Mechanistic insights on lignin-mediated plant defense against pathogen infection

Lignin, a pivotal constituent of plant secondary cell walls, plays a versatile role as a structural and biochemical defense compound against biotic stress. This phenolic biopolymer is integral in defending plants and neutralizing invading pathogens, including necrotrophs, biotrophs, and hemibiotrophs. Lignin monomer units vary from plant to plant and it has been observed that this variability plays a crucial role in the resistance and susceptibility of a particular plant type. The complex network and cross-linking of lignin units have been observed to disintegrate under the influence of a fungal or bacterial pathogen attack. The quantitative variation of lignin in various plant parts has also been correlated with pathogen colonization. The secondary metabolites, specifically phytoalexins that are offshoots of lignin biosynthetic pathways such as coumarins, stilbenes, resveratrol, caffeic acid, and chlorogenic acid, have been demonstrated to play a crucial role in restricting several plant pathogens. Strategies are being developed to induce defense mechanisms involving endophytes, melatonin phytohormone, and chitosan, which can play a role in enhancing lignin biosynthesis and deposition under pathogen attack. Transcriptomic and proteomic studies have elucidated the functions of lignin in plant resistance, offering insights into potential future strategies for enhanced resistance to biotic stresses. In this review, we have highlighted the role of lignin biosynthesis and associated metabolites in the alleviation of a diverse range of pathogens. Emphasis has been given to the discussion of regulatory networks and environmental stimulants that alter lignin biosynthesis, which can lead to sustainable management approaches for crop protection.