Modifying lignin to improve bioenergy feedstocks: strengthening the barrier against pathogens?

Abstract

Lignin is a ubiquitous polymer present in cell walls of all vascular plants, where it rigidifies and strengthens the cell wall structure through covalent cross-linkages to cell wall polysaccharides. The presence of lignin makes the cell wall recalcitrant to conversion into fermentable sugars for bioenergy uses. Therefore, reducing lignin content and modifying its linkages have become major targets for bioenergy feedstock development through either biotechnology or traditional plant breeding. In addition, lignin synthesis has long been implicated as an important plant defense mechanism against pathogens, because lignin synthesis is often induced at the site of pathogen attack. This article explores the impact of lignin modifications on the susceptibility of a range of plant species to their associated pathogens, and the implications for development of feedstocks for the second-generation biofuels industry. Surprisingly, there are some instances where plants modified in lignin synthesis may display increased resistance to associated pathogens, which is explored in this article.

Keywords: CAD; COMT; brown midrib; lignin; monolignol pathway; plant pathogens.

FIGURE 1

A model for monolignol pathway. In phenylpropanoid metabolism, there are 10 enzymatic steps (green) leading to hydroxycinnamyl alcohols which are polymerized into lignin, namely; phenylalanine ammonia lyase (PAL), cinnamate 4-hydroxylase (C4H), 4-coumarate-CoA ligase (4CL), hydroxycinnamoyl CoA:shikimate transferase (HCT), p-coumarate 3-hydroxylase (C’3H), caffeoyl CoA O-methyltransferase (CCoAOMT), cinnamyl CoA reductase (CRR), ferulate 5-hydroxylase (F5H), caffeic acid O-methyltransferase (COMT), and cinnamyl alcohol dehydrogenase (CAD).