| Members of the legume family can enter into symbiotic interactions with soil bacteria of the genus Rhizobium, characterized by the formation of nitrogen-fixing root nodules. Bacteria invade the root and are released into cells of the developing root nodule where they differentiate into nitrogen fixing bacteroids. The root nodule is a specialized organ which provides the bacteria with a suitable environment in which to reduce atmospheric dinitrogen to ammonia, and is the site where reduced nitrogen is absorbed by the plant. Initiation of the symbiosis between the free living plant and bacteria partners occurs by reciprocal exchange of signaling molecules. The plant root exudes flavonoids into the soil that are perceived by symbiotically compatible bacteria and induce the transcription of a suite of genes, termed Nod genes. The gene products of many Nod genes are enzymes which catalyze steps in the biosynthesis of a class of signaling molecules, termed Nod factors. Nod factor is both necessary and sufficient for nodule organogenesis, and is required, but not sufficient, for infection of the root by Rhizobium . Nod factors trigger a number of cellular responses in root hairs of compatible legume hosts, which include periodic, transient increases in cytosolic calcium levels, termed calcium spiking. I screened 11 pharmaceutical modulators of eukaryotic signal transduction for effects on Nod factor-induced calcium spiking. Cyclopiazonic acid inhibits Nod factor-induced calcium spiking, implicating the requirement for a type IIA calcium ATPase in Nod factor signal transduction. Complete and partial gene sequences of ten calcium ATPase genes from the model legume Medicago truncatula are determined as the initial stage in identification of the specific calcium ATPase gene(s) required for Nod factor signal transduction. The phytohormones cytokinin and ethylene have been implicated as regulators of nodulation. I isolated an M. truncatula mutant, sickle1-2, that is insensitive to ethylene and exhibits a supernodulation phenotype, demonstrating that ethylene is an endogenous regulator of nodulation. I also demonstrate that treatment of M. truncatula roots with medium containing Nod factor elicits an increase in transcript levels of the ACC Synthase1 gene, which encodes a putative ethylene biosynthesis enzyme. |
