Abscisic acid (ABA) plays a crucial role to integrate plant response to abiotic stress (particularly drought and salinity) into the regulation of plant growth and development. An increase in ABA levels and the subsequent plant response to the hormone are key components of the adaptive mechanism to resist/avoid those forms of abiotic stress. Accordingly, the characterization of ABA signaling offers a high biotechnological potential to improve plant tolerance to drought and salinity.
Because of its essential function in plant stress physiology, elucidating the abscisic acid (ABA) signaling pathway holds enormous promise for application in agriculture. A breakthrough in ABA signaling occurred in 2009, i.e. the discovery of the 14-member PYR/PYL/RCAR family of ABA receptors (Park et al., 2009; Ma et al., 2009; Santiago et al., 2009). Control of ABA signaling by PYR/PYL/RCAR ABA-receptors involves direct ABA-dependent inhibition of clade A phosphatases type-2C (PP2Cs), for instance, ABI1, HAB1, PP2CA, which are key negative regulators of the pathway (Saez et al., 2006; Rubio et al., 2009). Inhibition of PP2Cs leads to activation of sucrose non-fermenting 1-related subfamily 2 (SnRK2) kinases, which regulate stomatal aperture and transcriptional response to ABA. Thus, a core signaling network for ABA has emerged from these findings (Fujii et al., 2009; Cutler et al., 2010).
Agrochemicals: Crystal structures are available for ABA receptors and receptor-ABA-phosphatase complexes, which reveal key details on the mode of interaction between the receptor, the hormone, and the protein phosphatase as well as the mechanism of activation of ABA signaling. This information will be used in the structure-assisted identification of synthetic molecules able to act as agonists of ABA receptors and activate ABA signaling in plants. These molecules might have the potential to improve the yield of crop plants under drought stress or any other properties modulated by the ABA pathway in crop or ornamental plants. To this end, we will clone and produce recombinant ABA receptors in tomato, orange, grapevine, and monocots as targets for screening small molecules capable of acting as agrochemicals through the activation of ABA receptors. Additionally, through collaboration with organic chemists and protein crystallographers, direct synthesis of small molecules that fit into the ABA binding ligand pocket has been performed. Recently, we achieved the structure-guided engineering of a receptor-agonist pair for inducible activation of the ABA adaptive response to drought, protected by the EP21382948 patent.
Molecular genetics: The ABA signaling group has played a key role in the discovery and characterization of the PYR/PYL/RCAR family of ABA receptors, and their connection with PP2Cs and SnRK2s (Rodriguez et al., 1998; Gonzalez-Guzman et al., 2002; Saez et al., 2004, 2006, 2008; Park et al., 2009; Santiago et al., 2009a, 2009b; Rubio et al., 2009; Vlad et al., 2009; Fujii et al., 2009; Cutler et al., 2010; Vlad et al., 2010; Dupeux et al., 2011a, 2011b; Antoni et al. 2012; Gonzalez-Guzman et al., 2012; Antoni et al., 2013; Merilo et al., 2013; Pizzio et al., 2013).
Some of these works are landmarks in ABA signaling and together with other results have brought about a breakthrough in our knowledge of the pathway. Later on, we provided insight into the subcellular location of ABA receptors and we discovered that C2-domain abscisic acid-related proteins mediate the interaction of PYR/PYL/RCAR receptors with the plasma membrane (Rodriguez et al., 2014; Diaz et al., 2016). Concerning ABA-induced transcriptional regulation, we have discovered a link between SWI/SNF chromatin remodeling complexes and core components of ABA signaling (Saez et al., 2008; Han et al., 2012; Peirats-Llobet et al., 2016). We have contributed several genetic strategies that enhance ABA signaling as a valuable tool for improving plant water use. Among them, the constitutive inactivation of PP2Cs (Saez et al., 2006), overexpression of monomeric ABA receptors (Santiago et al., 2009a; Gonzalez-Guzman et al., 2014), and the generation of mutated ABA receptors that enhance ABA-dependent inhibition of PP2Cs (Pizzio et al., 2013). As a result, three patents were filled to cover these findings.
We have played a pioneering role in studies that address the turnover of core ABA signaling components, particularly ABA receptors and PP2Cs (Bueso et al., 2014; Irigoyen et al., 2014; Wu et al., 2016; Belda-Palazon et al., 2016, 2018 and 2019; Fernandez et al., 2020; Coego et al., 2021). We uncovered the unique role of PYL8 in root ABA signaling, which involves a non-cell-autonomous mechanism like mobile transcription factors, ABA-induced stabilization, and predominant nuclear localization (Belda-Palazon et al., 2018). We have further studied the mechanisms that affect subcellular localization and half-life of ABA receptors. We have uncovered a novel route for endosomal degradation of ABA receptors through the ESCRT pathway (Belda-Palazón et al., 2016; Yu et al., 2016; Garcia-Leon et al., 2019) and we have identified two novel families of E3 ligases that mediate the turnover of PP2Cs (Wu et al., 2016; Belda-Palazon et al., 2019; Julian et al., 2019). Physiological studies and structural biology of crop ABA receptors are a long-lasting interest of our group (Gonzalez-Guzman et al., 2014), to elucidate the formation of receptor-ABA-phosphatase complexes (Moreno-Alvero et al., 2017) and the key role of PYL8-like receptors in crop response to abiotic stress (Garcia-Maquilon et al., 2021; Pizzio et al., 2022). International collaborations (4 publications in Nature Plants, 1 Science Advances, 1 Dev Cell) have provided key findings on the role of SnRK2s-PP2Cs-ABA receptors in root hydrotropism (Dietrich et al., 2017; Miao et al., 2021), regulation of plant growth via SnRK1 and TOR (Belda-Palazon et al., 2020), specific roles of ABA receptors in stomatal response to ABA and high CO2 (Dittrich et al., 2019) and moonlight roles of FYVE1/FREE1 in repression of ABA signaling (Li et al., 2019).