INVESTIGATING THE ROLES OF PHENYLPROPANOID PATHWAY IN PLANT DEFENSE AGAINST PATHOGEN ATTACK
The plant phenylpropanoid pathway is initiated from deamination of phenylalanine to form cinnamic acid followed by hydroxylation and methylation of the aromatic ring to generate a variety of phenolic compounds including lignin monomers, flavonoid compounds and sinapate esters. The incorporation of phenylpropanoid metabolism served as a key step in the early land-colonization of plants from aqueous environment since phenolic compounds play important roles in plant development and abiotic/biotic stress responses. Lignin is a heteropolymer of hydroxycinnamyl alcohols that are derived from the major branch of plant phenylpropanoid pathway. The main function of lignin is to enhance the strength of plant cell wall and waterproof the vascular system for long-distance transportation of water and solutes. In addition, lignin is also involved in protecting plants against pathogen attack. My Ph.D. research is to investigate how lignin biosynthesis contributes to plant immunity. The results showed that the expression of major lignin biosynthetic genes was induced upon host fungal pathogen infection. Moreover, a mutant disrupted in the lignin gene F5H1 showed enhanced susceptibility when challenged with several fungal pathogens. F5H1 encodes a ferulic acid 5-hydroxylase that is uniquely present in angiosperm plants, leading to the biosynthesis of syringyl lignin monomer, which is not present in gymnosperm plants. Subsequent research demonstrated that f5h1 mutation impaired the penetration (pre-invasion) resistance but did not impact post-invasion resistance. Furthermore, the pathogen-induced expression of lignin genes was independent of well-characterized defensive signaling pathways, and regulated by a novel regulating mechanism. F5H1 contributes to pmr2-mediated resistance but acts independently of other molecular components of penetration resistance including PEN1, PEN2, and PEN3. In contrast to f5h1, a knockout mutant of flavonoid pathway gene chalcone isomerase (CHI/TT5) showed enhanced resistance to host anthracnose pathogen Colletotrichum higginsianum in a salicylic acid (SA)-dependent manner. Taken together, our results for the first time provide genetic evidence demonstrating that lignin biosynthetic gene F5H1 plays critical roles in plant penetration resistance and that an uncharted pathway in flavonoid metabolism confers an SA-dependent resistance pathway in Arabidopsis.
Arabidopsis thaliana, phenylpropanoid metabolism, lignin, flavonoid compounds, plant immunity, fungal pathogens
Doctor of Philosophy (Ph.D.)