Inter-organ defense networking: Leaf whitefly sucking elicits plant immunity to crown gall disease caused by Agrobacterium tumefaciens.

Plants have elaborate defensive machinery to protect against numerous pathogens and insects. Plant hormones function as modulators of defensive mechanisms to maintain plant resistance to natural enemies. Our recent study suggests that salicylic acid (SA) is the primary phytohormone regulating plant responses to Agrobacterium tumefaciens infection. Tobacco (Nicotiana benthamiana Domin.) immune responses against Agrobacterium-mediated crown gall disease were activated by exposure to the sucking insect whitefly, which stimulated SA biosynthesis in aerial tissues; in turn, SA synthesized in aboveground tissues systemically modulated SA secretion in root tissues. Further investigation revealed that endogenous SA biosynthesis negatively modulated Agrobacterium-mediated plant genetic transformation. Our study provides novel evidence that activation of the SA-signaling pathway mediated by a sucking insect infestation has a pivotal role in subsequently attenuating Agrobacterium infection. These results demonstrate new insights into interspecies cross-talking among insects, plants, and soil bacteria.