Seed quality
ABSCISIC ACID (ABA)
controls many plant
processes including stress
responses, development
and reproduction
Dormancy
Germination
Abscisic Acid
Development
Biotic stress
response
Gene expression
Stomatal aperture
Stress tolerance
Adapted with permission from RIKEN
© 2015 American Society of Plant Biologists
Biosynthesis, homeostasis and
transport
ABA levels
increase
during stress
but decrease
when stress
is relieved
Jan Zeevaart (1930-2009) was a
major contributor to our understanding
of ABA synthesis and homeostasis.
Image courtesy of Michigan State University-Department of Energy Plant Research Lab; Zeevaart, J.A.D. (1980). Changes in the levels of
abscisic acid and its metabolites in excised leaf blades of Xanthium strumarium during and after water stress. Plant Physiol. 66: 672-678.
© 2015 American Society of Plant Biologists
ABA is synthesized in the plastid
and cytoplasm and is derived from
zeaxanthin, a plant pigment
Zeaxanthin
ABA2
ABA
Zeaxanthin is
abundant in green
tissues but can be
limiting for ABA
synthesis in roots
Reprinted from Nambara, E., and Marion-Pol, A. (2003) ABA action and interactions in
seeds. Trends Plant Sci. 8: 213-217 with permission from Elsevier.
© 2015 American Society of Plant Biologists
ABA accumulation and homeostasis
are tightly controlled
Zeaxanthin
Water stress
Developmental signals
Irreversible
inactivation
NCED
Rehydration
Developmental signals
9-cisxanthoxin
expoxycarotenoids
NCED (9-cis-epoxycarotenoid
dioxygenase) expression is
closely correlated with the rate
of ABA synthesis
[ABA]
Reversible
inactivation
-Glc
ABA-glucosyl ester
© 2015 American Society of Plant Biologists
ABA movement from root to shoot
may help regulate stomatal aperture
ABA
translocation
and root
hydraulic
signals may
be involved in
signaling
from root to
shoot
Well-watered
plant with
open stomata
and high
transpiration
rate
Water-stressed
plant with closed
stomata and low
transpiration rate
© 2015 American Society of Plant Biologists
Biosynthesis, homeostasis and
transport - summary
•ABA synthesis increases with drought
stress and during seed maturation
•In most but not all tissues NCED is rate
limiting for ABA synthesis
•ABA can be degraded to phaseic acid or
reversibly conjugated to ABA-GE
•ABA can be transported within the plant,
from root to shoot and from vascular tissues
to guard cells
© 2015 American Society of Plant Biologists
Perception and Signaling
PYR1
PYR/RCAR receptors
ABA
Phosphatase
PP2C Protein phosphatases
(including ABI1)
Protein kinases (including
SnRK2s and CDPKs)
Kinase
P
The core
signaling
pathway
P
TF
ABA RESPONSES
© 2015 American Society of Plant Biologists
The PYR/RCAR ABA receptors are
necessary for ABA responses
Wild-type plants fail
to germinate on
ABA-containing
medium
Pyrobactin-insensitive
mutants are ABAinsensitive and so
germinate on ABAcontaining medium
The ABA-insensitive
mutant abi1
germinates on ABAcontaining medium
From Park, S.-Y., et al., and Cutler, S.R. (2009). Abscisic acid inhibits type 2C protein phosphatases via the
PYR/PYL family of START proteins. Science 324: 1068-1071 reprinted with permission from AAAS.
© 2015 American Society of Plant Biologists
PYR/RCAR receptors bind ABA in a
complex with ABI1 or other PP2Cs
NO ABA
ABA
PYR1
PYR1
PP2C
PP2C
Reprinted from Raghavendra, A.S., Gonugunta, V.K., Christmann, A., and Grill, E. (2010) ABA
perception and signalling. Trends Plant Sci. 15: 395-401 with permission from Elsevier.
© 2015 American Society of Plant Biologists
PP2Cs interfere with the action of
SnRK2 protein kinases
NO ABA
PYR1
ABI1
SnRK2
In the absence of
ABA, SnRK2 protein
kinase activity is
inhibited by PP2C
phosphatases
P
NO ABA
RESPONSES
© 2015 American Society of Plant Biologists
ABA / PYR1 binding sequesters
PP2C and permits SnRK2 activity
PYR1
PYR1, ABA and PP2C form a
complex that inactivates PP2C
This permits SnRK2 activation.
Phosphorylation targets include
SnRK2s, ion channels and
transcription factors
PP2C
(ABI1)
P
SnRK2
SnRK2
P
P
TF
Ion
channel
P
ABA RESPONSES
© 2015 American Society of Plant Biologists
SnRK2s are protein kinases that
promote ABA responses
The CPDK-SnRK
superfamily of protein
kinases
P
SnRK2
P
P
TF
Ion
channel
The SnRK2 subfamily
P
ABA RESPONSES
Hrabak, E.M., Chan, C.W.M., Gribskov, M., Harper, J.F., Choi, J.H., Halford, N., Kudla, J., Luan, S., Nimmo, H.G., Sussman, M.R., Thomas, M., WalkerSimmons, K., Zhu, J.-K., and Harmon, A.C. (2003). The Arabidopsis CDPK-SnRK superfamily of protein kinases. Plant Physiol. 132: 666-680.
© 2015 American Society of Plant Biologists
ABA signaling contributed to evolution
of drought tolerance in land plants
Reprinted from Umezawa, T., Nakashima, K., Miyakawa, T., Kuromori, T., Tanokura, M., Shinozaki, K., and Yamaguchi-Shinozaki, K. (2010). Molecular basis of the core
regulatory network in ABA responses: Sensing, signaling and transport. Plant Cell Physiol. 51: 1821-1839 with permission from the Japanese Society of Plant Physiologists.
© 2015 American Society of Plant Biologists
Transcription factors (TFs) are major
targets of CDPKs and SnRK2s
Some TFs were identified genetically
PYR1
ABA
PP2C
P
SnRK2
Some TFs were identified biochemically
SnRK2
CDPKs are
cyclin-dependent
protein kinases
CDPK
P
TF
ABA RESPONSES
© 2015 American Society of Plant Biologists
Transcriptional targets
Signaling genes
Stress and dehydration induced genes
Seedspecific genes
Genes involved in
ABA metabolism
A major output of ABA
signaling is changes in
transcription patterns. Many of
the transcriptionally
upregulated genes have
functions in osmoprotection
© 2015 American Society of Plant Biologists
ABA signaling - review
PYR1
PYR/RCAR receptors
ABA
PP2C Protein phosphatases
(including ABI1)
Phosphatase
Protein kinases (including
SnRK2s and CDPKs)
Kinase
P
PP
Ion
channel
The core
signaling
pathway
TF
TF
P
ABA RESPONSES
© 2015 American Society of Plant Biologists
ABA’s roles in whole-plant
processes
•Guard cell responses
•Root growth
•Vegetative dehydration
responses and osmoprotectants
•Seed development
•Biotic stress responses
•Drought-tolerant plants
© 2015 American Society of Plant Biologists
Guard cells are regulated portals for
gas exchange and transpiration
Turgid guard cells
= open stomata
= gas exchange
+ transpiration
Flaccid guard cells
= closed stomata
= decreased gas exchange
+ decreased transpiration
COC2
Sirichandra, C., Wasilewska, A., Vlad, F., Valon, C., and Leung, J. (2009a). The guard cell as a single-cell model towards
understanding drought tolerance and abscisic acid action. J. Exp. Bot. 60: 1439-1463. by permission of Oxford University Press.
© 2015 American Society of Plant Biologists
ABA regulates guard cell turgor
through complex signals
Guard cell turgor is regulated by a
ABA
complex network of interacting
NO
INNER WALL second messengers, pH, membrane
potential, protein phosphorylation,
ion channel activity – and more!!
H2O2
PP2C
Ca2+
CDPK
SnRK2
K+
K+
A-
K+
K+
© 2015 American Society of Plant Biologists
Water stress and ABA promote root
growth at the expense of shoot growth
Increasing water stress
Sharp, R.E., Silk, W.K., and Hsiao, T.C. (1988). Growth of the maize primary root at low water potentials : I.
Spatial distribution of expansive growth. Plant Physiol. 87: 50-57.
© 2015 American Society of Plant Biologists
Desiccation-tolerant plants reveal
cellular mechanisms
Watered control
A few plants, such as
these “resurrection
plants” can stay alive
even when 90% of their
water content is lost
Water withheld 5 days
Rewatered
Craterostigma
plantagineum
Studies of desiccation
tolerant plants contributes to
our understanding of cellular
desiccation responses
Rewatered
Selaginella tamariscina
Liu, M.-S., Chien, C.-T., and Lin, T.-P. (2008). Constitutive Components and Induced Gene Expression are Involved in the Desiccation Tolerance of
Selaginella tamariscina. Plant and Cell Physiology 49: 653-663, by permission of the Japanese Society of Plant Physiologists; Bohnert, H.J. (2000).
What makes desiccation tolerable? Genome Biology, published by BioMed Central.
© 2015 American Society of Plant Biologists
ABA controls seed maturation,
dormancy and desiccation
Seed dormancy
and desiccation
tolerance is
correlated with
high levels of ABA
synthesis and
accumulation
ABA
GA
Germination
involves catabolism
of ABA and
synthesis of GA
Embryonic
patterning
Reserve
accumulation
Desiccation
tolerance
Reserve
mobilization
Cell expansion
© 2015 American Society of Plant Biologists
Towards drought-tolerant plants
Many approaches are being investigated to
breed drought-tolerant plants:
•Modification of ABA synthesis and inactivation
to reduce transpiration
•Increased ABA sensitivity of guard cells to
reduce transpiration
•Increase root growth for better water uptake
•Drought-inducible expression of desiccation
tolerance genes
© 2015 American Society of Plant Biologists
ABA - summary
The hormone ABA and its signaling
pathway were instrumental in the
evolution of land plants
ABA participates in
physiological, developmental
and defense responses
throughout the plant body
Studying ABA is important
for the development of
drought-tolerant crops
© 2015 American Society of Plant Biologists