RNAi‐suppression of barley caffeic acidO‐methyltransferase modifies lignin despite redundancy in the gene family

Abstract

Caffeic acid O-methyltransferase (COMT), the lignin biosynthesis gene modified in many brown-midrib high-digestibility mutants of maize and sorghum, was targeted for downregulation in the small grain temperate cereal, barley (Hordeum vulgare), to improve straw properties. Phylogenetic and expression analyses identified the barley COMT orthologue(s) expressed in stems, defining a larger gene family than in brachypodium or rice with three COMT genes expressed in lignifying tissues. RNAi significantly reduced stem COMT protein and enzyme activity, and modestly reduced stem lignin content while dramatically changing lignin structure. Lignin syringyl-to-guaiacyl ratio was reduced by ~50%, the 5-hydroxyguaiacyl (5-OH-G) unit incorporated into lignin at 10--15-fold higher levels than normal, and the amount of p-coumaric acid ester-linked to cell walls was reduced by ~50%. No brown-midrib phenotype was observed in any RNAi line despite significant COMT suppression and altered lignin. The novel COMT gene family structure in barley highlights the dynamic nature of grass genomes. Redundancy in barley COMTs may explain the absence of brown-midrib mutants in barley and wheat. The barley COMT RNAi lines nevertheless have the potential to be exploited for bioenergy applications and as animal feed.

Lignin engineering to improve saccharification and digestibility in grasses

“…Together, these demonstrate changes in COMT-suppressed transgenics consistent with those in bm3 plants, i.e., moderate reductions to lignin (6-14%), with more dramatic reductions in S units (20-46%) accompanied by a decrease in S/G. Some reports record reduced levels of ester-linked p-coumarate in lignin with normal levels of ferulate [37,40] and/or the appearance of 'signature' 5-OH-G units [35,37]. Growth is generally normal although field-grown sugarcane had normal biomass at 6% lignin reduction, but decreased biomass at 8-12% lignin reduction [40].…”

“…Maize and rice have been produced with manipulated CCoAOMT or F5H, but digestibility was not reported [33,34]. By comparison, COMT, the gene underlying improved digestibility in bm3 mutants, has been an obvious target for transgenic manipulation in biomass and forage grasses and cereal crops [e.g., [35][36][37][38] with several evaluations progressing to the field [38][39][40][41][42]. Together, these demonstrate changes in COMT-suppressed transgenics consistent with those in bm3 plants, i.e., moderate reductions to lignin (6-14%), with more dramatic reductions in S units (20-46%) accompanied by a decrease in S/G.…”

“…However, a maize mutant in chalcone synthase C2 (acting at the entry point to the flavonoid pathway) with dramatically reduced tricin had increased leaf lignin due to redirection of flux from the blocked flavonoid pathway into lignin, reducing leaf saccharification efficiency [48]. COMT is involved in later steps in tricin biosynthesis and its suppression led to reduced tricin [37,49,50]; conversely suppression of C3H in rice increased tricin [32], yet both types of manipulation improved saccharification. A rice mutant in the FNSII gene specific to the flavonoid pathway and devoid of tricin had enhanced saccharification efficiency, but also unexpectedly showed reduced lignin S/G, as well as incorporating the intermediate naringenin in place of tricin [51].…”

Association mapping identifies quantitative trait loci (QTL) for digestibility in rice straw

“…Alterations in cell wall components can affect the recalcitrance of lignocellulosic biomass, and thus improve its saccharification with the potential to improve energy crops through plant breeding [9][10][11]. While reducing lignin can decrease recalcitrance in grasses [12], several publications also indicate that alterations in hydroxycinnamic esters can have a significant effect on recalcitrance [13]. In rice and Brachypodium, decreased levels of ferulic acid accompany increases in lignocellulose digestibility [14][15][16] Recently, important advances that lay the foundations for engineering or breeding plants for biofuel production have been made.…”

Overexpression of TaCOMT Improves Melatonin Production and Enhances Drought Tolerance in Transgenic Arabidopsis

“…Melatonin plays important roles in regulating plant growth and development and enhancing the resistance of plants against biotic and abiotic stresses [9,10,11,12]. For example, melatonin treatment significantly enhances the drought tolerance of wheat seedlings with the decreasing of membrane damage, increasing photosynthetic rate, maintaining intact grana lamella of chloroplast, and increasing water holding capacity [13].…”