Transgenic Approaches to Study Nodulation in the Model Legume, Lotus japonicus

The soybean apyrase, GS52, characterized as an early nodulin, was further investigated for its possible role in nodulation. GS52 is expressed in roots and localized to the plasma membrane. In addition, it is rapidly induced upon rhizobial inoculation. Treatment of soybean roots with anti-GS52 antibodies blocked nodulation by Bradyrhizobium japonicum. Transgenic Lotus japonicus plants were generated expressing gs52 and showed enhanced nodulation and infection thread formation upon inoculation with Mesorhizobium loti that correlated with expression of the transgene. Surprisingly, expression of GS52 allowed L. japonicus plants to be infected but not nodulated by B. japonicum, the natural symbiont of soybean. The data presented supports a critical role for the GS52 apyrase in nodulation and control of infection host specificity. Similarly, the role of the plant defense response in nodulation was investigated via generation of transgenic plants. Salicylic acid (SA) is a central molecule in the plant defense response and plants that express salicylate hydroxylase, an enzyme that degrades SA to catechol, are deficient in SA cannot mount the defense response. NahG L. japonicus plants, expressing salicylate hydroxylase, were generated and characterized for their nodulation phenotype. NahG plants demonstrated enhanced nodulation and infection thread formation when inoculated with M. loti. However, other phenotypes were observed, such as increased root growth, that complicated interpretation of the results.