T-DNA GENES RESPONSIBLE FOR INDUCING A NECROTIC RESPONSE ON GRAPE VINES

Agrobacterium tumefaciens supervirulent strain A281 induces a progress ive necrotic response, rather than tumor formation, when inoculated on stems of several grape cultivars, The Ti plasmid, and specifically it s T-DNA, is required for the process, In the present study, 40 T-DNA i nsertion mutants of A281 were generated via transposon mutagenesis and tested for their necrosis-inducing ability on grape stems in vitro. T en mutants were attenuated in inducing necrogenesis. Restriction mappi ng and DNA sequencing revealed that at least two genes, tms1 and 6b, w hose gene products are involved in the synthesis and activity modulati on of auxin, are responsible for inducing necrogenesis, Double mutants of tms1 and 6b were totally non-necrogenic. The orientation of grapev ine stem explants showed strong effects on the occurrence and progress of necrogenesis. Inoculation of Agrobacterium on physiological basal ends resulted in the greatest degree of necrogenesis. In addition, gen e 5 of T-DNA, which modulates auxin responses in plants by the autoreg ulated synthesis of an auxin antagonist, was found to be separated fro m other TL-DNA genes by a novel insertion sequence, IS1312. Since a T- DNA borderlike sequence occurs in IS1312, gene 5 might not always be t ransferred into plants. Based on the accumulated data, we propose that the necrogenesis induced by Agrobacterium results from the sensitivit y of grapevine cells to elevated levels of auxin or a precursor of aux in.