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Role of Extracellular Polysaccharide and Endoglucanase in Root Invasion and Colonization of Tomato Plants by Ralstonia solanacearum

Ralstonia solanacearum is a soilborne plant pathogen that normally invades hosts through their roots and then systemically colonizes aerial tissues. Previous research using wounded stem infection found that the major factor in causing wilt symptoms was the high-molecular-mass acidic extracellular polysaccharide (EPS I), but the β-1,4-endoglucanase (EG) also contributes to virulence. We investigated the importance of EPS I and EG for invasion and colonization of tomato by infesting soil of 4-week-old potted plants with either a wild-type derivative or genetically well-defined mutants lacking EPS I, EG, or EPS I and EG. Bacteria of all strains were recovered from surface-disinfested roots and hypocotyls as soon as 4 h after inoculation; that bacteria were present internally was confirmed using immunofluorescence microscopy. However, the EPS-minus mutants did not colonize stems as rapidly as the wild type and the EG-minus mutant. Inoculations of wounded petioles also showed that, even though the mutants multiplied as well as the wild type in planta, EPS-minus strains did not spread as well throughout the plant stem. We conclude that poor colonization of stems by EPS-minus strains after petiole inoculation or soil infestation is due to reduced bacterial movement within plant stem tissues.