INDUCTION OF SYSTEMIC RESISTANCE OF TOBACCO TO TOBACCO NECROSIS VIRUS BY THE ROOT-COLONIZING PSEUDOMONAS-FLUORESCENS STRAIN CHA0 - INFLUENCE OF THE GACA GENE AND OF PYOVERDINE PRODUCTION

Pseudomonas fluorescens strain CHAO, which suppresses various plant diseases caused by soilborne pathogens, also can restrict leaf disease. Plants of Nicotiana glutinosa and of two cultivars of N. tabacum were grown in autoclaved natural soil previously inoculated with strain CHAO. After 6 wk, all the plants tested showed resistance in leaves to infection with tobacco necrosis virus (TNV) to the same extent as plants previously immunized with TNV (induced resistance control). Polyacrylamide gel electrophoresis and enzyme assays showed that the same amount of PR proteins (PR-1 group proteins, beta-1,3-glucanases, and endochitinases) was induced in the intercellular fluid of leaves of plants grown in the presence of strain CHAO as in the intercellular fluid of leaves of plants immunized by a previous TNV inoculation on a lower leaf. Strain CHAO was reisolated from the roots but could not be detected in stems or leaves. Strain CHA96, a gacA (global activator)-negative mutant of strain CHAO defective in the production of antibiotics and in the suppression of black root rot of tobacco, had the same capacity to induce PR proteins and resistance against TNV as did the wild-type strain. CHA400, a pyoverdine-negative mutant of strain CHAO with the same capacity to suppress black root rot of tobacco and take-all of wheat as the wild-type strain, was able to induce PR proteins but only partial resistance against TNV. P3, another P. fluorescens wild-type strain, does not suppress diseases caused by soilborne pathogens and induced neither resistance nor PR proteins in tobacco leaves. Root colonization of tobacco plants with strain CHAO and its derivatives as well as leaf infection with TNV caused an increase in salicylic acid in leaves. These results show that colonization of tobacco roots by strain CHAO reduces TNV leaf necrosis and induces physiological changes in the plant to the same extent as does induction of systemic resistance by leaf inoculation with TNV. We further conclude that the bacterial gacA gene, which is important in the protection of the roots, has no influence on the induction of leaf resistance and that the bacterial production of the siderophore pyoverdine, which has no role in the protection of the roots, is involved in the induction of resistance in the leaves.