Genetic transformation of cotton with a harpin-encoding gene hpa Xoo confers an enhanced defense response against different pathogens through a priming mechanism

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Abstract
Background: The soil-borne fungal pathogenVerticillium dahliaeKleb causesVerticilliumwilt in a wide range of crops including cotton (Gossypium hirsutum). To date, most upland cotton varieties are susceptible toV. dahliaeand the breeding for cotton varieties with the resistance toVerticilliumwilt has not been successful.Results: Hpa1Xoois a harpin protein fromXanthomonas oryzaepv.oryzaewhich induces the hypersensitive cell death in plants. Whenhpa1Xoowas transformed into the susceptible cotton line Z35 throughAgrobacterium-mediated transformation, the transgenic cotton line (T-34) with an improved resistance toVerticillium dahliaewas obtained. Cells of the transgenic T-34, when mixed with the conidia suspension ofV. dahliae, had a higher tolerance toV. dahliaecompared to cells of untransformed Z35. Cells of T-34 were more viable 12 h after mixing withV. dahliaeconidia suspension. Immunocytological analysis showed that Hpa1Xoo, expressed in T-34, accumulated as clustered particles along the cell walls of T-34. In response to the infection caused byV. dahliae, the microscopic cell death and the generation of reactive oxygen intermediates were observed in leaves of T-34 and these responses were absent in leaves of Z35 inoculated withV. dahliae. Quantitative RT-PCR analysis indicated that five defense-related genes,ghAOX1, hin1, npr1, ghdhg-OMT, andhsr203J, were up-regulated in T-34 inoculated withV. dahliae. The up-regulations of these defense-relate genes were not observed or in a less extent in leaves of Z-35 after the inoculation.Conclusions: Hpa1Xooaccumulates along the cell walls of the transgenic T-34, where it triggers the generation of H2O2as an endogenous elicitor. T-34 is thus in a primed state, ready to protect the host from the pathogen. The results of this study suggest that the transformation of cotton withhpa1Xoocould be an effective approach for the development of cotton varieties with the improved resistance against soil-borne pathogens.

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