Key Laboratory of Monitoring and Management of Pathogens and Insect Pests, Ministry of Agriculture of China and Department of Plant Pathology, Nanjing Agricultural University, 1 Weigang, Nanjing, 210095, China
HpaGXooc, produced by Xanthomonas oryzae pv. oryzicola, is a member of harpin group of proteins that stimulate plant growth, hypersensitive cell death (HCD), and pathogen defense. The protein contains two copies of the glycine-rich motif (GRM), a characteristic of harpins, and a cysteine, which is absent in other harpins. Genetic modification generated the pro-tein mutants HpaGXoocMG (MG) by deleting GRMs and HpaGXoocC47T (C47T) by replacing cysteine with threonine. When applied to tobacco plants, C47T and MG were 1.2- and 1.7-fold stronger, respectively, than HpaGXooc in inducing HCD, which occurred consistently with expression of the marker genes hin1 and hsr203. The proteins markedly alleviated infection of tobacco by Tobacco mosaic virus and Arabidopsis and tomato by Pseudomonas syringae. Treating tobacco plants with HpaGXooc, C47T, and MG decreased the viral infection by 58, 81, and 92%, respectively. In Arabidopsis and tomato plants treated with HpaGXooc, C47T, or MG, P. syringae multiplication was inhibited; bacterial population multiplied in 5 days in these plants were ca. 160-, 1,260-, or 15,860-fold smaller than that in control plants. So pathogen defense was induced in both plants. Defense-related genes Chia5, NPR1, and PR-1a were expressed consistently with resistance. In response to HpaGXooc, C47T, and MG, aerial parts and roots of tomato plants increased growth by 15 and 53%, 25 and 77%, and 46 and 106%, relative to controls. The expansin gene, EXP2, involved in the cell expansion and plant growth was expressed coordinately with plant growth promotion. These results suggest that the presence of GRM and cysteine in HpaGXooc represses the effects of the protein in plants.