A mechanistic link between legume-rhizobium symbiosis and plant immunity
K. TOTH (1), G. Stacey (1) (1) University of Missouri, U.S.A.

The legume-rhizobium symbiosis provides around 200 million tons of nitrogen per year. In order to better understand the biology of this mutualistic interaction, we conducted a phosphoproteomic study on soybean roots and root hairs in response to the symbiont Bradyrhizobium japonicum. Among the proteins phosphorylated rapidly upon inoculation was a protein previously characterized as playing a key, functional role in plant innate immunity. RNAi-mediated silencing of the expression of this protein resulted in a significant reduction in nodule formation. In order to further study the specific role of phosphorylation, we introduced phosphomimetic or phospho-minus point mutations into the gene and tested the resulting phenotype after constitutive expression in soybean transgenic roots. In the case of one, specific phosphorylation site, the expression of the phosphomimic version resulted in significantly fewer nodules, while the expression of the phospho-minus version had no measurable response on nodule numbers. Sequence comparison of an additional phosphorylation site suggested that it lies within a legume specific region, not found in the same protein from non-leguminous plants. Such a location suggests a legume-specific function, perhaps nodulation. Indeed, when a phospho-minus version of this phosphorylation site was introduced into soybean, the transgenic roots produced significantly fewer nodules, suggesting that the site might be required for symbiotic signaling. Additional data suggest that one or more of the phosphorylation sites is involved in the very early stages of nodulation.

Abstract Number: P1-3
Session Type: Poster