HRP(-) MUTANTS OF PSEUDOMONAS-SOLANACEARUM AS POTENTIAL BIOCONTROL AGENTS OF TOMATO BACTERIAL WILT

There have been many attempts to control bacterial with with antagonis tic bacteria or spontaneous nonpathogenic mutants of Pseudomonas solan acearum that lack the ability to colonize the host, but they have met with limited success. Since a large gene cluster (hrp) is involved in the pathogenicity of P. solanacearum, we developed a biological contro l strategy using genetically engineered Hrp(-) mutants of P. solanacea rum. Three pathogenic strains collected in Guadeloupe (French West Ind ies) were rendered nonpathogenic by insertion of an Omega-Km interposo n within the hrp gene cluster of each strain. The resulting Hrp(-) mut ants were tested for their ability to control bacterial wilt in challe nge inoculation experiments conducted either under growth chamber cond itions or under greenhouse conditions in Guadeloupe. Compared with the colonization by a pathogenic strain which spread throughout the tomat o plant, colonization by the mutants was restricted to the roots and t he lower part of the stems. The mutants did not reach the fruit. Moreo ver, the presence of the mutants did not affect fruit production. When the plants were challenge inoculated with a pathogenic strain, the pr esence of Hrp(-) mutants within the plants was correlated with a reduc tion in disease severity, although pathogenic bacteria colonized the s tem tissue at a higher density than the nonpathogenic bacteria. Challe nge inoculation experiments conducted under growth chamber conditions led, in some cases, to exclusion of the pathogenic strain from the aer ial part of the plant, resulting in high protection rates. Furthermore , there was evidence that one of the pathogenic strains used for the c hallenge inoculations produced a bacteriocin that inhibited the in vit ro growth of the nonpathogenic mutants.