Lignin and Its Pathway-Associated Phytoalexins Modulate Plant Defense against Fungi

Abstract

Fungi infections cause approximately 60-70% yield loss through diseases such as rice blast, powdery mildew, Fusarium rot, downy mildew, etc. Plants naturally respond to these infections by eliciting an array of protective metabolites to confer physical or chemical protection. Among plant metabolites, lignin, a phenolic compound, thickens the middle lamella and the secondary cell walls of plants to curtail fungi infection. The biosynthesis of monolignols (lignin monomers) is regulated by genes whose transcript abundance significantly improves plant defense against fungi. The catalytic activities of lignin biosynthetic enzymes also contribute to the accumulation of other defense compounds. Recent advances focus on modifying the lignin pathway to enhance plant growth and defense against pathogens. This review presents an overview of monolignol regulatory genes and their contributions to fungi immunity, as reported over the last five years. This review expands the frontiers in lignin pathway engineering to enhance plant defense.

Keywords: defense metabolites; lignin; pathway enzymes; plant immunity.

Figure 1

Monolignols biosynthesis and polymerization. The various enzymes leading to monolignol formation are based on current understanding: traditional monolignols (black) and recently discovered monolignols in some plant species (blue). Stage 1: Phenylalanine escapes from the chorismate pathway in the plastid into the cytosol. Stage 2: Enzymatic activities that occur prior to monolignol formation. Stage 3: Monolignols are transported into the apoplast. Stage 4: PRX/LAC encodes monolignol polymerization into lignin. Lignin fills up intercellular voids to enhance cell wall rigidity. Proposed mechanism of monolignol transport: (a) ABC transporters mediate active trafficking of monolignols. (b) Trans-membrane diffusion of monolignols/channels-facilitated membrane transport. (c) ABC transporters channel monolignol glycoside into vacuoles for release at cell death.

Figure 2

Defense metabolites associated with the lignin pathway and encoded by the pathway enzymes Defense metabolites are illustrated in orange, highlighting their biosynthesis routes in the pathway. The distribution of the metabolites is based on the current knowledge of their biosynthesis. Steps 1, 2, 3, and 4 are the same as in Figure 1. The proposed monolignol transport mechanisms (a–c) are also the same as in Figure 1.

Figure 3

Non-lignin defense metabolites associated with the lignin biosynthetic pathway. This Figure was created using ChemDraw Professional, version 20.0.41, and the structures were analyzed and confirmed using https://pubchem.ncbi.nlm.nih.gov/ structure inquiry (accessed on 16 December 2022).