Overexpression of SSBXoc, a Single-Stranded DNA-Binding Protein From Xanthomonas oryzae pv. oryzicola, Enhances Plant Growth and Disease and Salt Stress Tolerance in Transgenic Nicotiana benthamiana

Abstract

We previously reported that SSB_Xoc, a highly conserved single-stranded DNA-binding protein from <i>Xanthomonas</i> spp., was secreted through the type III secretion system (T3SS) and functioned as a harpin-like protein to elicit the hypersensitive response (HR) in the non-host plant, tobacco. In this study, we cloned <i>Ssb_Xoc</i> gene from <i>X. oryzae</i> pv. <i>oryzicola</i> (<i>Xoc</i>), the causal agent of bacterial leaf streak in rice, and transferred it into <i>Nicotiana benthamiana</i> via Agrobacterium-mediated transformation. The expression of <i>Ssb_Xoc</i> in transgenic <i>N</i>. <i>benthamiana</i> enhanced growth of both seedling and adult plants. When inoculated with the harpin Hpa1 or the pathogen <i>Pseudomonas syringae</i> pv. <i>tomato</i> DC3000 (<i>Pst</i> DC3000), the accumulation of reactive oxygen species (ROS) was increased more in <i>Ssb_Xoc</i> transgenic lines than that in wild-type (WT) plants. The expression of pathogenesis-related protein genes (<i>PR1a</i> and <i>SGT1</i>), HR marker genes (<i>HIN1</i> and <i>HSR203J</i>) and the mitogen-activated protein kinase pathway gene, <i>MPK3</i>, was significantly higher in transgenic lines than in WT after inoculation with <i>Pst</i> DC3000. In addition, <i>Ssb_Xoc</i> transgenic lines showed the enhanced resistance to the pathogenic bacteria <i>P</i>. <i>s</i>. <i>tabaci</i> and the improved tolerance to salt stress, accompanied by the elevated transcription levels of the defense- and stress-related genes. Taken together, these results indicate that overexpression of the <i>Ssb_Xoc</i> gene in <i>N</i>. <i>benthamiana</i> significantly enhanced plant growth and increased tolerance to disease and salt stress via modulating the expression of the related genes, thus providing an alternative approach for development of plants with improved tolerance against biotic and abiotic stresses.