Sustainable Phosphorus Summit,
Montpellier Sep 2014
“Understanding the genetic
control of rhizosheath
formation and impacts of
multiple stress on
phosphorus acquisition
Tim George
Ecological Sciences
The James Hutton Istitute
Productive agricultural land – scarce resource
Only 30% of the Earth’s surface is land,
and only 9% of this land area is cultivated,
which will not increase
Nearly 33% of the worlds arable land lost
to erosion or pollution in the last 40 years.
Erosion rates from ploughed fields average
10-100 times greater than rates of soil
formation.
LARGE SCALE ARABLE
PRODUCTION
UPLAND PADDY RICE
SYSTEMS
LARGE SCALE PIVOT IRRIGATION SYSTEMS
Food Security – Demand for Food
Population Growth
Balanced “Western” Diet
We need to produce as much food in the next 50
years as we have in the last 10000 with fewer inputs
The need to improve the P-use efficiency of plants
• Many soils are P-deficient
- agricultural systems respond significantly to P application
• Efficiency of P fertilizer use is poor
- 10 to 50% recovery of applied P
- fixation of P in soil & accumulation of total soil P
• Environmental problems with P mismanagement
- eutrophication of aquatic environments
- need to reduce the P-load on a landscape scale
• Future trends towards P-deficit
- Rock-P is a finite resource
- ‘Nutritional drought’ caused by water deficit
- agriculture will reach nutrient limited productivity ceilings
Key traits identified from regulatory pathways
controlling plant response to P-deficiency
Replacement
phospholipids
Decreased
Photosynthesis
Increased Shoot Sucrose
Increased Root Sucrose
Decreased Shoot
Growth
Decreased Shoot P
Decreased Root P
PHR1
Cascade
Pi Transporters
Phosphatase
Organic Acid Efflux
Increased Root Growth
Root Hair Production
Mycorrhizal
Symbiosis
Hammond & White 2008 JXB 59:93
First account as “a peculiar sheath,
composed of agglutinated particles
of sand” critical for tolerance to
severe drought – Price 1911
New Phytologist 10:328-340
Aristida pungens – Sahara, North Africa
SEM of Maize Rhizosheath –
Thinner than most cereal
Rhizosheaths
Intimate interaction between
root hairs and mucilage
McCully 1999 Ann Rev Plant Phys Plant Mol Biol 50:695-718
Large genotypic variation in rhizosheath formation
250
Rhizosheath Weight (g g-1 root)
200
150
100
50
0
Elite and Mutant Spring Barley Varieties
144 Genotype = 12.1-fold variation (mutants - red)
5.1-fold variation (association population - blue)
George et al. New Phytologist 203:195-205
Rhizosheath improves growth in sub-optimal P and
dry conditions
Biomass Production (g)
6
5
4
P0
3
P250
2
P500
1
0
Small (20 g g-1)
Moderate (102 g g-1)
Rhizosheath Size
Large (197 g g-1)
Rhizosheath mapped to chromosome 2 using AMP
-log10fp
Putative candidate genes include:
• calcium/calmodulin-dependent protein kinase (OsCDPK7)
• glutamate receptor (GLR3.1)
• QTL’s on 2H
Chromosomal Position (cM)
Drought tolerance
Root elongation
Root length, Root dry weight
Relationship between rhizosheath and root hair
length in populations
Root hair length v Rhizosheath weight
Mutant Population
1.0
y = 0.1281x - 0.0103
R2 = 0.8979**
Root hair length (mm)
0.8
0.6
0.4
0.2
Association Population
0.0
0
2
4
6
Rhizosheath weight (g)
Rhizosheath Weight g cm-1 root
70
60
50
40
30
20
10
0
0
0.5
1
1.5
Root Hair Length
George et al. New Phytologist 203:195-205
2
2.5
3
Similar observations in wheat by Delhaize et al.
Aluminium
Tolerance
allele of
TaALMT1
Delhaize et al. 2012 New Phytologist 195: 609-619
Rhizosheath Phylogeny
George et al. (2014) New Phytologist 203:195-205
Poales
Asparagales
•
•
•
5-fold geneotypic variation in barley
QTL for trait on Chromosome 2H
Candidate genes include root growth genes and
drought tolerance
Rhizosheath is only found in Poales?
42 species across 9 orders screened
Asterales
Summary
 The ability of crops to cope with abiotic stress needs to be improved.
 Plants have a number of ways to improve P acquisition:
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Root Morphology
Organic acids
Phosphatases
Interaction with microorganisms – mycorrhizal symbiosis
Root hair formation
 Root hair presence is key to maintaining yield under stress and root hair length
is strongly related to rhizosheath formation in controlled and field conditions.
 Large genotypic variation and genetic association for rhizosheath formation
exists and impacts P acquisition.
 A putative QTL for rhizosheath has been mapped to chromosome 2- a number
of candidate genes present in this region.
 Rhizosheath is present in a range of groups across the phylogeny – so relevant
to a range of crops not just cereals.
 Future crops will benefit from maintenance or enhancement of the rhizosheath
trait.
Acknowledgements
Plant Soil Ecology
Sub-Programme
Lawrie Brown
Philip White
Lionel Dupuy
Personal Research Fellowship
Glyn Bengough
Barley Genetics
Bill Thomas
Luke Ramsay
RESAS Workpackage 3.3
Joanne Russell
Induced Mutations Grant
Root hair mutant phenotypes
No Root Hairs
(NRH)
Short Root Hairs
(SRH)
Long Root Hairs
(LRH)
Brown et al. 2012 Annals Botany 110: 319-
Rhizosheath reduced in some mutants
LRH
Rhizosheath weight by genotype
8
P0
P500
LSD p< 0.05
6
4
2
Wild-Type
LRH-3
LRH-2
LRH-1
SRH-3
SRH-2
SRH-1
NRH-3
NRH-2
0
NRH-1
Rhizosheath weight (g)
NRH
Genotype
Brown et al. 2012 Annals Botany 110: 319-
Shoot Biomass (mg plant-1)
Shoot P Conc (mg P g-1)
P accumulation (mg plant-1)
40.8
1100.0
x27
0.6
7.5
x13
24.2
8200.1
x340
Barley
Alfisol
26 days
No P
P Added
In-vitro mutant screen for root phenotype
Analyse mutants with desired
phenotype (size, root length,
root hairs) to elucidate genetic
basis for the trait
Screen SCRI Optic barley mutant population (9000 -22000
individuals) under controlled conditions of low P and a range of
water contents
No Root Hairs
Short Root Hairs
Long Root Hairs
In-vitro mutant screen for root phenotype
5%
2%2%1%
As Standard =
More Root Hairs +
25%
7%
Longer Root Hairs +
Agravitropic +
Slightly Longer Roots +
Very Dense Root Hairs +
Short Roots -
19%
5%
2%
30%
458 Mutant Lines
Short Root Hairs Slightly Shorter Roots -
1%
Fewer Root Hairs -
1%
Slightly Shorter Root Hairs -
0%
Hairless Very Short Root Hairs -
Root hair length reduced in some mutants
P0
1.6
a
P500
1.4
b
Root hair length (mm)
1.2
bc
c bc
1
d
0.8
0.6
e
e
0.4
0.2
f
f
0
NRH
SRH
LRH
Phenotypes
WT
P
% Relative effectiveness of phenotype on yield
Absence of root hairs limits P-deficient yield
120
100
LSD (p,0.05)
80
60
40
20
0
NRH
SRH
LRH
Phenotype
WT
Rhizosheath trait mapped to chromosome 2
-log10fp
Significant Association
Chromosome 2
Map Distance cM
Wheat Rhizosheath – picture M. Watt
Rhizosheath only found in Poales
which include all cereals
Duell & Peacock 1985
Crop Science 25:880-883
BUT some suggestion may exist
in other orders
Presence of root hairs enhances rhizosheath in the
field
Similar variation in chromosome substitution lines
Presence of rhizosheath allows tolerance to extreme
combined stress
Rhizosheath Size
Moderate
80
100
70
% Fie
60
ld Ca
pacity
50
0
Microarray analysis done under
drought conditions in contrasting
genotypes
g -1
)
500
m
400
20
300
P
200
90
40
g
timal P)
Biomass (wrt op
g -1
)
m
0
60
0
200
90
80
100
70
% Fie
60
ld Ca
pacity
50
0
(
P
50
0
300
80
ad
de
d
100
70
100
Relative Shoot
80
% Fie
60
ld Ca
pacity
ad
de
d
200
90
500
400
20
P
0
(
g
P
300
m
400
20
40
g
g -1
)
500
60
(
40
80
ad
de
d
tim
Biomass (wrt op
60
100
Relative Shoot
80
LRH
P
al P)
P)
optimal
t Biomass (wrtbiomass
Relative
Relative Shooshoot
100
0
20
40
60
80
100
Large
SRH
NRH
P
Small
Genotypes with no rhizosheath have a different
transcriptional response
 248 genes differentially regulated under extreme combined P
and water stress specific to genotypes without rhizosheath
 Differential regulation of putative PUE and rooting habit genes
 Others include genes involved in:
 P-deficit response
 biological stress tolerance
 water-stress tolerance
 oxidative stress
 tip growth
 membrane restructuring