Rhizobia Can Induce Nodules in White Clover by “Hijacking” Mature Cortical Cells Activated During Lateral Root Development

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Abstract
We examined a range of responses of root cortical cells to Rhizobium sp. inoculation to investigate why rhizobia preferentially nodulate legume roots in the zone of emerging root hairs, but generally fail to nodulate the mature root. We tested whether the inability to form nodules in the mature root is due to a lack of plant flavonoids to induce the bacterial genes required for nodulation or a failure of mature cortical cells to respond to Rhizobium spp. When rhizobia were inoculated in the zone of emerging root hairs, changes in beta-glucuronidase (GUS) expression from an auxin-responsive promoter (GH3), expression from three chalcone synthase promoters, and the accumulation of specific flavonoid compounds occurred in cortical cells prior to nodule formation. Rhizobia failed to induce these responses when inoculated in the mature root, even when co-inoculated with nod gene-inducing flavonoids. However, mature root hairs remained responsive to rhizobia and could support infection thread formation. This suggests that a deficiency in signal transduction is the reason for nodulation failure in the mature root. However, nodules could be initiated in the mature root at sites of lateral root emergence. A comparison between lateral root and nodule formation showed that similar patterns of GH3:gusA expression, chalcone synthase gene expression, and accumulation of a particular flavonoid compound occurred in the cortical cells involved in both processes. The results suggest that rhizobia can "hijack" cortical cells next to lateral root emergence sites because some of the early responses required for nodule formation have already been activated by the plant in those cells.