The function of the tomato Pto gene in tobacco and the identification and characterization of two genes induced by the Pto pathway
Abstract
The Pto gene encodes a serine-threonine kinase that confers resistance in tomato to Pseudomonas syringae pv. tomato strains expressing the avirulence gene avrPto. I examined the ability of Pto to function in tobacco. The Pto-transformed tobacco plants exhibited a significant increase in resistance to the avirulent P. s. tabaci strain compared with wild-type tobacco as indicated by: (1) more rapid development of the hypersensitive response (HR) at high inoculum concentrations; (2) lessened severity of disease symptoms at moderate inoculum concentrations; and (3) reduced growth of avindent P. s. tabaci in inoculated leaves. The results indicate that essential components of a Pto-mediated signal transduction pathway are conserved in tobacco. Salicylic acid (SA) is an important signal molecule involved in the plant disease resistance. I examined the role of SA in Pto mediated resistance using tobacco plants that express both the Pto and the nahG transgenes. The nahG gene, which encodes salicylate hydroxylase, attenuates local resistance and blocks systemic acquired resistance in tobacco and Arabidopsis. Inoculated tobacco leaves expressing Pto and nahG exhibit an increase in symptom development and avirulent bacterial growth compared to tobacco plants expressing Pto alone. This reduction in resistance is correlated with a delay in the expression of pathogenesis related (PR) genes within inoculated tissue. Despite these changes in resistance, the Pto/nahG plants exhibit an unaltered HR and allow less bacterial growth in their leaves than nahG plants that lacked the Pto transgene. In addition, Pto/ nahG plants express the hin1 and hsr203J genes to similar levels as control plants. Expression of nahG effectively blocked systemic acquired resistance and systemic PR gene expression that is activated by the Pto-mediated HR. These results suggest that Pto-mediated resistance involves both SA dependent and independent pathways. The Pto-mediated defense response involves the induction of PR genes and potentially other defense genes. I used the mRNA differential display technique to identify two genes in tomato that are specifically induced by the Pto pathway. Transcripts of Prg1 (Pto responsive gene 1) and Le-Eli3 rapidly accumulate in Pto-resistant plants inoculated with the avirulent pathogen. Prg1 is a single copy gene and lacks homology to any known proteins in the database. Le-Eli3 is a member of a gene family in tomato and has significant homology to the ELI3 defense response genes from Arabidopsis and parsley. The expression of Prg1 and Le-Eli3 in response to SA and their developmental expression in different tomato tissues was examined.
Degree
Ph.D.
Advisors
Martin, Purdue University.
Subject Area
Agronomy|Plant pathology|Microbiology
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