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During symbiosis between leguminous plants and rhizobia, flavonoids exuded by the plants act as chemoattractants and nodulation (nod) gene regulators in the other partner. To better understand the role of these compounds during the early steps of the alfalfa-Sinorhizobium meliloti symbiosis and the regulation of their production we have isolated nod gene inducers from alfalfa roots. All the compounds that we identified in this study as nod gene inducers in the root are flavonoids, indicating that other compounds with nod gene activator capacity may have little contribution, if any, to nod gene activation. Most of the intermediates of the flavonoid pathway were found in Medicago sativa roots and nodules, but only end products of the flavonoid pathway were identified in the root exudate. We have also studied flavonoid production in different parts of the root and found that it is developmentally regulated during root growth. Finally, we have shown that coumestrol and medicarpin, present in the exudates and previously described as phytoalexins, possess nod gene repressing activity, indicating that the in vivo nod gene inducing activity of the root exudate results from positive as well as negative controls of nod gene expression by the flavonoids.

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A compound, which accumulates when trypt-IN vivo feeding experiments with labelled potential preamine (I) and secologanin (11) are incubated with an cursors have led to the formulation of generally accepted enzyme preparation from Catharanthus roseus cell suspen-schemes for the biosynthesis of indole a1kaloids.l *2 The sion cultures, was identified as 20,2 1-didehydroaj malicine limitations of in vivo experiments were recently overcome (cathenamine) (IVa), and was shown to be a central when it was shown for the first time that complex alkaloids, intermediate in the enzymatic production of ajmalicine such as geissoschizine and ajmalicine (Va), were synthesized (Va) ,19-epi-aj malicine (Vb) and tetrahydroalstonine (Vc) . by crude cell-free preparations of Catharanthus roseus