Molecular genetic analysis of subterranean clover-microbe interactions

Abstract

Trifolium subterraneum (subterranean clover) is of considerable economic importance to the Australian rural industries as a pasture legume. In addition to its commercial value, it has a number of specific attributes-such as small seed size, diploidy, self-fertilization, the ability to be transformed and small genome-which make it a prime target for the modern techniques of molecular genetics. We report genetic and physiological factors that control the production and excretion of the lipooligosaccharide molecules formed by Rhizobium leguminosarum bv. trifolii in the formation of the symbiosis with subterranean clover. These molecules, synthesized by the products of the nodulation (nod) genes, are a major determinant of nodule occupancy and the strain selection imposed by the host plant. In addition, we have investigated which plant genes and proteins are activated in subterranean clover when they are either physically wounded, infected with Rhizobium, or attacked by red-legged earth mites. To analyse these interactions more precisely, we have cloned plant genes involved in the phenylpropanoid pathway and used their promoters to construct transgenic subterranean clover plants. Our studies provide an insight into the nature and consequences of the chemical exchange between plants and invading microbes.