Characterization of Aberrant Infection Events Induced on Trifolium subterraneum by Rhizobium trifolii Region II Mutants

Abstract

The early infection events of subterranean clover by the wild-type Rhizobium trifolii strain ANU843 was compared to the infective ability of two nodulation-defective mutants derived from this strain (261 and 262). Strain ANU843 readily infected the newly emerging root hair cells which were present behind the root tip at the time of inoculation although the zone of maximum susceptibility occurred behind the root tip in an area where no root hairs had yet emerged. Both mutant strains were capable of infecting the most susceptible root hair cells at the time of inoculation and inducing nodules but did so with a>50 % reduction in efficiency and nodules were slow to appear and develop. After direct «spot inoculation» of root cells in the most susceptible zone, the progress of the infections initiated by ANU843 and mutant 261 was assessed at 24, 48, 72 h and several days post inoculation using microscopic observation. The rate of the infection process initiated by strain ANU843 on subterranean clovers was similar to those described for the infection of other legumes: marked root hair curling and the initial penetration of the root hair was already apparent after 24 h; extensive cortical cell division ahead of the infection thread was prominent at 72 h and by this time, the infection thread had penetrated to the root cortex. In contrast, infection threads initiated by mutant 261 were still confined to the root hair cell even after 72 h and the infection thread induced rarely penetrated to the root cortex. Microscopic analysis indicated that the development of the infection thread was slow. Cortical cell division was initiated by the mutant strain but was not as extensive as that induced by the parent strain. The poor infective and nodulation ability of mutant strain 261 suggests that the gene(s) affected are required by the bacterium to initiate normal infection thread development in the plant host.