into external plant hormone treatments aimed at improving growth, we can
look into how gibberellic acid can affect different areas of growth. For example, experiments
conducted on crops such as soybeans and tomatoes have revealed that exogenous GA 3 promotes
vertical growth of internode cells without any significant changes in the width of the cells. The
internode cells contain the part of the stem connected to two nodes, meaning the xylem and pith
will also grow vertically. The width involves the secondary growth of the phloem and vascular
cambium, which does not correspond with the exogenous GA3 treatments (Shan et al. 2021).
This research raises the question as to why exogenous GA3 selectively affects primary growth in
certain crops and not secondary growth.
Our study also highlights the need for further research into hormones that regulate secondary
growth processes. Understanding the mechanisms that secondary growth regulation can provide
for the future enhancements of plant growth and development is crucial.
Lastly, our findings exhibit the importance of identifying plant genotypes that can synthesize
GA3 for height benefit versus those that may require external hormonal treatment for growth
enhancement. This knowledge can allow for the development of more efficient and sustainable
agricultural practices, paving the way for future studies that aim to maximize crop production
while also producing sustainably.
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