Expression of the legume symbiotic lectin genes psl and gs52 promotes rhizobial colonization of roots in rice

Transgenic rice ( Oryza sativa L. cv. Murasaki) carrying genes encoding pea ( Pisum sativum) lectin (PSL) or wild-soybean ( Glycine soja) lectin-nucleotide phosphohydrolase (GS52) were inoculated with Rhizobium leguminosarum bv. v iciae or Bradyrhizobium japonicum USDA110, respectively, as well as with Rhizobium sp. NGR234, and root colonization was assessed in comparison to comparably inoculated control plants. The data showed that expression of PSL and GS52 significantly promoted rhizobial colonization of root epidermal cells including root hairs in rice. In addition, in the case of R. leguminosarum bv. v iciae and B. japonicum USDA110 colonization of the psl and gs52 transgenic rice plants, respectively, the bacterial cells were found to preferentially home towards and aggregate maximally at the root hair tip regions rather than on the root hair “stalks”. The above data suggest that the lectins PSL and GS52, which participate in rhizobial recognition by root epidermal cells in pea and soybean, respectively, are also able to facilitate rhizobial attachment and colonization of the epidermal cells in rice roots. Moreover, aggregation of R. leguminosarum bv. v iciae and B. japonicum USDA110 cells preferentially at root hair tip regions suggest that similar to legumes, the PSL and GS52 lectins are targeted to the root hair tips in transgenic rice, enabling higher bacterial attachment/colonization at the tip region. Rhizobial colonization at root hair tips in the psl and gs52 rice plants frequently led to the localized dissolution of the cell wall creating perforations at the tip region. It is likely that the presence of lectins, such as PSL and GS52 leads to structural modifications in cell wall organization of the root hair/epidermal cells, making them prone to localized dissolution by the hydrolytic activity of compatible rhizobia to permit invasion of the root cells.