Abstract:
Root nodules plays an important role in legumes such as soybean’s ability to fix atmospheric nitrogen into an accessible form that plants can utilise. Root nodules’ lifespan is affected by drought stress periods and causes premature nodule senescence. Delaying the initiation of nodule senescence could prolong active nitrogen fixation and improve soybean yield. Cysteine proteases and cystatins are important role players in premature senescence and possible candidate genes for marker assisted breeding. Soybean plants were subjected to different levels of drought stress (60 % Vermiculite Water Content (VWC), 40 % VWC and 30 % VWC) and premature senescence was initiated in 40 % VWC and 30 % VWC. Nodule formation, moisture content and water potential was seemingly negatively affected by drought stress. Nodule tissue was seemingly negatively affected by drought as was visible in the colour change observed from an active pink colour to an inactive brown colour. A gene expression profile analysis was conducted using RNA-Sequencing (RNA-Seq). Eight C1 cysteine proteases, three C13 cysteine proteases and four cystatins were seen to be involved in drought-induced nodule senescence. These genes were able to recover to pre-drought expression levels after rehydration confirming their ability to be used as drought molecular markers. A functional analysis of vacuolar processing enzyme genes (VPE), done on Arabidopsis mutants, indicated that ɑ-VPEs affect the activity of C1 cysteine proteases after a lower activity in Cathepsin-L like cysteine proteases were observed. Other genes such as late embryogenesis abundant genes (LEA) and defensin like genes were also identified as possible candidates for marker assisted breeding approaches. Future investigations should include in vivo interaction of the cysteine protease-cystatin system and localised down-regulation of ɑ-VPEs and other candidate genes to investigate if it reduces the consequences of drought stress in root nodules and improves nitrogen fixation.
