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.

Drought is a major limiting factor for plant growth and crop productivity. Many Calcineurin B-like interacting protein kinases (CIPKs) play crucial roles in plant adaptation to environmental stresses. It is particularly essential to find the phosphorylation targets of CIPKs and to study the underlying molecular mechanisms. In this study, we demonstrate that CIPK11 acts as a novel component to modulate drought stress in plants. The overexpression of CIPK11 (CIPK11OE) in Arabidopsis resulted in the decreased tolerance of plant to drought stress. When compared to wild type plants, CIPK11OE plants exhibited higher leaf water loss and higher content of reactive oxygen species (ROS) after drought treatment. Additionally, a yeast two hybrid screening assay by using CIPK11 as a bait captures Di19-3, a Cys2/His2-type zinc-finger transcription factor that is involved in drought stress, as a new interactor of CIPK11. Biochemical analysis revealed that CIPK11 interacted with Di19-3 in vivo and it was capable of phosphorylating Di19-3 in vitro. Genetic studies revealed that the function of CIPK11 in regulating drought stress was dependent on Di19-3. The transcripts of stress responsive genes, such as RAB18, RD29A, RD29B, and DREB2A were down-regulated in the CIPK11OE plants. Whereas overexpression of CIPK11 in di19-3 mutant background, expression levels of those marker genes were not significantly altered. Taken together, our results demonstrate that CIPK11 partly mediates the drought stress response by regulating the transcription factor Di19-3.

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Uridine diphosphate‐glucosyltransferases (UGTs) maintain abscisic acid (ABA) homeostasis in Arabidopsis thaliana by converting ABA to abscisic acid‐glucose ester (ABA‐GE). UGT71C5 plays an important role in the generation of ABA‐GE. Abscisic acid receptors are crucial upstream components of the ABA signaling pathway, but how UGTs and ABA receptors function together to modulate ABA levels is unknown. Here, we demonstrated that the ABA receptors RCAR12/13 and UGT71C5 maintain ABA homeostasis in Arabidopsis following rehydration under drought stress. Biochemical analyses show that UGT71C5 directly interacted with RCAR8/12/13 in yeast cells, and the interactions between UGT71C5 and RCAR12/13 were enhanced by ABA treatment. Enzyme activity analysis showed that ABA‐GE contents were significantly elevated in the presence of RCAR12 or RCAR13, suggesting that these ABA receptors enhance the activity of UGT71C5. Determination of the content of ABA and ABA‐GE in Arabidopsis following rehydration under drought stress revealed that ABA‐GE contents were significantly higher in Arabidopsis plants overexpressing RCAR12 and RCAR13 than in non‐transformed plants and plants overexpressing RCAR11 following rehydration under drought stress. These observations suggest that RCAR12 and RCAR13 enhance the activity of UGT71C5 to glycosylate excess ABA into ABA‐GE following rehydration under drought stress, representing a rapid mechanism for regulating plant growth and development.