THE ROLE OF PHYTOHORMONES IN PLANT-MICROBE SYMBIOSES

Plant hormones, especially auxin, cytokinin, and ethylene, have long b een implicated in nodule development. In addition, plant hormones have been shown to have increased concentrations in mycorrhizal associatio ns. We show that the early nodulin (ENOD) genes can be used as indicat ors for the status of endogenous hormones in symbiotic root tissues. T ranscripts for ENOD2 and ENOD40 genes are shown to accumulate in unino culated, cytokinin-treated alfalfa roots, even in roots of the non-nod ulating alfalfa mutant MN1008, which is unresponsive to Rhizobium meli loti inoculation and to Nod factor treatment. Transcripts for these EN OD genes also accumulate in mycorrhizal roots of alfalfa. A model desc ribing the involvement of cytokinin and auxin in stimulating cell divi sions in the inner cortex which leads to nodule formation is presented .