Abscisic acid (ABA) regulates numerous developmental processes and adaptive stress responses in plants. Many ABA signaling components have been identified, but their interconnections and a consensus ...on the structure of the ABA signaling network have eluded researchers. Recently, several advances have led to the identification of ABA receptors and their three-dimensional structures, and an understanding of how key regulatory phosphatase and kinase activities are controlled by ABA. A new model for ABA action has been proposed and validated, in which the soluble PYR/PYL/RCAR receptors function at the apex of a negative regulatory pathway to directly regulate PP2C phosphatases, which in turn directly regulate SnRK2 kinases. This model unifies many previously defined signaling components and highlights the importance of future work focused on defining the direct targets of SnRK2s and PP2Cs, dissecting the mechanisms of hormone interactions (i.e., cross talk) and defining connections between this new negative regulatory pathway and other factors implicated in ABA signaling.
Abstract
The phytohormone ABA mediates many physiological and developmental responses, and its key role in plant water relations has fueled efforts to improve crop water productivity by manipulating ...ABA responses. ABA’s core signaling components are encoded by large gene families, which has hampered functional studies using classical genetic approaches due to redundancy. Chemical approaches can complement genetic approaches and have the advantage of delivering both biological probes and potential agrochemical leads; these benefits have spawned the discovery and design of new chemical modulators of ABA signaling and biosynthesis, which have contributed to the identification of ABA receptors and helped to define PYR1 and related subfamily III receptors as key cellular targets for chemically manipulating water productivity. In this review, we provide an overview of small molecules that have helped dissect both ABA signaling and metabolic pathways. We further discuss how the insights gleaned using ABA probe molecules might be translated to improvements in crop water productivity and future opportunities for development of small molecules that affect ABA metabolism and signaling.
•The significant global investment in GM technology to improve plant response to water deficit has produced few products.•Significant progress has been made identifying genes that have a positive ...effect on plant response to water deficit.•There is a notable disconnect between simulated water deficit in pots and the impact of water deficit on crop productivity.•Field research confines drought tolerance trait evaluation to one season per hemisphere per year, which slows progress.•The cost of bringing a GM product to market limits the development of drought tolerance GM traits to large acreage crops.
Since the dawn of modern biotechnology public and private enterprise have pursued the development of a new breed of drought tolerant crop products. After more than 20 years of research and investment only a few such products have reached the market. This is due to several technical and market constraints. The technical challenges include the difficulty in defining tractable single-gene trait development strategies, the logistics of moving traits from initial to commercial genetic backgrounds, and the disconnect between conditions in farmer’s fields and controlled environments. Market constraints include the significant difficulty, and associated costs, in obtaining access to markets around the world. Advances in the biology of plant water management, including response to water deficit reveal new opportunities to improve crop response to water deficit and new genome-based tools promise to usher in the next era of crop improvement. As biotechnology looks to improve crop productivity under drought conditions, the environmental and food security advantages will influence public perception and shift the debate toward benefits rather than risks.
Rising temperatures and lessening fresh water supplies are threatening agricultural productivity and have motivated efforts to improve plant water use and drought tolerance. During water deficit, ...plants produce elevated levels of abscisic acid (ABA), which improves water consumption and stress tolerance by controlling guard cell aperture and other protective responses. One attractive strategy for controlling water use is to develop compounds that activate ABA receptors, but agonists approved for use have yet to be developed. In principle, an engineered ABA receptor that can be activated by an existing agrochemical could achieve this goal. Here we describe a variant of the ABA receptor PYRABACTIN RESISTANCE 1 (PYR1) that possesses nanomolar sensitivity to the agrochemical mandipropamid and demonstrate its efficacy for controlling ABA responses and drought tolerance in transgenic plants. Furthermore, crystallographic studies provide a mechanistic basis for its activity and demonstrate the relative ease with which the PYR1 ligand-binding pocket can be altered to accommodate new ligands. Thus, we have successfully repurposed an agrochemical for a new application using receptor engineering. We anticipate that this strategy will be applied to other plant receptors and represents a new avenue for crop improvement.
•The CRISPR/Cas9 system is a versatile tool for genome engineering.•A customizable gRNA and a nuclease are the core components of the CRISPR/Cas9 system.•The CRISPR/Cas9 system has been validated in ...Arabidopsis, tobacco, rice, wheat, and sorghum.•Researchers should be aware of potential off-target mutations when using the CRISPR/Cas9 system.
The recent development of tools for precise editing of user-specified sequences is rapidly changing the landscape for plant genetics and biotechnology. It is now possible to target mutations and regulatory proteins to specific sites in a genome using zinc-finger nucleases (ZFNs), transcription activator-like endonucleases (TALENs), or the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 (Cas9) system. Here we provide an update of recent developments in CRISPR/Cas9 technology and highlight online resources that will help biologists adopt new genome-editing tools.
Abscisic acid (ABA) is an essential molecule in plant abiotic stress responses. It binds to soluble pyrabactin resistance1/PYR1-like/regulatory component of ABA receptor receptors and stabilizes them ...in a conformation that inhibits clade A type II C protein phosphatases; this leads to downstream SnRK2 kinase activation and numerous cellular outputs. We previously described the synthetic naphthalene sulfonamide ABA agonist pyrabactin, which activates seed ABA responses but fails to trigger substantial responses in vegetative tissues in Arabidopsis thaliana . Here we describe quinabactin, a sulfonamide ABA agonist that preferentially activates dimeric ABA receptors and possesses ABA-like potency in vivo. In Arabidopsis , the transcriptional responses induced by quinabactin are highly correlated with those induced by ABA treatments. Quinabactin treatments elicit guard cell closure, suppress water loss, and promote drought tolerance in adult Arabidopsis and soybean plants. The effects of quinabactin are sufficiently similar to those of ABA that it is able to rescue multiple phenotypes observed in the ABA-deficient mutant aba2 . Genetic analyses show that quinabactin’s effects in vegetative tissues are primarily mediated by dimeric ABA receptors. A PYL2-quinabactin-HAB1 X-ray crystal structure solved at 1.98-Å resolution shows that quinabactin forms a hydrogen bond with the receptor/PP2C “lock” hydrogen bond network, a structural feature absent in pyrabactin-receptor/PP2C complexes. Our results demonstrate that ABA receptors can be chemically controlled to enable plant protection against water stress and define the dimeric receptors as key targets for chemical modulation of vegetative ABA responses.
Drought causes crop losses worldwide, and its impact is expected to increase as the world warms. This has motivated the development of small-molecule tools for mitigating the effects of drought on ...agriculture. We show here that current leads are limited by poor bioactivity in wheat, a widely grown staple crop, and in tomato. To address this limitation, we combined virtual screening, x-ray crystallography, and structure-guided design to develop opabactin (OP), an abscisic acid (ABA) mimic with up to an approximately sevenfold increase in receptor affinity relative to ABA and up to 10-fold greater activity in vivo. Studies in
reveal a role of the type III receptor
for the antitranspirant efficacy of OP. Thus, virtual screening and structure-guided optimization yielded newly discovered agonists for manipulating crop abiotic stress tolerance and water use.
Water availability is a key determinant of terrestrial plant productivity. Many climate models predict that water stress will increasingly challenge agricultural yields and exacerbate projected food ...deficits. To ensure food security and increase agricultural efficiency, crop water productivity must be increased. Research over past decades has established that the phytohormone abscisic acid (ABA) is a central regulator of water use and directly regulates stomatal opening and transpiration. In this study, we investigated whether the water productivity of wheat could be improved by increasing its ABA sensitivity. We show that overexpression of a wheat ABA receptor increases wheat ABA sensitivity, which significantly lowers a plant's lifetime water consumption. Physiological analyses demonstrated that this water-saving trait is a consequence of reduced transpiration and a concomitant increase in photosynthetic activity, which together boost grain production per litre of water and protect productivity during water deficit. Our findings provide a general strategy for increasing water productivity that should be applicable to other crops because of the high conservation of the ABA signalling pathway.
Type 2C protein phosphatases (PP2Cs) are vitally involved in abscisic acid (ABA) signaling. Here, we show that a synthetic growth inhibitor called pyrabactin functions as a selective ABA agonist. ...Pyrabactin acts through PYRABACTIN RESISTANCE 1 (PYR1), the founding member of a family of START proteins called PYR/PYLs, which are necessary for both pyrabactin and ABA signaling in vivo. We show that ABA binds to PYR1, which in turn binds to and inhibits PP2Cs. We conclude that PYR/PYLs are ABA receptors functioning at the apex of a negative regulatory pathway that controls ABA signaling by inhibiting PP2Cs. Our results illustrate the power of the chemical genetic approach for sidestepping genetic redundancy.
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