THE ISOFLAVONOID PHYTOALEXIN PATHWAY - FROM ENZYMES TO GENES TO TRANSCRIPTION FACTORS

The pterocarpan phytoalexins of the Leguminosae are synthesized from L -phenylalanine via a minimum of 11 enzymatic steps involving the centr al phenylpropanoid pathway, three reactions of flavonoid biosynthesis, and the isoflavonoid branch pathway. The extractable activities of al l these enzymes, and of enzymes supplying precursors from primary meta bolism, increase in response to fungal infection or exposure of plant cells to elicitor macromolecules isolated from the cell walls of yeast or plant pathogenic fungi. The involvement of reductases and cytochro me P450 hydroxylases places a high demand for NADPH on elicited cells. The NADPH is most likely supplied by activation of the pentose phosph ate pathway. Genes or cDNAs encoding 7 of the enzymes involved in the synthesis of the phytoalexin medicarpin have been cloned from alfalfa and/or other species. Induction of enzyme activity results from transc riptional activation of the corresponding genes, leading to increased steady state levels of translatable mRNAs. This transcriptional activa tion is programmed through the interaction of sets of elicitor/infecti on-modulated transcription factors with their cognate cis elements in the promoters of the phytoalexin biosynthetic genes. Gene activation o ccurs through generation of intracellular signals which lead to modula tion of transcription factor activity, through either increased synthe sis of the factor(s), activation via reversible post-translational mod ification (e.g. phosphorylation/dephosphorylation), translocation of f actors from cytoplasm to nucleus, or combinations of these. Coordinate d induction of the enzymes of phytoalexin synthesis may involve multip le signals and factors for transcriptional activation, as well as feed back and feed-forward fine controls at both transcriptional and post-t ranscriptional levels. In beneficial mycorrhizal interactions, inducti on of early pathway genes is uncoupled from that of later, phytoalexin -specific genes.