Fig. 1

Monolignol and ethylene biosynthesis pathways. Molecules and enzymes of the pathways are indicated. Met methionine; SAM S-adenosylmethionine; ACC 1-aminocyclopropane-1-carboxylic acid; ACS ACC-synthase, ACO ACC-oxidase; Phe phenylalanine, PAL phenylalanine ammonia lyase; C4H cinnamate-4-hydroxylase; C3H coumarate 3-hydroxylase; HCT hydroxycinnamoyl transferase; 4CL 4-coumarate ligase; CAD cinnamyl alcohol dehydrogenase; CSE caffeoyl shikimate esterase; CCoAOMT caffeoyl-CoA 3-O-methyltransferase; CCR cinnamoyl CoA reductase; COMT caffeate O-methyltransferase; F5H ferulate 5-hydroxylase. Ethylene influences the activity of PAL (indicated as a lightning bold) and thereby affects monolignol biosynthesis. The enzyme CSE together with 4CL bypasses the second HCT reaction and appears to be important in the lignin biosynthetic pathway of M. truncatula [28]