METABOLIC ENGINEERING - PROSPECTS FOR CROP IMPROVEMENT THROUGH THE GENETIC MANIPULATION OF PHENYLPROPANOID BIOSYNTHESIS AND DEFENSE RESPONSES - A REVIEW

In leguminous plants such as the forage legume alfalfa, products of th e phenylpropanoid pathway of secondary metabolism are involved in inte ractions with beneficial microorganisms (flavonoid inducers of the Rhi zobium symbiosis), and in defense against pathogens (isoflavonoid phyt oalexins). In addition, the phenylpropane polymer lignin is a major st ructural component of secondary vascular tissue and fibers in higher p lants. The recent isolation of genes encoding key enzymes of the vario us phenylpropanoid branch pathways opens up the possibility of enginee ring important crop plants such as alfalfa for: (a) improved forage di gestibility, by modification of lignin composition and/or content; (b) increased or broader-spectrum disease resistance, by introducing nove l phytoalexins or structural variants of the naturally occurring phyto alexins, or by modifying expression of transcriptional regulators of p hytoalexin pathways; and (c) enhanced nodulation efficiency, by engine ering over-production of flavonoid nod gene inducers. The basic bioche mistry and molecular biology underlying these strategies is briefly re viewed, and recent progress with transgenic plants summarized. The pot ential importance of metabolic compartmentation for attempts to engine er phenylpropanoid biosynthetic pathways is also discussed. Over-expre ssion of an alfalfa glucanase-encoding gene confers significant protec tion against Phytophthora in alfalfa, possibly via indirect effects on phenylpropanoid metabolism.