The role of Sinorhizobium meliloti Type IVb pili protein on legume nodule morphology and infection

The symbiotic relationship between legumes and rhizobial soil bacteria leads to the conversion of nitrogen gas to biologically available forms of nitrogen. A critical step in this interaction is bacterial attachment to the host root. A variety of factors are involved in attachment, such as the Type IV pili (Tfp). In pathogens, Tfpb mediates tight adherence to host cells, promoting colonization, however in symbiotic interactions, the role of Tfpb–encoded by pilA genes–is not well understood. To assess the role of Tfpb pili during symbiosis, Melilotus alba plants were inoculated with wild type Sinorhizobium meliloti or ΔpilA deletion mutants; roots were harvested at different stages of development. Plants inoculated with ΔpilA mutants showed abnormalities in root nodule development, including patchy infection patterns, irregularly shaped nodules, and clusters of partially infected or uninfected nodules. However, when evaluating plant growth overall, the plants infected with ΔpilA mutants looked comparable, if not healthier than the wild type controls, indicating that the nodule abnormalities caused by the pilA deficiency might be beneficial; the nodule clusters might ultimately accommodate more bacteria to fix nitrogen. Expression of genes associated with plant defense and symbiosis was also evaluated. Preliminary results indicate that plants inoculated with ΔpilA mutants have reduced expression of DNF2, a gene that is necessary for the development and maintenance of nodules.