1. Abscisic Acid Signaling 严涛 04 May 2018 College of Agriculture & Biotechnology Zhejiang University

2. Abscisic Acid Biosynthesis ABA is a sesquiterpenoid synthesized from zeaxanthin in a five-step biosynthetic process, catalyzed by the proteins ABA1, ABA4, NCED, ABA2, and ABA3. Zeaxanthin and initial biosynthetic reactions are localized in plastids while the reactions after xanthoxin (ABA2, ABA3) are localized in the cytoplasm. The hormone is catabolized either by CYP707A mediated oxidative degradation or conjugation with glucose. Alternative biosynthesis pathways exist in fungi. Hauser et al. (2017) Cell, 171(7):1708

3. ABA Receptor Signal Transduction Core ABA binds to the intracellular PYR/RCAR receptors. Binding of ABA to these receptors enhances their interaction with and inhibition of clade A PP2C protein phosphatases. PP2C inhibition enables the activation of SnRK2/OST1 (Open Stomata 1) protein kinases. These kinases phosphorylate and activate targets, including ion channels that mediate stomatal closure and transcription factors that function in transcriptional networks, which leads to the synthesis of proteins and molecules capable of preventing damage caused by abiotic stresses. The PP2C phosphatases also dephosphorylate additional downstream targets, including channels and transcription factors. Hauser et al. (2017) Cell, 171(7):1708

4. Guard Cell ABA Signaling and ABA Signaling during Seed Germination ABA regulates guard cell ion fluxes to mediate stomatal pore closure and reduce transpirational water loss. In the absence of ABA or at low basal ABA levels, PP2Cs are active and directly inhibit the ABA-activated protein kinase OST1, as well as the S-type anion channel SLAC1 and other ion channels. High ABA concentrations in seeds activate transcription factors, including ABI3/ABI4/ABI5, and suppress germination (i.e., stabilize dormancy). Decreasing ABA concentrations along with rising gibberellin concentration initiates the transition from dormancy to germination Hauser et al. (2017) Cell, 171(7):1708

5. ABA-Marker Gene Expression RAB18 and RD29B are marker genes for ABA-induced gene expression. The heatmaps show expression patterns of RAB18 and RD29B at basal native ABA concentrations. Based on these data, ABA-dependent gene expression combined with tissue-dependent basal ABA levels is pronounced in guard cells and seeds and less active in roots under non-stressed conditions. Abiotic stress and ABA further enhance RAB18/RD29B expression Hauser et al. (2017) Cell, 171(7):1708

6. ABA Transport Members of the ATP-binding cassette (ABCG) transporter family, the NRT1/PTR Family (NPF), and the DTX/MATE-type transporter DTX50 have been characterized as ABA transporters. These ABA transporters localize in the plasma membrane and either export ABA from source tissues where ABA is synthesized, such as vascular tissues and the seed endosperm, or import ABA into sink tissues, such as embryos or guard cells. Although several ABA transporters of the NPF family require further characterization, NPF4.6 functions in retaining ABA in source vascular tissues. Guard cells synthesize ABA and express both ABA importers (ABCG40 and ABCG22) and exporters (DTX50). Hauser et al. (2017) Cell, 171(7):1708

7. Protein Turnover Regulation in ABA Signaling Protein abundance of key components in the ABA signal transduction network is regulated by ubiquitination-mediated degradation by the 26S-proteasome or vesicle-mediated delivery and degradation in vacuoles. Hauser et al. (2017) Cell, 171(7):1708

8. ABA Control of Root Growth ABA can function synergistically or antagonistically with other hormones and signals, including auxin, ethylene, and ROS, to fine-tune root architecture. New evidence suggests that ABA regulates directional (tropic) root growth by modulating differential growth in cortical cells of the elongation zone to promote hydrotropic bending. Hauser et al. (2017) Cell, 171(7):1708

9. New insights on ABA  Recently, A research shows that short-day induced dormancy is regulated by the plant hormone abscisic acid which activates (among others) the production of the callose that is used to block the plasmodesmata. Once blocked, a long exposure to low temperatures is needed to slowly re-open the plasmodesmata again, so that the growth-inducing signals can reach the buds and stimulate the growth in the buds in the spring.  It is clear that ABA levels that have been increased by winter buds induce the closure of plasmodesmata, which prevents cell-to-cell communication of growth-promoting signals into the shoot apical meristem of the winter buds. So the function of ABA in winter buds is to prevent the release of dormancy by intermittent warm days during winter.  Some of the facets of the dormancy regulation mechanism described in the paper have been observed in winter wheat as well as characean algae, suggesting that this mechanism is probably ancient and evolutionarily conserved. Tylewicz et al. (2018) Science:eaan8576

10. New insights on ABA Ubiquitin–Proteasome System in ABA Signaling Perception of ABA signaling is conducted by the soluble ABA receptors, PYR/PYL/RACR type proteins. PYR/PYL/RACR receptors bind to ABA, which leads to or enhances the interaction of the receptors and PP2C phosphatases, and forms a complex of ABA, receptors, and PP2Cs. Recruitment of PP2Cs by PYR/PYL/RACR leads to the dissociation of PP2Cs and SnRK2s. The released SnRK2s can phosphorylate downstream ABA regulators, such as bZIP transcription factors, ion channels, and oxidases. Yu et al. (2016) Molecular Plant, 9(1):21-33.

11. New insights on ABA Chen et al. demonstrate that Arabidopsis EL1-like (AEL) protein, a that regulates various physiological processes, phosphorylate PYR/PYLs to promote their ubiquitination and degradation, resulting in suppressed ABA responses. Chen et al. (2018) Molecular Plant