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[EN] The hormone abscisic acid (ABA) orchestrates the plant stress response and regulates sophisticated metabolic and physiological mechanisms essential for survival in a changing environment. Plant ABA receptors were described more than 10 years ago, and a considerable amount of information is available for the model plant Arabidopsis thaliana. Unfortunately, this knowledge is still very limited in crops that hold the key to feeding a growing population. In this review, we summarize genomic, genetic and structural data obtained in crop ABA receptors. We also provide an update on ABA perception in major food crops, highlighting specific and common features of crop ABA receptors., This research was funded by the grants SP20180340 (PAID-06-18), from Universitat Politecnica de Valencia, and GV/2020/159, from Generalitat Valenciana, to J.L.-J. and FONDOCYT2018 2C1-182 grant from FONDOCYT to S.M.R. and J.L.-J.

[ES] En condiciones de estrés, las plantas producen la hormona ácido abscísico (ABA) que es responsable de la adaptación y la supervivencia a condiciones adversas, en especial a la sequía. En la célula vegetal, el ABA es percibido a través de su unión a los receptores solubles PYR/PYL/RCAR. La señalización de ABA es orquestada a través de un módulo que incluye la familiar de receptores de ABA PYR/PYL/RCAR, las fosfatasas de la familia PP2C, las quinasas de la familia SnRK2 y los factores de transcripción de respuesta a ABA. La percepción y señalización del ABA requiere la unión del ligando (ABA) por los receptores PYR/PYL/RCAR y la formación de un complejo ternario con las PP2Cs. La formación del complejo inhibe la actividad fosfatasa de las PP2Cs, dejando que las SnRK2 permanezcan fosforiladas y activas para mediar la activación de los factores de trascripción de respuesta a ABA y de diferentes transportadores de iones. La activación de esta cascada de señalización promueve la adaptación a estrés. Es más, la aplicación de ABA es capaz de mejorar el rendimiento de las cosechas afectadas por la sequía. Sin embargo, el alto coste de producción del ABA así como su reducida estabilidad ha limitado su comercialización con este propósito. Este proyecto tiene como objetivo caracterizar moléculas químicas sintéticas con pr¬¬¬¬¬opiedades similares al ABA, y evaluar su potencial para controlar la tolerancia al estrés hídrico. Para ello el estudiante de master realizará ensayos moleculares, bioquímicos y fisiológicos con la especie modelo de dicotiledóneas Arabidopsis thaliana y de monocotiledóneas, Setaria viridis. El candidato estará involucrado en un proyecto multidisciplinar que cuenta con especialistas en biología estructural, química orgánica, genética y biología molecular de plantas con el objetivo de desarrollar nuevas moléculas químicas que sean capaces de activar una completa resistencia al estrés en plantas de cosecha., [EN] Drought is the factor that causes the biggest economic losses in agriculture. To deal with these hostile conditions, plants have developed some resistance mechanisms that protect themselves from water shortage. Abscisic acid (ABA) is a phytohormone that plays an essential role in the regulation of stomatal closure. Its application reduces the transpiration of plants, preventing water loss. The high cost of synthesis and the photolability of ABA make it difficult to be applied in the field. New strategies have emerged based on the development of agrochemicals that mimic the effect of ABA, thus reducing the damage caused by drought. The objective of this work is to characterize synthetic agonist of ABA receptors and compare their effect on two model plants Arabidopsis thaliana and Setaria viridis. Our results show that each agnonist has a certain preference for the activation of a specific set of PYR / PYL / RCAR receptors in the different species studied. However, this activation does not always correlate with an improvement in drought resistance. While in Arabidopsis thaliana the compounds quinabactin, AMF4 and zeabactin-m reduce transpiration and increase drought resistance, in Setaria viridis none of these compounds can improve drought resistance, with the exception of ABA. However, these agonist compounds can activate Setaria viridis ABA receptors in vitro. These data suggest the existence of a mechanism that regulates and inhibits the response caused by these agrochemicals. Our work demonstrates that the effectiveness of agonist compounds designed to improve drought resistance is plant specie specific. And it shows an important difference between the activity of these synthetic ligands in eudicots and monocots.

Pyrenophoric acid (P-Acid), P-Acid B, and P-Acid C are three phytotoxic sesquiterpenoids produced by the ascomycete seed pathogen Pyrenophora semeniperda, a fungus proposed as a mycoherbicide for biocontrol of cheatgrass, an extremely invasive weed. When tested in cheatgrass bioassays, these metabolites were able to delay seed germination, with P-Acid B being the most active compound. Here, we have investigated the cross-kingdom activity of P-Acid B and its mode of action, and found that it activates the abscisic acid (ABA) signaling pathway in order to inhibit seedling establishment. P-Acid B inhibits seedling establishment in wild-type Arabidopsis thaliana, while several mutants affected in the early perception as well as in downstream ABA signaling components were insensitive to the fungal compound. However, in spite of structural similarities between ABA and P-Acid B, the latter is not able to activate the PYR/PYL family of ABA receptors. Instead, we have found that P-Acid B uses the ABA biosynthesis pathway at the level of alcohol dehydrogenase ABA2 to reduce seedling establishment. We propose that the fungus P. semeniperda manipulates plant ABA biosynthesis as a strategy to reduce seed germination, increasing its ability to cause seed mortality and thereby increase its fitness through higher reproductive success., The fungus Pyrenophora semeniperda produces pyrenophoric acid B, a small molecule that exploits the plant ABA biosynthetic pathway to reduce seed germination, increasing its reproductive success.

Abscisic acid (ABA) is a major phytohormone that mediates the adaptation of plants to environmental stresses such as drought and regulates developmental signals such as seed maturation. Studies on ABA signaling have progressed rapidly since the recent discovery of PYR/PYL receptor proteins as soluble ABA receptors. In plant cells, the receptor receives ABA to inhibit the phosphatase activity of type 2C protein phosphatase (PP2C), which is the major negative regulator in ABA signaling. SNF1-related protein kinase 2 (SnRK2) is then released from negative regulation by PP2C, turning on ABA signals by the phosphorylation of downstream factors. Insights into the regulation of PYR/PYL receptor proteins is therefore required in order to control drought-stress tolerance in plants. This article reviews the regulatory mechanism of the ABA receptor by ABA and its selective agonist. Structural analyses of PYR/PYL receptors have clearly elucidated the mechanism of ABA perception of the receptor or the mechanism of interaction with PP2C that leads to inhibition of its phosphatase activity. Moreover, the structures of PYR/PYL receptors complexed with pyrabactin, a selective ABA agonist, have provided the structural basis of ABA agonism and antagonism.

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. Copyright © 2010 by Annual Reviews. All rights reserved.