The role of phytohormones in plant-microbe symbioses

Plant hormones, especially auxin, cytokinin, and ethylene, have long been implicated in nodule development. In addition, plant hormones have been shown to have increased concentrations in mycorrhizal associations. We show that the early nodulin (ENOD) genes can be used as indicators for the status of endogenous hormones in symbiotic root tissues. Transcripts for ENOD2 and ENOD40 genes are shown to accumulate in uninoculated, cytokinin-treated alfalfa roots, even in roots of the non-nodulating alfalfa mutant MN1008, which is unresponsive to Rhizobium meliloti inoculation and to Nod factor treatment. Transcripts for these ENOD genes also accumulate in mycorrhizal roots of alfalfa. A model describing the involvement of cytokinin and auxin in stimulating cell divisions in the inner cortex which leads to nodule formation is presented.