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 central phenylpropanoid pathway, three reactions of flavonoid biosynthesis, and the isoflavonoid branch pathway. The extractable activities of all these enzymes, and of enzymes supplying precursors from primary metabolism, 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 cytochrome P450 hydroxylases places a high demand for NADPH on elicited cells. The NADPH is most likely supplied by activation of the pentose phosphate 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 transcriptional activation of the corresponding genes, leading to increased steady state levels of translatable mRNAs. This transcriptional activation is programmed through the interaction of sets of elicitor/infection-modulated transcription factors with their cognate cis elements in the promoters of the phytoalexin biosynthetic genes. Gene activation occurs through generation of intracellular signals which lead to modulation of transcription factor activity, through either increased synthesis of the factor(s), activation via reversible post-translational modification (e.g. phosphorylation/dephosphorylation), translocation of factors from cytoplasm to nucleus, or combinations of these. Coordinated induction of the enzymes of phytoalexin synthesis may involve multiple signals and factors for transcriptional activation, as well as feedback and feed-forward fine controls at both transcriptional and post-transcriptional levels. In beneficial mycorrhizal interactions, induction of early pathway genes is uncoupled from that of later, phytoalexin-specific genes.