Modulators of the Abscisic Acid Core Signaling Pathway

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Increases in the human population of the Earth, coupled with increased carbon emissions, are leading to changes in our global climate that threaten agriculture just as our collective demands grow. Climate change is manifest as severe weather events ranging from warming and drought to major storms and flooding in different regions. The combination of severe drought during growing seasons and excessive moisture during cereal grain maturation can lead to many billions of dollars in annual yield losses worldwide. A better understanding of the genetic mechanisms controlling plant stress tolerance and productivity could help us to create or breed plants that could better withstand unfavorable climate events. The hormone abscisic acid (ABA) is a key regulator of stress signaling. Within the last seven years a "core ABA signaling pathway" has emerged that mechanistically links many ABA signaling components that were previously identified genetically. This proposal addresses the function of a group of proteins that interact directly with multiple central components of this core pathway, possibly functioning as signaling hubs. Understanding the interactions controlling ABA responses could be instrumental for ensuring the world's future food security. The central components of the ABA core signaling pathway include four families of proteins: the PYL/PYR/RCAR family of ABA receptors, the clade A family of protein phosphatase (PP)2Cs, the Snf-Related Kinase (SnRK)2 family,and the ABI5/ABF/AREB clade of ABA response element binding basic leucine zipper (bZIP) class transcription factors. An additional family, the ABI Five binding Protein (AFP)s, interact directly with the bZIPs, PP2Cs and SnRK2s, and also show interactions with chromatin modifiers. This project will address the mechanisms and implications of these interactions by determining 1) whether AFPs act as transcriptional repressors of ABA response element (ABRE)-containing genes by sequestering bZIPs at low ABA concentrations; (2) whether AFPs interact with TPL-like co-repressor(s) or other chromatin modifying protein(s) to alter histone acetylation and activity of genes regulated by bZIPs; and 3) whether AFP function and localization is regulated by changes in phosphorylation mediated by PP2Cs and SnRK2s or other kinases. The proposed experiments will test how the AFPs contribute to a feedback system that permits plants to resume growth when developmentally or stress-induced ABA levels decline. In addition, they will address the biochemical function(s) of a family still annotated only as proteins of unknown function.