br Acknowledgement This study was supported by Vetenskapsr d
Acknowledgement This study was supported by Vetenskapsrådet, Sweden (Grant 2017-02918).
Introduction Soybean is a major source of protein and oil used for food and feed, as well as for the production of industrial products like lubricants and hydraulic fluids (Choudhary and Tran, 2011; Hsien, 2015). Predicted changes in the climate will result in significant higher temperatures and extended drought periods, likely resulting in severe soybean yield losses (Ku et al., 2013; Zinta et al., 2014; Daryanto et al., 2015). Selection of more drought-tolerant soybean cultivars is therefore required to address this imminent threat to food and protein security (Ku et al., 2013). A recent study on drought-stressed soybean roots identified Cyclosporin D changes of genes associated with osmo-protectant biosynthesis, as well as a study identifying genes coding for kinases, transcription factors controlling root growth and phosphatase 2C proteins (Ha et al., 2015, Song et al., 2016). These studies are the first steps in characterizing the soybean transcriptome under drought. Soybean, symbiotically fixes nitrogen via rhizobial bacteria housed in determinate root nodules (Herridge et al., 2008; Foyer et al., 2016). Extended drought periods lower nodule water potential, decrease nitrogenase activity and also affects nodule formation and life span (Fernandez-Luquen et al., 2008; Gil-Quintana et al., 2015). Adaptive mechanisms, which are activated during drought, include the expression of proteases to activate the plant’s nitrogen reserves through proteolysis (Simova-Stoilova et al., 2010; Kidrič et al., 2014). However, relatively little research on protease expression during drought has been reported for soybean nodules. Past research has provided strong evidence that nodule cysteine proteases are involved in the regulation of the bacterial symbiosis. They degrade leghemoglobin involved in nitrogen fixation and their expression have been shown to increase during nodule senescence (Kardailsky and Brewin, 1996; Van de Velde et al., 2006; Li et al., 2008; Vorster et al., 2013; van Wyk et al., 2014; Marquez-Garcia et al., 2015). In particular, expression of cysteine proteases belonging to the C1 (papain-like) and C13 (legumain-like or VPE) cysteine protease families are induced during nodule senescence (van Wyk et al., 2014). The C1 family proteases are associated with many plant processes including senescence and resistance against diseases and pests (Kidrič et al., 2014). Nine C1 cysteine proteases subfamilies exist based on functional and structural characteristics. These characteristics include presence of a NPIR and KDEL localization signal and existence of a carboxyl-terminal granulin domain (Richau et al., 2012). Individual C1 proteases have various cathepsin activities (L-, B-, H-, and F-like) depending on their gene structure and phylogenetic relationship (Roberts et al., 2012; Diaz and Martinez, 2013; Díaz-Mendoza et al., 2014). Inhibitor studies based on the over-expression of cysteine proteases inhibitors, such as the rice cysteine protease inhibitor OCI (Quain et al., 2015) in transgenic plants have provided evidence for possible C1 protease targets. The reduction of C1 cysteine protease activity by an inhibitor can alter plant development, enhances stress tolerance and better protects photosynthesis (Quain et al., 2015). Vacuoles are also involved in senescence-associated cellular degradation (Martínez et al., 2008). C13 (legumain-like) cysteine proteases, also called vacuolar processing enzymes (VPEs) (Okamoto and Minamikawa, 1995), are involved in many vacuolar proteolytic processes (Diaz and Martinez, 2013; Rawlings et al., 2016, van Wyk et al., 2014). Most vacuolar soluble proteins, such as C1 proteases, are synthesized on the endoplasmic reticulum as a larger precursor, which is then transported into vacuoles where they are converted into their respective mature forms by VPEs (Okamoto and Minamikawa, 1995; Simova-Stoilova et al., 2010). VPEs play an important role in programmed cell death (PCD). Specific inhibitors of caspase-3 activity have recently also been instrumental in the identification of a further important cysteine protease, cathepsin B, responsible for PCD (Ge et al., 2016). Cathepsin B is normally bound to an endogenous cysteine protease inhibitor, but is released upon perception of PCD triggers. VPEs function as key molecules of plant PCD through disrupting the vacuole in pathogenesis and development. Cell death triggered by vacuolar collapse is unique to plants and has not been seen in animals, it suggested that plants might have an evolved VPE-mediated vacuolar system which functions as a cellular suicide strategy (Hatsugai et al., 2006).