1995 and Beyond

Abstract

We present the following hypothesis; that lipo­ oligochitin Nod-factors can act in an elicitor-Iike fash­ ion inducing, amongst other effects, a plant chitolytic enzyme, capable of hydrolysing the oligochitin chain 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 down­ regulated in order to avoid further, unwanted, ell· citation and the consequent abortion of the symbiosis. The plant-derived compounds Inhibiting the synthesis of bacterial Nod-factors are limiting in root tissue, leading to residual elicitation and the abortion of infec­ tion thread formation. Nod-gene anti-induction is, fur­ thermore, inactivated by both light and nitrate, thus contributing to the inhibition of nodulation under these conditions. In nitrogen-fixing nodules, the bacteroids are exposed to both nOd-gene inducing and repressing compounds. The slow accumulation of Nod-factors within the peribacteroid space eventually results in the elicitation of phytoalexin synthesis and nodule