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Identification of Specific Fragments of HpaG_Xooc, a Harpin from Xanthomonas oryzae pv. oryzicola, that Induce Disease Resistance and Enhance Growth in Plants

First, second, third, fourth, fifth, sixth, seventh, eighth, ninth, and twelfth authors: Plant Growth and Defense Signaling Laboratory, Group of Key Laboratory of Monitoring and Management of Plant Pathogens and Insect Pests, Ministry of Agriculture of P. R. China, Nanjing Agricultural University, Nanjing 210095, China; third author: Horticulture Department, Northeast Agricultural University, Harbin 150030, China; and tenth and eleventh authors: Department of Plant Pathology, Cornell University, Ithaca, NY 14853.

Harpin proteins from gram-negative plant-pathogenic bacteria can stimulate hypersensitive cell death (HCD) and pathogen defense as well as enhance growth in plants. Two of these diverse activities clearly are beneficial and may depend on particular functional regions of the proteins. Identification of beneficial and deleterious regions might facilitate the beneficial use of harpin-related proteins on crops without causing negative effects like cell death. Here, we report the identification and testing of nine functional fragments of HpaG_Xooc, a 137-amino-acid harpin protein from Xanthomonas oryzae pv. oryzicola, the pathogen that causes bacterial leaf streak of rice. Polymerase chain reaction-based mutagenesis generated nine proteinaceous fragments of HpaG_Xooc; these caused different responses following their application to Nicotiana tabacum (tobacco) and Oryza sativa (rice). Fragment HpaG_62-137, which spans the indicated amino acid residues of the HpaG, induced more intense HCD; in contrast, HpaG_10-42 did not cause evident cell death in tobacco. However, both fragments stimulated stronger defense responses and enhanced more growth in rice than the full-length parent protein, HpaG_Xooc. Of the nine fragments, the parent protein and one deletion mutant of HpaG_Xooc tested, HpaG_10-42, stimulated higher levels of rice growth and resulted in greater levels of resistance to X. oryzae pv. oryzae and Magnaporthe grisea. These pathogens cause bacterial leaf blight and rice blast, respectively, the two most important diseases of rice world-wide. HpaG_10-42 was more active than HpaG_Xooc in inducing expression of several genes that regulate rice defense and growth processes and activating certain signaling pathways, which may explain the greater beneficial effects observed from treatment with that fragment. Overall, our results suggest that HpaG_10-42 holds promise for practical agricultural use to induce disease resistance and enhance growth of rice.

Additional keywords:hypersensitive response (HR), systemic acquired resistance (SAR).