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  • Currently it is not known if

    2019-07-17

    Currently, it is not known if drought induces changes in the cysteine protease adrenergic receptors of soybean nodules. The available annotation of the soybean genome, in Phytozome (www.phytozome.net) has identified 706 C1 cysteine protease sequences and 75 C13 cysteine proteases. We have previously reported expression of several C1 and C13 cysteine proteases during soybean nodule development (van Wyk et al., 2014). Our current study aimed to establish an expression profile of C1 and C13 cysteine proteases in soybean nodules following exposure to drought. A RNA-seq analysis was used as a first step for gene discovery in drought-stressed nodules. Detected changes cysteine protease expression was then validated by qPCR. In addition, the possibility that any cysteine protease was uniquely induced by drought was addressed by comparing drought induced expression with expression during natural senescence. This comparison was done to find support for the view that drought causes premature senescence with accelerating tissue senescence (Muchero et al., 2009, Wehner et al., 2016). Arabidopsis thaliana mutant lines deficient ɑ-VPE expression (Shimada et al., 2003) was finally characterized to determine the possible function of ɑ-VPE in activating C1 proteases. Overall, by investigating cysteine protease expression in soybean nodules we found that drought causes premature nodule senescence with differential increase in C1 and C13 cysteine protease expression and activity. Such an increase was further associated with a decreased nodule number and a lower water potential in nodules.
    Materials and methods
    Results
    Discussion This study is the first analysis of the transcriptome of drought-stressed soybean crown nodules. Previously, only the transcriptome of developing non-stressed crown nodules has been investigated (van Wyk et al., 2014). In our study drought stress decreased soybean plant growth but resulted in larger root weight (Appendix Table 3A) which has previously also been reported (Fenta et al., 2011). As the roots, due to drought stress, become deeper and denser, the biomass increases but negatively affected the formation of root nodules and nodule activity. Following drought exposure, nodule formation terminated and nodules were inactivated, resulting in a colour change in nodule tissue from red to greenish (Puppo et al., 2005; Fenta et al., 2011 and Fenta et al., 2014). Further, the nodule water potential decreased due to drought. A low water potential negatively affects nodule respiratory capacity resulting in a decline in nodule permeability (Purcell and Sinclair, 1995). When cell turgor is lost in the nodule cortex, the microbial partner is eliminated due to limited O2 diffusion (Guerin et al., 1990). The induction of changes in expression of a high number of genes under severe drought conditions has recently been reported for soybean roots (Song et al., 2016). In this study, drought also induced higher expression of nodule genes including various cysteine proteases. The majority of cysteine proteases expressed in nodules belonged to the C1 cysteine protease (papain-like) family. Contribution of C1 cysteine proteases to the total cysteine protease transcriptome further changed when a more severe drought treatment was applied (30% mWHC). C1 cysteine proteases represent about 50% of all cysteine proteases in the soybean transcriptome (Severin et al., 2010). Only the expression of one C1 cysteine protease Glyma.14G085800 and of two VPEs, Glyma.17G230700 and Glyma.05G055700, were highly up-regulated under our most severe drought condition (30% mWHC). However, although expressed in non-stressed nodules, expression of the C1 cysteine protease Glyma.14G085800 did not increase in our study when nodules senesced. The C1 cysteine protease Glyma.14G085800 is also highly expressed in all other investigated soybean tissues (Severin et al., 2010; van Wyk et al., 2014) and is also responsive to phosphorus deficiency (Sha et al., 2016).