Induction of lateral root structure formation on petunia roots: A novel effect of GMI1000 Ralstonia solanacearum infection impaired in Hrp mutants

Ralstonia solanacearum is a soilborne plant pathogen that invades its host via roots. As in many gram-negative bacterial plant pathogens, the R. solanacearum Hrp type III secretion system is essential for interactions of the bacterium with plants; however, the related mechanisms involved in disease expression are largely unknown. In this work, we examined the effects of infection by R. solanacearum GMI1000 and Hrp mutants on the root system of petunia plants. Both the wild-type and mutant strains disturbed the petunia root architecture development by inhibiting lateral root elongation and provoking swelling of the root tips. In addition, GMI100 but not the Hrp mutants induced the formation of new root lateral structures (RLS). This ability is shared by other, but not all, R. solanacearum strains tested. Like lateral roots, these new structures arise from divisions of pericycle founder cells which, nevertheless, exhibit an abnormal morphology. These RLS are efficient colonization sites allowing extensive bacterial multiplication. However, they are not required for the bacterial vascular invasion that leads to the systemic spread of the bacterium through the whole plant, indicating that, instead, they might play a role in the rhizosphere-related stages of the R. solanacearum life cycle.