SYNTHESIS, RELEASE, AND TRANSMISSION OF ALFALFA SIGNALS TO RHIZOBIAL SYMBIONTS

In addition to the flavonoids exuded by many legumes as signals to their rhizobial symbionts, alfalfa (Medicngo sativa L.) releases two betaines, trigonelline and stachydrine, that induce nodulation (nod) genes in Rhizobium meliloti. Experiments with C-14-phenylalanine in the presence and absence of phenylalanine ammonia-lyase inhibitors show that exudation of flavonoid nod-gene inducers from alfalfa roots is linked closely to their concurrent synthesis. In contrast, flavonoid and betaine nod-gene inducers are already present on mature seeds before they are released during germination. Alfalfa seeds and roots release structurally different nod-gene-inducing signals in the absence of rhizobia. When R. meliloti is added to roots, medicarpin, a classical isoflavonoid phytoalexin normally elicited by pathogens, and a nod-gene-inducing compound, formononetin-7-O-(6''-O-malonylglycoside), are exuded. Carbon flow through the phenylpropanoid pathway and into the flavonoid pathway via chalcone synthase is controlled by complex cis-acting sequences and trans-acting factors which are not completely understood. Even less information is available on molecular regulation of the two other biosynthetic pathways that produce trigonelline and stachydrine. Presumably the three separate pathways for producing nod-gene inducers in some way protect the plant against fluctuations in the production or transmission of the two classes of signals. Factors influencing transmission of alfalfa nod-gene inducers through soil are poorly defined, but solubility differences between hydrophobic flavonoids and hydrophilic betaines suggest that the diffusional traits of these molecules are not similar. Knowledge derived from studies of how legumes regulate rhizobial symbionts with natural plant products offers a basis for defining new fundamental concepts of rhizosphere ecology.