Absence Of Abscisic Acid Research Articles

Abscisic acid-induced H2O2 production positively regulates the activity of SAPK8/9/10 through oxidation of the type one protein phosphatase OsPP47.

Subclass III sucrose nonfermenting1-related protein kinase 2s (SnRK2s) are positive regulators of abscisic acid (ABA) signaling and abiotic stress responses. However, the underlying activation mechanisms of osmotic stress/ABA-activated protein kinase 8/9/10 (SAPK8/9/10) of rice (Oryza sativa) subclass III SnRK2s in ABA signaling remain to be elucidated. In this study, we employed biochemical, molecular biology, cell biology, and genetic approaches to identify the molecular mechanism by which OsPP47, a type one protein phosphatase in rice, regulates SAPK8/9/10 activity in ABA signaling. We found that OsPP47 not only physically interacted with SAPK8/9/10 but also interacted with ABA receptors PYLs. OsPP47 negatively regulated ABA sensitivity in seed germination and root growth. In the absence of ABA, OsPP47 directly inactivated SAPK8/9/10 by dephosphorylation. In the presence of ABA, ABA-bound OsPYL2 formed complexes with OsPP47 and inhibited its phosphatase activity, partially releasing the inhibition of SAPK8/9/10. SAPK8/9/10-mediated H2O2 production inhibited OsPP47 activity by oxidizing Cys-116 and Cys-256 to form OsPP47 oligomers, resulting in not only preventing the OsPP47-SAPK8/9/10 interaction but also blocking the inhibition of SAPK8/9/10 activity by OsPP47. Our results reveal novel pathways for the inhibition of SAPK8/9/10 in the basal state and for the activation of SAPK8/9/10 induced by ABA in rice.

Ethylene-mediated stomatal responses to dehydration and rehydration in seed plants.

Ethylene, a plant hormone that significantly influences both plant growth and response to stress, plays a well-established role in stress signaling. However, its impact on stomatal opening and closure during dehydration and rehydration remains relatively unexplored and is still debated. Exogenous ethylene has been proven to induce stomatal closure through a series of signaling pathways, including the accumulation of reactive oxygen species, subsequent synthesis of nitric oxide and hydrogen sulfide, and SLOW ANION CHANNEL-ASSOCIATED 1 activation. Thus, it has been suggested that ethylene might function to induce stomatal closure synergistically with abscisic acid (ABA). Furthermore, it has also been shown that increased ethylene can inhibit ABA- and jasmonic acid-induced stomatal closure, thus hindering drought-induced closure during dehydration. Simultaneously, other stresses, such as chilling, ozone pollution, and K+ deficiency, inhibit drought- and ABA-induced stomatal closure in an ethylene synthesis-dependent manner. However, ethylene has been shown to take on an opposing role during rehydration, preventing stomatal opening in the absence of ABA through its own signaling pathway. These findings offer novel insights into the function of ethylene in stomatal regulation during dehydration and rehydration, giving a better understanding of the mechanisms underlying ethylene-induced stomatal movement in seed plants.

MePP2C24, a cassava (Manihot esculenta) gene encoding protein phosphatase 2C, negatively regulates drought stress and abscisic acid responses in transgenic Arabidopsis thaliana

Abscisic acid (ABA) signaling plays a crucial role in plant development and response to abiotic/biotic stress. However, the function and regulation of protein phosphatase 2C (PP2C), a key component of abscisic acid signaling, under abiotic stress are still unknown in cassava, a drought-tolerant crop. In this study, a cassava PP2C gene (MePP2C24) was cloned and characterized. The MePP2C24 transcripts increased in response to mannitol, NaCl, and ABA. Overexpression of MePP2C24 in Arabidopsis resulted in increased sensitivity to drought stress and decreased sensitivity to exogenous ABA. This was demonstrated by transgenic lines having higher levels of malondialdehyde (MDA), ion leakage (IL), and reactive oxygen species (ROS), lower activities of catalase (CAT) and peroxidase (POD), and lower proline content than wild type (WT) under drought stress. Moreover, MePP2C24 overexpression caused decrease in expression of drought-responsive genes related to ABA signaling pathway. In addition, MePP2C24 was localized in the cell nucleus and showed self-activation. Furthermore, many MePYLs (MePYL1, MePYL4, MePYL7-9, and MePYL11-13) could interact with MePP2C24 in the presence of ABA, and MePYL1 interacted with MePP2C24 in both the presence and absence of ABA. Additionally, MebZIP11 interacted with the promoter of MePP2C24 and exerted a suppressive effect. Taken together, our results suggest that MePP2C24 acts as a negative regulator of drought tolerance and ABA response.

Maduración y germinación de embriones somáticos de guayabo var. Enana Roja Cubana EEA-1840

Somatic embryogenesis is an alternative for the accelerated propagation of promising guava (Psidium guajava L.) materials of agronomic interest. However, low maturation and germination rates of somatic embryos are some of the aspects that limit its application in breeding programs for different purposes. In this sense, the effect of three concentrations (0, 1 and 1.5 mg.L-1) of abscisic acid (ABA) on embryo maturation and two concentrations of sucrose (3 and 5 %) and Murashige and Skoog (MS) macronutrients (50 and 100 %) on the germination of somatic embryos of guava var. Cuban Red Dwarf EEA-1840 were studied. After six weeks of culture, ABA had a negative effect on somatic embryo maturation in culture media supplemented with 1 or 1.5 mg.L-1, whereas 3.95 to 5.49 times more mature embryos were observed in the absence of ABA. Germination of somatic embryos was significantly improved when the concentration of macronutrient in the culture medium was reduced independently of the sucrose concentration [MS 50 % + 3 % of sucrose (73,3 %) and MS 50 % + 5 % de sucrose (55,0 %)]. It is concluded that the simplification of the culture media with reduction of the macroelements MS and sucrose to standard concentration favors the germination of mature embryos of guava var. Cuban Red Dwarf EEA-1840.

A dual function of SnRK2 kinases in the regulation of SnRK1 and plant growth

Adverse environmental conditions trigger responses in plants that promote stress tolerance and survival at the expense of growth1. However, little is known of how stress signalling pathways interact with each other and with growth regulatory components to balance growth and stress responses. Here, we show that plant growth is largely regulated by the interplay between the evolutionarily conserved energy-sensing SNF1-related protein kinase 1 (SnRK1) protein kinase and the abscisic acid (ABA) phytohormone pathway. While SnRK2 kinases are main drivers of ABA-triggered stress responses, we uncover an unexpected growth-promoting function of these kinases in the absence of ABA as repressors of SnRK1. Sequestration of SnRK1 by SnRK2-containing complexes inhibits SnRK1 signalling, thereby allowing target of rapamycin (TOR) activity and growth under optimal conditions. On the other hand, these complexes are essential for releasing and activating SnRK1 in response to ABA, leading to the inhibition of TOR and growth under stress. This dual regulation of SnRK1 by SnRK2 kinases couples growth control with environmental factors typical for the terrestrial habitat and is likely to have been critical for the water-to-land transition of plants.

To grow or not to grow ….

To grow or not to grow ….

Forcing vine regrowth to delay ripening and its association to changes in the hormonal balance

The quality and typicality of wines, strongly depends on the management techniques used for grapevine cultivation. Actually, the increment in the average world temperature due to climate change induces not only bigger irrigation necessities, but also earlier grape-ripening processes, which take place in warmer days and shorter nights. Thus, with the aim of delaying grape ripeness of at least two months, a technique has been proposed based on forcing vine regrowth. This technique consists on forcing vine regrowth from the formed latent buds after cutting the green shoots between the second and the third node; lateral shoots, leaves, and primary clusters are also removed. In this study, a forcing treatment was carried out at three different phenological stages (G, I and J). Depending on the phenological stage of vines during the forcing treatment, we wanted to determine the berry ripening delay and to explore how this mechanical pruning interacts with the hormonal balance to modulate bud growth just before shoot decapitation (Control) and later, within the following 7 and 14 days after cutting the green shoots. Forcing treatments carried out at stages G, I and J succeeded to delay ripening 18, 27 and 45 days respectively, as compared to unforced plants. Vine yield was significantly reduced in all treatments as compared to control plants, resulting in a high level of acidity in berries which might be associated with the loss of flowers, a reduction in the fruit set percentage or a combination of both. Endogenous cytokinin (CK) content in control latent buds decreased during the vine vegetative cycle. Contrarily, abscisic acid (ABA) and Jasmonic acid (JA) increased, while minor changes were found in the concentration of gibberellins (GAs), salicylic acid (SA) and the ethylene precursor 1-aminocyclopropane-1-car-boxilic acid (ACC). Moreover, a clear modification of the hormonal balance was found in latent buds 7 and 14 days after forcing regrowth. CK content significantly increased while ABA rapidly decreased after pruning in all treatments. Thus, vine regrowth from the formed latent buds might have been upregulated by CK and promoted by the absence of ABA.

Rebuilding core abscisic acid signaling pathways of Arabidopsis in yeast

The phytohormone abscisic acid (ABA) regulates plant responses to abiotic stress, such as drought and high osmotic conditions. The multitude of functionally redundant components involved in ABA signaling poses a major challenge for elucidating individual contributions to the response selectivity and sensitivity of the pathway. Here, we reconstructed single ABA signaling pathways in yeast for combinatorial analysis of ABA receptors and coreceptors, downstream‐acting SnRK2 protein kinases, and transcription factors. The analysis shows that some ABA receptors stimulate the pathway even in the absence of ABA and that SnRK2s are major determinants of ABA responsiveness by differing in the ligand‐dependent control. Five SnRK2s, including SnRK2.4 known to be active under osmotic stress in plants, activated ABA‐responsive transcription factors and were regulated by ABA receptor complexes in yeast. In the plant tissue, SnRK2.4 and ABA receptors competed for coreceptor interaction in an ABA‐dependent manner consistent with a tight integration of SnRK2.4 into the ABA signaling pathway. The study establishes the suitability of the yeast system for the dissection of core signaling cascades and opens up future avenues of research on ligand‐receptor regulation.

Genome-wide characterization and expression analysis of PP2CA family members in response to ABA and osmotic stress in Gossypium.

Clade A type 2C protein phosphatases (PP2CAs), as central regulators of abscisic acid (ABA) signaling, negative control growth, development and responses to multiple stresses in plants. PP2CA gene families have been characterized at genome-wide levels in several diploid plants like Arabidopsis and rice. However, the information about genome organization, phylogenesis and putative functions of PP2CAs in Gossypium is lacking. Here, PP2CA family members were comprehensively analyzed in four Gossypium species including the diploid progenitor Gossypium arboreum, G. raimondii and the tetraploid G. hirsutum and G. barbadense, and 14, 13, 27, and 23 PP2CA genes were identified in the genomic sequences of these plants, respectively. Analysis results showed that most Gossypium PP2CAs were highly conserved in chromosomal locations, structures, and phylogeny among the four cotton species. Segmental duplication might play important roles in the formation of the PP2CAs, and most PP2CAs may be under purifying selection in Gossypium during evolution. The majority of the PP2CAs were expressed specifically in diverse tissues, and highly expressed in flowers in G. hirsutum. The GhPP2CAs displayed diverse expression patterns in responding to ABA and osmotic stress. Yeast-two hybrid assays revealed that many GhPP2CAs were capable of interaction with the cotton ABA receptors pyrabactin resistance1/PYR1-like/regulatory components of ABA receptors (PYR1/PYL/RCAR) GhPYL2-2D (Gh_D08G2587), GhPYL6-2A (Gh_A06G1418), and GhPYL9-2A (Gh_A11G0870) in the presence and/or absence of ABA. These results gave a comprehensive view of the Gossypium PP2CAs and are valuable for further studying the functions of PP2CAs in Gossypium.

Precise control of ABA signaling through post-translational protein modification

Abscisic acid (ABA) plays a key role in plant growth and development and during stress responses. Plants respond to ABA through recognition, signal transduction, and response cascades. The core ABA signaling pathway consists of ABA receptors (RCAR/PYL/PYRs), protein phosphatases (PP2Cs), kinases (SnRK2s), transcription factors and ion channel proteins. Protein phosphorylation plays a key role in this pathway. In the absence of ABA, PP2Cs inhibit SnRK2s activities by dephosphorylating SnRK2s. When ABA binds to RCAR/PYL/PYRs, the complex then binds to PP2Cs, resulting in inactivation of the PP2Cs and release of the SnRK2s, which then phosphorylate a series of substrates to activate ABA responses. Selective protein degradation by the ubiquitin–proteasome system also contributes to regulation of ABA homeostasis, transport, signaling, and desensitization. The small ubiquitin-like modifier (SUMO) enhances the stability of ABI5 but also inhibits its transcription. ABA-induced reactive nitrogen and oxygen species regulate multiple key components of the ABA signaling pathways through redox-induced modifications (REDOX), such as oxidation, nitration, and nitrosylation, forming a feedback regulation mechanism that precisely regulates ABA signaling. This review will detail the role of these post-translational modifications in the core ABA signaling pathway.

Abscisic Acid Inhibits Rice Protein Phosphatase PP45 via H2O2 and Relieves Repression of the Ca2+/CaM-Dependent Protein Kinase DMI3.

In plants, Ca2+/calmodulin-dependent protein kinase (CCaMK) is a positive regulator of abscisic acid (ABA) responses, including root growth, antioxidant defense, and tolerance of both water stress and oxidative stress. However, the underlying molecular mechanisms are poorly understood. Here, we show a direct interaction between DMI3 (Doesn't Make Infections 3), a rice (Oryza sativa) CCaMK and PP45, a type 2C protein phosphatase in rice (PP2C). This interaction involves the CaM binding domain of DMI3 and the PP2C domain of PP45. In the absence of ABA, PP45 directly inactivates DMI3 by dephosphorylating Thr-263 in DMI3. However, in the presence of ABA, ABA-induced H2O2 production by the NADPH oxidases RbohB/E inhibits the activity of PP45 not only by inhibiting the expression of PP45 but also by oxidizing Cys-350 and Cys-428 residues to form PP45 intermolecular dimers. ABA-induced oxidation of Cys-350 and Cys-428 in PP45 blocked the interaction between PP45 and DMI3 and substantially prevented PP45-mediated inhibition in DMI3 activity. Genetic analysis indicated that PP45 is an important negative regulator of ABA signaling. These results reveal important pathways for the inhibition of DMI3 under the basal state and for its ABA-induced activation in rice.

Insights into ABA-mediated regulation of guard cell primary metabolism revealed by systems biology approaches

Despite the fact that guard cell abscisic acid (ABA) signalling pathway is well documented, our understanding concerning how and to which extent ABA regulates guard cell metabolism remains fragmentary. Here we have adopted different systems approaches to investigate how ABA modulates guard cell central metabolism by providing genes that are possibly ABA-regulated. By using previous published Arabidopsis guard cell transcript profiling data, we carried out an extensive co-expression network analysis using ABA-related genes and those related to the metabolism and transport of sugars, starch and organic acids. Next, we investigated the presence of ABA responsive elements (ABRE) in the promoter of genes that are highly expressed in guard cells, responsive to ABA and co-expressed with ABA-related genes. Together, these analyses indicated that 44 genes are likely regulated by ABA and 8 of them are highly expressed in guard cells in both the presence and absence of ABA, including genes of the tricarboxylic acid cycle and those related to sucrose and hexose transport and metabolism. It seems likely that ABA may modulate both sucrose transport through guard cell plasma membrane and sucrose metabolism within guard cells. In this context, genes associated with sucrose synthase, sucrose phosphate synthase, trehalose-6-phosphate, invertase, UDP-glucose epimerase/pyrophosphorylase and different sugar transporters contain ABRE in their promoter and are thus possibly ABA regulated. Although validation experiments are required, our study highlights the importance of systems biology approaches to drive new hypothesis and to unravel genes and pathways that are regulated by ABA in guard cells.

Genome-wide identification of ABA receptor PYL family and expression analysis of PYLs in response to ABA and osmotic stress in Gossypium

Abscisic acid (ABA) receptor pyrabactin resistance1/PYR1-like/regulatory components of ABA receptor (PYR1/PYL/RCAR) (named PYLs for simplicity) are core regulators of ABA signaling, and have been well studied in Arabidopsis and rice. However, knowledge is limited about the PYL family regarding genome organization, gene structure, phylogenesis, gene expression and protein interaction with downstream targets in Gossypium. A comprehensive analysis of the Gossypium PYL family was carried out, and 21, 20, 40 and 39 PYL genes were identified in the genomes from the diploid progenitor G. arboretum, G. raimondii and the tetraploid G. hirsutum and G. barbadense, respectively. Characterization of the physical properties, chromosomal locations, structures and phylogeny of these family members revealed that Gossypium PYLs were quite conservative among the surveyed cotton species. Segmental duplication might be the main force promoting the expansion of PYLs, and the majority of the PYLs underwent evolution under purifying selection in Gossypium. Additionally, the expression profiles of GhPYL genes were specific in tissues. Transcriptions of many GhPYL genes were inhibited by ABA treatments and induced by osmotic stress. A number of GhPYLs can interact with GhABI1A or GhABID in the presence and/or absence of ABA by the yeast-two hybrid method in cotton.

The Arabidopsis Phytocystatin AtCYS5 Enhances Seed Germination and Seedling Growth under Heat Stress Conditions.

Phytocystatins (PhyCYSs) are plant-specific proteinaceous inhibitors that are implicated in protein turnover and stress responses. Here, we characterized a PhyCYS from Arabidopsis thaliana, which was designated AtCYS5. RT-qPCR analysis showed that the expression of AtCYS5 in germinating seeds was induced by heat stress (HS) and exogenous abscisic acid (ABA) treatment. Analysis of the expression of the β-glucuronidase reporter gene under the control of the AtCYS5 promoter showed that AtCYS5 expression during seed germination was induced by HS and ABA. Constitutive overexpression of AtCYS5 driven by the cauliflower mosaic virus 35S promoter led to enhanced HS tolerance in transgenic Arabidopsis, which was characterized by higher fresh weight and root length compared to wild-type (WT) and knockout (cys5) plants grown under HS conditions. The HS tolerance of At-CYS5-overexpressing transgenic plants was associated with increased insensitivity to exogenous ABA during both seed germination and post-germination compared to WT and cys5. Although no HS elements were identified in the 5′-flanking region of AtCYS5, canonical ABA-responsive elements (ABREs) were detected. AtCYS5 was upregulated in ABA-treated protoplasts transiently co-expressing this gene and genes encoding bZIP ABRE-binding factors (ABFs and AREB3). In the absence of ABA, ABF1 and ABF3 directly bound to the ABREs in the AtCYS5 promoter, which activated the transcription of this gene in the presence of ABA. These results suggest that an ABA-dependent pathway plays a positive role in the HS-responsive expression of AtCYS5 during seed germination and post-germination growth.

Brachypodium distachyon BdPP2CA6 Interacts with BdPYLs and BdSnRK2 and Positively Regulates Salt Tolerance in Transgenic Arabidopsis.

The phytohormone abscisic acid (ABA) is essential in plant responding to biotic and abiotic stresses. Although ABA signaling model is well established in Arabidopsis, ABA receptor PYL family and clade A PP2C subfamily are not yet characterized in monocot model plant Brachypodium distachyon. In this study, we identified 12 PYLs and 8 clade A PP2Cs from B. distachyon genome and successfully cloned 12 PYLs and 7 clade A PP2Cs. Bioinformatic and expression analyses showed that most of the identified genes respond to several signal molecules and abiotic stresses. Protein–protein interaction analysis revealed that many BdPYLs and BdPP2CAs participate in the classic ABA-PYL-PP2C-SnRK2 signaling pathway. A clade A PP2C, designated BdPP2CA6, interacted with BdPYL11 in the absence of ABA and localized in nucleus. Most clade A PP2C members from Arabidopsis showed negatively regulation in ABA signaling pathway, whereas BdPP2CA6-overexpression transgenic Arabidopsis showed ABA hypersensitive phenotype, resulting in enhanced stomatal closure and salinity tolerance. Our results indicate that BdPP2CA6 positively regulates ABA and stress signal pathway in transgenic Arabidopsis plant seedlings.

A Direct Link between Abscisic Acid Sensing and the Chromatin-Remodeling ATPase BRAHMA via Core ABA Signaling Pathway Components

Optimal response to drought is critical for plant survival and will affect biodiversity and crop performance during climate change. Mitotically heritable epigenetic or dynamic chromatin state changes have been implicated in the plant response to the drought stress hormone abscisic acid (ABA). The Arabidopsis SWI/SNF chromatin-remodeling ATPase BRAHMA (BRM) modulates response to ABA by preventing premature activation of stress response pathways during germination. We show that core ABA signaling pathway components physically interact with BRM and post-translationally modify BRM by phosphorylation/dephosphorylation. Genetic evidence suggests that BRM acts downstream of SnRK2.2/2.3 kinases, and biochemical studies identified phosphorylation sites in the C-terminal region of BRM at SnRK2 target sites that are evolutionarily conserved. Finally, the phosphomimetic BRMS1760D S1762D mutant displays ABA hypersensitivity. Prior studies showed that BRM resides at target loci in the ABA pathway in the presence and absence of the stimulus, but is only active in the absence of ABA. Our data suggest that SnRK2-dependent phosphorylation of BRM leads to its inhibition, and PP2CA-mediated dephosphorylation of BRM restores the ability of BRM to repress ABA response. These findings point to the presence of a rapid phosphorylation-based switch to control BRM activity; this property could be potentially harnessed to improve drought tolerance in plants.

Shoot apical meristem ontogeny inArabidopsisembryo explants treated with abscisic acid

The formation of meristemoids and the ontogeny of the shoot apical meristem (SAM) were studied using cultured zygotic embryos of Arabidopsis thaliana (L.) Heynh. LER ecotype. In the callus induction treatment, the procambial cells within the cotyledons of the embryo explants proliferated and gave rise to callus tissues. At the end of the treatment, a band of small cytoplasmic-rich cells derived from the procambium was produced and located at the outer surface of the callus. Upon transfer to the shoot induction medium (SIM) in the absence of abscisic acid (ABA), the cytoplasmic cells differentiated mainly into vascular elements and vacuolated parenchyma cells. This pattern of development negatively affected the explants’ ability to produce meristemoids and SAMs. Contrary to the control, the inclusion of ABA in the SIM resulted first in starch synthesis and accumulation in the surface cytoplasmic cells. This was followed by the formation of cytoplasmic cells among the starch-rich cells; further proliferation of the cytoplasmic cells resulted in the formation of meristemoids. The formation of tracheary elements was suppressed in the ABA-containing SIM. Upon transferring to the shoot development medium, which lacked plant growth regulators, some meristemoids differentiated into apical meristem cells. These cells had distinct nuclei and nucleoli, with little starch present. Additional cell divisions increased the size of the future SAM. Shoot buds with distinct SAMs were clearly delineated with the appearance of leaf primordia.

Structural basis and functions of abscisic acid receptors PYLs.

Abscisic acid (ABA) plays a key role in many developmental processes and responses to adaptive stresses in plants. Recently, a new family of nucleocytoplasmic PYR/PYL/RCAR (PYLs) has been identified as bona fide ABA receptors. PYLs together with protein phosphatases type-2C (PP2Cs), Snf1 (Sucrose-non-fermentation 1)-related kinases subfamily 2 (SnRK2s) and downstream substrates constitute the core ABA signaling network. Generally, PP2Cs inactivate SnRK2s kinases by physical interaction and direct dephosphorylation. Upon ABA binding, PYLs change their conformations and then contact and inhibit PP2Cs, thus activating SnRK2s. Here, we reviewed the recent progress in research regarding the structures of the core signaling pathways of ABA, including the (+)-ABA, (−)-ABA and ABA analogs pyrabactin as well as 6AS perception by PYLs, SnRK2s mimicking PYLs in binding PP2Cs. PYLs inhibited PP2Cs in both the presence and absence of ABA and activated SnRK2s. The present review elucidates multiple ABA signal perception and transduction by PYLs, which might shed light on how to design small chemical compounds for improving plant performance in the future.

Identification and characterization of ABA receptors in Oryza sativa.

Abscisic acid (ABA) is an essential phytohormone that regulates plant stress responses. ABA receptors in Arabidopsis thaliana (AtPYLs) have been extensively investigated by structural, biochemical, and in vivo studies. In contrast, relatively little is known about the ABA signal transduction cascade in rice. Besides, the diversities of AtPYLs manifest that the information accumulated in Arabidopsis cannot be simply adapted to rice. Thus, studies on rice ABA receptors are compulsory. By taking a bioinformatic approach, we identified twelve ABA receptor orthologs in Oryza sativa (japonica cultivar-group) (OsPYLs), named OsPYL1–12. We have successfully expressed and purified OsPYL1–3, 6 and 10–12 to homogeneity, tested the inhibitory effects on PP2C in Oryza sativa (OsPP2C), and measured their oligomerization states. OsPYL1–3 mainly exhibit as dimers and require ABA to inhibit PP2C’s activity. On the contrary, OsPYL6 retains in the monomer-dimer equilibrium state and OsPYL10–11 largely exist as monomers, and they all display an ABA-independent phosphatase inhibition manner. Interestingly, although OsPYL12 seems to be a dimer, it abrogates the phosphatase activity of PP2Cs in the absence of ABA. Toward a further understanding of OsPYLs on the ABA binding and PP2C inhibition, we determined the crystal structure of ABA-OsPYL2-OsPP2C06 complex. The bioinformatic, biochemical and structural analysis of ABA receptors in rice provide important foundations for designing rational ABA-analogues and breeding the stress-resistant rice for commercial agriculture.

Abscisic Acid- and Stress-Induced Highly Proline-Rich Glycoproteins Regulate Root Growth in Rice

In the root of rice (Oryza sativa), abscisic acid (ABA) treatment, salinity, or water deficit stress induces the expression of a family of four genes, REPETITIVE PROLINE-RICH PROTEIN (RePRP). These genes encode two subclasses of novel proline-rich glycoproteins with highly repetitive PX₁PX₂ motifs, RePRP1 and RePRP2. RePRP orthologs exist only in monocotyledonous plants, and their functions are virtually unknown. Rice RePRPs are heavily glycosylated with arabinose and glucose on multiple hydroxyproline residues. They are significantly different from arabinogalactan proteins that have glycan chains composed of arabinose and galactose. Transient and stable expressions of RePRP-green fluorescent protein reveal that a fraction of this protein is localized to the plasma membrane. In rice roots, ABA treatment increases RePRP expression preferentially in the elongation zone. Overexpression of RePRP in transgenic rice reduces root cell elongation in the absence of ABA, similar to the effect of ABA on wild-type roots. Conversely, simultaneous knockdown of the expression of RePRP1 and RePRP2 reduces the root sensitivity to ABA, indicating that RePRP proteins play an essential role in ABA/stress regulation of root growth and development. Moreover, rice RePRPs specifically interact with a polysaccharide, arabinogalactan, in a dosage-dependent manner. It is suggested that RePRP1 and RePRP2 are functionally redundant suppressors of root cell expansion and probably act through interactions with cell wall components near the plasma membrane.