Understanding the Origins of Nodulation by Studying Rhizobia-Root Interactions in Basal Legumes

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The nitrogen-fixing mutualism between rhizobia and legumes is one of the best-studied interactions between plants and bacteria. Two major groups of rhizobia, 1) alpha-proteobacteria, i.e. the family Rhizobiaceae (alpha-rhizobia), and 2) certain beta-proteobacteria, Burkholderia and Cupriavidus (beta-rhizobia), inhabit legume root nodules, converting inert atmospheric nitrogen into ammonia, which is assimilated by plants into proteins and nucleic acids. Surprisingly, little is known about the interactions with beta-rhizobia, especially for the Caesalpinioid legumes, the most basal of the three sub-classes of the legume family. Although more than 80% of Mimosoid and Papilionoid legumes nodulate, less than 30% of the Caesalpinioid species form nodules. Thus, they represent a transition from non-nodulating to nodulating legumes and can help tease out the evolutionary origins of nodule formation. This project will investigate two Caesalpinioid legumes: 1) Gleditsia triacanthos, in which bacteria enter roots and fix nitrogen without nodule formation, a mutualism that probably predates the evolution of nodules, and 2) Chamaecrista fasciculata, one of the few caesalpinioid legumes that nodulates. Experiments will be carried out to determine how the bacteria establish these associations with basal legumes and identify orthologs of genes known to be critical for nodule formation in the advanced legumes. After identifying the gene orthologs, they will be mutated to test their function. A hairy root transformation system for implementing RNAi mutagenesis has been established with the help of an undergraduate student, and is a major advance for studying gene function in basal legumes. The project will provide other opportunities for student training. Caesalpinioid legumes comprise the major flora of the subtropics and tropics, serving as important food/forestry crops and replenishing nitrogen to N-limited soils. Studying these basal legumes will bring us closer not only to learning more about the genes responsible for nodulation, but also to educating students about the value of basal plants to the nitrogen economy of subtropical and tropical soils; these are threatened because of increasing human populations and climate change. Soil degradation could lead to the extinction of these environmentally important legumes.