Distinct cinnamoyl CoA reductases involved in parallel routes to lignin in Medicago truncatula

Abstract

Cinnamoyl CoA reductases (CCR) convert hydroxycinnamoyl CoA esters to their corresponding cinnamyl aldehydes in monolignol biosynthesis. We identified two CCR genes in the model legume Medicago truncatula. CCR1 exhibits preference for feruloyl CoA, but CCR2 prefers caffeoyl and 4-coumaroyl CoAs, exhibits sigmoidal kinetics with these substrates, and is substrate-inhibited by feruloyl and sinapoyl CoAs. M. truncatula lines harboring transposon insertions in CCR1 exhibit drastically reduced growth and lignin content, whereas CCR2 knockouts grow normally with moderate reduction in lignin levels. CCR1 fully and CCR2 partially complement the irregular xylem gene 4 CCR mutation of Arabidopsis. The expression of caffeoyl CoA 3-O-methyltransferase (CCoAOMT) is up-regulated in CCR2 knockout lines; conversely, knockout of CCoAOMT up-regulates CCR2. These observations suggest that CCR2 is involved in a route to monolignols in Medicago whereby coniferaldehyde is formed via caffeyl aldehyde which then is 3-Omethylated by caffeic acid O-methyltransferase.

Salicylic acid mediates the reduced growth of lignin down-regulated plants

“…This phenomenon has been observed in plants with partial knockdown of enzyme activities through expression of antisense or RNAi constructs (5, 7) and is more apparent in genetic knockouts, which are often highly dwarfed (4,27,28). If these growth effects are primarily caused by altered vascular morphology and function, it will be difficult to reverseengineer low-lignin plants for improved growth.…”

Characterization of 5-Hydroxyconiferaldehyde O-Methyltransferase in Oryza sativa

“…From these experiments together with characterization of transgenic plant chemotypes in which genes encoding these enzymes were downregulated, the 5-HCAld pathway from CAld to SAlc via 5-HCAld and SAld is considered a major pathway for syringyl lignin biosynthesis (Figure 1; Li et al 2000;Umezawa 2010). In addition, caffealdehyde (CaAld) conversion to CAld has also been proposed for syringyl lignin biosynthesis in medicago (Figure 1) (family Fabaceae, Zhou et al 2010).…”

Developmental- and stress-mediated expression analysis of cinnamoyl-CoA reductase 1 (CCR1) fromHibiscus cannabinus

“…The AtCCR2 expression level is generally lower in wild type, although its higher expression was observed in ccr1 knockout lines and function of AtCCR1 was partly compensated (Mir Derikvand et al 2008). Transposon insertion in Medicago CCR1 showed highly reduced lignin contents and growth, whereas ccr2 knockouts exhibited no significant changes in growth (Zhou et al 2010). Taken together, these results suggest that only a few CCR homologs function as the actual gene, whereas the others are backups.…”

“…This diverse gene family can be classified as CCR and CCR-like genes, which differ in function and biochemical properties (Raes et al 2003;Barakat et al 2011). For example, Arabidopsis has only two real CCR genes (Zhou et al 2010): AtCCR1, which is involved in developmental lignification, and AtCCR2, which is associated with stress and elicitor response (Raes et al 2003). The ccr1 knockout mutants displayed altered developmental program, dwarf phenotype and reduced lignin contents in Arabidopsis (Mir Derikvand et al 2008).…”

“…Lignin biosynthesis primarily consists of two major steps, monolignol biosynthesis and cross-linking of the monolignols by peroxidases and laccases (Xu et al 2009). Cinnamoyl-CoA reductase (CCR) is an important enzyme for lignin biosynthesis that catalyzes the first committed step in monolignol biosynthesis (Zhou et al 2010). Hence, it can be a very good target for genetically modified plants with reduced lignin contents.…”