Understanding ABSIC Acid (ABA) in Plant Processes

1. Seed quality ABSCISIC ACID (ABA) controls many plant processes including stress responses, development and reproduction Dormancy Germination Stress tolerance Abscisic Acid Development Biotic stress response Gene expression Stomatal aperture Adapted with permission from RIKEN

2. 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.

3. ABA is synthesized in the plastid and cytoplasm and is derived from zeaxanthin, a plant pigment Zeaxanthin Zeaxanthin is abundant in green tissues but can be limiting for ABA synthesis in roots ABA ABA2 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.

4. ABA accumulation and homeostasis are tightly controlled Water stress Developmental signals Zeaxanthin Irreversible inactivation NCED 9-cis-expoxycarotenoids xanthoxin Rehydration Developmental signals Reversible inactivation NCED (9-cis-epoxycarotenoid dioxygenase) expression is closely correlated with the rate of ABA synthesis [ABA] -Glc ABA-glucosyl ester

5. 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

6. 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

7. Perception and Signaling PYR/RCAR receptors The core signaling pathway PYR1 ABA PP2C Protein phosphatases (including ABI1) Phosphatase Protein kinases (including SnRK2s and CDPKs) P P Kinase TF ABA RESPONSES

8. The PYR/RCAR ABA receptors are necessary for ABA responses Wild-type plants fail to germinate on ABA-containing medium Pyrobactin-insensitive mutants are ABA-insensitive and so germinate on ABA-containing medium The ABA-insensitive mutant abi1 germinates on ABA-containing 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.

9. PYR/RCAR receptors bind ABA in a complex with ABI1 or other PP2Cs PYR1 NO ABA ABA 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.

10. PP2Cs interfere with the action of SnRK2 protein kinases PYR1 NO ABA In the absence of ABA, SnRK2 protein kinase activity is inhibited by PP2C phosphatases ABI1 SnRK2 P NO ABA RESPONSES

11. 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) SnRK2 P P P P SnRK2 TF Ion channel ABA RESPONSES

12. SnRK2s are protein kinases that promote ABA responses The CPDK-SnRK superfamily of protein kinases P P P P SnRK2 TF Ion channel The SnRK2 subfamily 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., Walker-Simmons, K., Zhu, J.-K., and Harmon, A.C. (2003). The Arabidopsis CDPK-SnRK superfamily of protein kinases. Plant Physiol. 132:666-680.

13. 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.

14. Transcription factors (TFs) are major targets of CDPKs and SnRK2s ABA Some TFs were identified genetically PYR1 PP2C SnRK2 P Some TFs were identified biochemically P SnRK2 CDPK CDPKs are cyclin-dependent protein kinases TF ABA RESPONSES

15. Transcriptional targets Stress and dehydration -induced genes Signaling genes A major output of ABA signaling is changes in transcription patterns. Many of the transcriptionally upregulated genes have functions in osmoprotection Seed- specific genes Genes involved in ABA metabolism

16. ABA signaling - review PYR/RCAR receptors The core signaling pathway PYR1 ABA PP2C Protein phosphatases (including ABI1) Phosphatase Protein kinases (including SnRK2s and CDPKs) P P P P Kinase TF TF Ion channel ABA RESPONSES

17. 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

18. 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 C CO2 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.

19. ABA regulates guard cell turgor through complex signals ABA Guard cell turgor is regulated by a complex network of interacting second messengers, pH, membrane potential, protein phosphorylation, ion channel activity – and more!! INNER WALL NO H2O2 PP2C Ca2+ CDPK SnRK2 K+ A- A- K+ A- A- K+ K+ K+ A-

20. 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.

21. 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 Studies of desiccation tolerant plants contributes to our understanding of cellular desiccation responses Rewatered Selaginella tamariscina Craterostigma plantagineum 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.

22. ABA controls seed maturation, dormancy and desiccation Seed dormancy and desiccation tolerance is correlated with high levels of ABA synthesis and accumulation Germination involves catabolism of ABA and synthesis of GA GA ABA Embryonic patterning Reserve accumulation Reserve mobilization Desiccation tolerance Cell expansion

23. 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 More crop per drop

24. 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