XYLEM COLONIZATION BY AN HRCV(-) MUTANT OF RALSTONIA-SOLANACEARUM IS A KEY FACTOR FOR THE EFFICIENT BIOLOGICAL-CONTROL OF TOMATO BACTERIAL WILT

Microscopic studies of the colonization of the vascular tissues of tom ato by an HrcV(-) (formerly HrpO(-)) mutant strain of Ralstonia solana cearum were carried out after either root inoculation of the mutant st rain alone or delayed challenge inoculation by a pathogenic strain. Th e use of two different marker genes, lacZ and uidA, introduced into ei ther mutant or wild-type strains, respectively, permitted histological observation for the presence of both strains simultaneously. In roots , both strains could be found together in infected root tips and in la teral root emergence sites (lateral root cracks), but these bacterial strains subsequently invaded separate xylem vessels in the root system . At the hypocotyl level, a never staining procedure, in conjunction w ith bacterial isolation and counting, showed three vascular colonizati on patterns: exclusive colonization by each of the competitors or simu ltaneous presence of each strain in separate xylem vessels. The relati ve frequencies of these patterns depended upon the root inoculation te chniques used. The presence of one population always influenced the de nsity of the other challenge-inoculated population. In plants inoculat ed with both wild-type and mutant strains, the population of the wildt ype strain is lower than in plants inoculated with the wild type alone . In contrast? growth of the HrcV- mutant strain was significantly inc reased in the presence of the pathogenic strain. Two agriculturally ac ceptable techniques for plant inoculation were tested. Inoculation of plants by transplanting them into soil amended with clay microgranules impregnated with the HrcV- mutant strain gave higher and more reprodu cible colonization of the plants than inoculation by watering a bacter ial suspension on the roots. Significant percentages of exclusive colo nization by the HrcV- mutant strain were only obtained after the clay microgranule inoculation technique. Competition for space in xylem ves sels is one of the possible explanations for the protective ability of the HrcV- mutant strain against subsequent invasion by a pathogenic s train.