A SIMPLE-MODEL BASED ON KNOWN PLANT DEFENSE REACTIONS IS SUFFICIENT TO EXPLAIN MOST ASPECTS OF NODULATION

We present the following hypothesis; that lipo-oligochitin Nod-factors can act in an elicitor-like fashion inducing, amongst other effects, a plant chitolytic enzyme, capable of hydrolysing the oligochitin chai n of the Nod-factor, Decorative groups on the oligochitin chain, e,g. sulphate, may confer partial resistance to hydrolysis upon particular Nod-factors, After entry into the plant, Nod-factor synthesis must be downregulated in order to avoid further, unwanted, elicitation and the consequent abortion of the symbiosis. The plant-derived compounds inh ibiting the synthesis of bacterial Nod-factors are limiting in root ti ssue, leading to residual elicitation and the abortion of infection th read formation. Nod-gene anti-induction is, furthermore, inactivated b y both light and nitrate, thus contributing to the inhibition of nodul ation under these conditions. In nitrogen-fixing nodules, the bacteroi ds are exposed to both nod-gene inducing and repressing compounds. The slow accumulation of Nod-factors within the peribacteroid space event ually results in the elicitation of phytoalexin synthesis and nodule s enescence.