29 Post-translational modification by SUMO is an essential process that has a major role in the 30 regulation of plant development and stress responses. Such diverse biological functions are 31 accompanied by functional diversification among the SUMO conjugation machinery components 32 and regulatory mechanisms that has just started to be identified in plants. In this review, we focus 33 on the current knowledge of the SUMO conjugation system in plants in terms of components, 34 substrate

more »

... icity, cognate interactions, enzyme activity and subcellular localization. In 35 addition, we analyze existing data on the role of SUMOylation in plant drought tolerance in model 36 plants and crop species, we discuss the genetic approaches used in order to stimulate or inhibit 37 endogenous SUMO conjugation. The role that potential SUMO targets identified in proteomic 38 analyses may have in drought tolerance is also discussed. Overall, the complexity of 39 SUMOylation and the multiple genetic and environmental factors that are integrated to confer 40 drought tolerance highlight the need for significant efforts to understand the interplay between 41 SUMO and drought. 42 43 65 In this review, we will focus on molecular aspects of the SUMO conjugation system in plants 66 and discuss the current knowledge of the role of SUMOylation in plant responses to drought 67 stress. Since its discovery 20 years ago, SUMO has received major attention due to its essential 68 cellular functions and its major role in human diseases, including cancer and neurological 69 disorders (Droescher et al., 2013; Seeler and Dejean, 2017). Numerous genetic and structural 70 studies performed in yeast and animal systems have contributed to identify the molecular 71 mechanisms involved in SUMO conjugation/deconjugation (Cappadocia and Lima, 2018). In 72 plants, SUMO was first identified as an interactor of the ethylene inducing xylanase (EIX) from 73 the fungus Trichoderma viride (Hanania et al., 1999). In the following years, the main components 74 of the SUMOylation system in Arabidopsis were characterized (Chosed et al., 2006; Kurepa et 75 al., 2003; Lois et al., 2003; Miura et al., 2005; Murtas et al., 2003). 76 In plants, as well as in animals, SUMO conjugation is essential during embryo development 77 (Nacerddine et al., 2005; Saracco et al., 2007). SUMOylation modulates plant hormone signaling 78 (Campanaro et al., 2016; Kim et al., 2015; Lois et al., 2003; Miura et al., 2010), root stem cell 79 maintenance (Xu et al., 2013), circadian clock (Hansen et al., 2017a; Hansen et al., 2017b), light 80 signaling (Lin et al., 2016; Sadanandom et al., 2015), plant immunity (Lee et al., 2007), plant 81 immunity and growth (Hammoudi et al., 2018), defense responses to necrotrophic fungal 82 4 pathogens (Castaño-Miquel et al., 2017) , thermotolerance (Yoo et al., 2006) and, virtually, any 83 aspect of plant development (Ishida et al., 2012; Ling et al., 2012; Liu et al., 2014). Considering 84 that SUMOylation regulates physiological processes that are key determinants for agriculture 85 productivity, uncovering the molecular insights into SUMO conjugation has a major interest for 86 providing new markers and/or biotechnological tools to the agro-food sector. 87 88 Components of the SUMO conjugation machinery in plants 89 The SUMO isoforms 90 The existence of distinctive SUMO isoforms and their attachment to substrates as monomers, 91 in single or multiple positions, or as polymers by building polySUMO chains, contribute to the high 92 complexity of the molecular consequences of SUMOylation. Among others, SUMOylation 93 regulates protein activity by inducing subcellular redistribution, modulating protein-protein 94 interactions, competing with other post-translational modifications or promoting conformational 95 changes. On the other hand, the most prevalent role of polySUMO chains seems to function as 96 substrate for ubiquitination (Tatham et al., 2008), so that the SUMOylated substrate is tagged for 97 degradation by the proteasome. 98 Arabidopsis genome encodes eight SUMO isoforms, although only SUMO1, 2, 3 and 5 are 99 expressed (Hammoudi et al., 2016; Kurepa et al., 2003; Novatchkova et al., 2004). SUMO1 and 100 SUMO2 are the most closely related isoforms sharing an 83% of amino acid sequence identity. 101 SUMO3 and SUMO5 display 42% and a 30% of amino acid sequence identity with SUMO1,