The plant hormone abscisic acid (ABA) acts as a developmental signal and as an integrator of environmental cues such as drought and cold. Key players in ABA signal transduction include the type 2C protein phosphatases (PP2Cs) ABI1 and ABI2, which act by negatively regulating ABA responses. In this study, we identify interactors of ABI1 and ABI2 which we have named regulatory components of ABA receptor (RCARs). In Arabidopsis, RCARs belong to a family with 14 members that share structural similarity with class 10 pathogen-related proteins. RCAR1 was shown to bind ABA, to mediate ABA-dependent inactivation of ABI1 or ABI2 in vitro, and to antagonize PP2C action in planta. Other RCARs also mediated ABA-dependent regulation of ABI1 and ABI2, consistent with a combinatorial assembly of receptor complexes.

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Abscisic acid (ABA) plays a key role in plant growth and development. The effect of ABA in plants mainly depends on its concentration, which is determined by a balance between biosynthesis and catabolism of ABA. In this study, we characterize a unique UDP-glucosyltransferase (UGT), UGT71C5, which plays an important role in ABA homeostasis by glucosylating ABA to abscisic acid-glucose ester (GE) in Arabidopsis (Arabidopsis thaliana). Biochemical analyses show that UGT71C5 glucosylates ABA in vitro and in vivo. Mutation of UGT71C5 and down-expression of UGT71C5 in Arabidopsis cause delay in seed germination and enhanced drought tolerance. In contrast, overexpression of UGT71C5 accelerates seed germination and reduces drought tolerance. Determination of the content of ABA and ABA-GE in Arabidopsis revealed that mutation in UGT71C5 and down-expression of UGT71C5 resulted in increased level of ABA and reduced level of ABA-GE, whereas overexpression of UGT71C5 resulted in reduced level of ABA and increased level of ABA-GE. Furthermore, altered levels of ABA in plants lead to changes in transcript abundance of ABA-responsive genes, correlating with the concentration of ABA regulated by UGT71C5 in Arabidopsis. Our work shows that UGT71C5 plays a major role in ABA glucosylation for ABA homeostasis.

The plant hormone abscisic acid (ABA) play essential roles in numerous physiological processes such as seed dormancy, seed germination, seeding growth and responses to biotic and abiotic stresses. Such biological processes are tightly controlled by a complicated regulatory network including ABA homoeostasis, signal transduction as well as cross-talking among other signaling pathways. It is known that ABA homoeostasis modulated by its production, inactivation, and transport pathways is considered to be of great importance for plant development and stress responses. Most of the enzymes and transporters involved in ABA homoeostasis have been largely characterized and they all work synergistically to maintain ABA level in plants. Increasing evidence have suggested that transcriptional regulation of the genes involved in either ABA production or ABA inactivation plays vital roles in ABA homoeostasis. In addition to transcription factors, such progress is also regulated by microRNAs and newly characterized root to shoot mobile peptide-receptor like kinase (RLKs) mediated long-distance signal transduction. Thus, ABA contents are always kept in a dynamic balance. In this review, we survey recent research on ABA production, inactivation and transport pathways, and summarize some latest findings about the mechanisms that regulate ABA homoeostasis.

Fibrillins are lipid-binding proteins of plastids that are induced under abiotic stress conditions. In response to environmental stress, plants generate abscisic acid (ABA) as an endogenous signal. We show that ABA treatment and fibrillin accumulation enhance the tolerance of photosystem II toward light stress-triggered photoinhibition in Arabidopsis. ABA induces fibrillin accumulation, and the ABA response regulators ABI1 and ABI2 regulate fibrillin expression. The abundance of fibrillin transcripts was specifically reduced in the ABA-insensitive abi1 mutant but not in the abi2 mutant. However, leaves of abi2 revealed in comparison to WT and abi1 enhanced fibrillin levels, pointing to a posttranscriptional control mechanism. Protein interaction analysis identified the protein phosphatase ABI2 to target the preprotein of fibrillin. Interaction was abrogated either by deleting the signal peptide of prefibrillin or by the single amino acid exchange present in the phosphatase-deficient abi2 protein. Thus, ABI1 and ABI2 seem to control fibrillin expression that is involved in mediating ABAinduced photoprotection.ABI1 ͉ ABI2 ͉ abiotic stress ͉ photoinhibition ͉ light stress F ibrillins are plastid-associated lipid-binding proteins that are ubiquitous in plants (1, 2). They have been primarily characterized from chromoplasts of tomato and pepper fruits and are known to accumulate during abiotic stress in plastids e.g., inflicted by high light, cold, and drought (3-5), and also during pathogen infection (6). The family of fibrillin-like proteins is unique to plants, and the members contain a hydrophobic domain that associates with or anchors within lipids (7). Fibrillins associate with stromal lamellae of thylakoids and fibrillic carotenoid-containing structures of chromoplasts (3,8). A model for the fibrillic structures predicts a layer of fibrillin shielding polar lipids and carotenoids (1). In potato, the fibrillin C40.4 protein is specifically associated with the photosystem II (PSII) light-harvesting complex with a presumptive role in the modulation of photosynthetic efficiency (9). A posttranslational control of fibrillin accumulation by abscisic acid (ABA) was indicated by studies on ABA-deficient tomato that failed to accumulate fibrillin despite normal transcript levels (5). Although fibrillin is induced in response to abiotic stress, its role in stress responses and the molecular mechanism regulating its accumulation are still elusive.ABA plays a major role in regulating plant growth and development and mediating adaptations to environmental stress such as cold, drought, and salinity (10). ABA regulates ion channel activities involved in osmoregulation and stomatal closure and influences gene expression at the transcriptional and posttranscriptional levels (11,12). Key regulators of diverse ABA-mediated responses are ABI1 and ABI2. Both proteins are members of a larger family of plant protein phosphatases 2C (PP2Cs), several of which act negatively in a partially redundant manner on ABA responses (13-15). The...