T-DNA Genes Responsible for Inducing a Necrotic Response on Grapevines. Wanyin Deng . Departments of Microbiology, SC-42, University of Washington, Seattle 98195, U.S.A. Xin-an Pu(2) R. N. Goodman(2) M. P. Gordon(3) and E.W. Nester(1,4). (1) Departments of Microbiology, SC-42,(3) Biochemistry, SJ-70 and (4)Botany, KB-154, University of Washington, Seattle 98195, U.S.A. and (2) Department of Plant Pathology, University of Missouri, Columbia 65211, U.S.A. MPMI 8:538-548. Accepted 13 March 1995. Copyright 1995 The American Phytopathological Society.

Agrobacterium tumefaciens supervirulent strain A281 induces a progressive necrotic response, rather than tumor formation, when inoculated on stems of several grape cul-tivars. The Ti plasmid, and specifically its T-DNA, is required for the process. In the present study, 40 T-DNA insertion mutants of A281 were generated via transposon mutagenesis and tested for their necrosis-inducing ability on grape stems in vitro. Ten mutants were attenuated in inducing necrogenesis. Restriction mapping and DNA sequencing revealed that at least two genes, tms 1 and 6b, whose gene products are involved in the synthesis and activity modulation of auxin, are responsible for inducing necrogenesis. Double mutants of tms1 and 6b were totally non-necrogenic. The orientation of grapevine stem ex-plants 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, gene 5 of T-DNA, which modulates auxin responses in plants by the autoregulated synthesis of an auxin antagonist, was found to be separated from 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 transferred into plants. Based on the accumulated data, we propose that the necrogenesis induced by Agrobacterium results from the sensitivity of grapevine cells to elevated levels of auxin or a precursor of auxin.