Rice xylanase inhibitor RIXI is a XIP-type inhibitor that belongs to the glycoside hydrolase family 18 (GH18), which includes plant class III chitinases (EC 184.108.40.206) known as PR-8 proteins. The aim of this study was to elucidate whether RIXI had any effect on rice endoxylanase and its role(s) in plant defence. RIXI encoding sequence was cloned from rice (Oryza sativa cv. Nipponbare), and was expressed in Escherichia coli. The activity of recombinant RIXI was investigated – among the tested xylanases, the GH11 xylanase from Aspergillus niger was the most inhibited, while rice endogenous xylanase OSX was least inhibited. Semi-quantitative real-time polymerase chain reaction (PCR) and quantitative real-time PCR showed that the xylanase inhibitor gene RIXI was up-regulated in rice seedlings infected by Magnaporthe grisea. The increased RIXI expression was also accompanied by significantly elevated expression of pathogenesis-related protein gene (PR-1) and class III chitinase gene (Cht-3). This suggested that RIXI might be involved in environmental responses such as defense against phytopathogens.
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