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

Pseudomonas fluorescens strain CHAO, which suppresses various plant di seases 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 CHA O. After 6 wk, all the plants tested showed resistance in leaves to in fection with tobacco necrosis virus (TNV) to the same extent as plants previously immunized with TNV (induced resistance control). Polyacryl amide gel electrophoresis and enzyme assays showed that the same amoun t of PR proteins (PR-1 group proteins, beta-1,3-glucanases, and endoch itinases) was induced in the intercellular fluid of leaves of plants g rown in the presence of strain CHAO as in the intercellular fluid of l eaves of plants immunized by a previous TNV inoculation on a lower lea f. 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 i n 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 s ame capacity to suppress black root rot of tobacco and take-all of whe at as the wild-type strain, was able to induce PR proteins but only pa rtial resistance against TNV. P3, another P. fluorescens wild-type str ain, does not suppress diseases caused by soilborne pathogens and indu ced neither resistance nor PR proteins in tobacco leaves. Root coloniz ation of tobacco plants with strain CHAO and its derivatives as well a s leaf infection with TNV caused an increase in salicylic acid in leav es. These results show that colonization of tobacco roots by strain CH AO reduces TNV leaf necrosis and induces physiological changes in the plant to the same extent as does induction of systemic resistance by l eaf inoculation with TNV. We further conclude that the bacterial gacA gene, which is important in the protection of the roots, has no influe nce on the induction of leaf resistance and that the bacterial product ion of the siderophore pyoverdine, which has no role in the protection of the roots, is involved in the induction of resistance in the leave s.