Comparison of the consequences on lignin content and structure of COMT and CAD downregulation in poplar and Arabidopsis thaliana

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English. Lignification, a major biological event in the plant kingdom, is still poorly understood due to the structural complexity and to the high variability of native lignins. Poplars underexpressing two enzymes of the monolignol biosynthetic pathway (caffeic acid/5 hydroxyferulic O-methyltransferase or cinnamyl alcohol dehydrogenase) or Arabidopsis thaliana knock-out T-DNA insertion mutants in the genes encoding these enzymes have been obtained and analysed for the quantity and quality of their lignins. COMT or CAD down-regulation induced in poplar, a woody plant, and in Arabidopsis, the annual crucifer model plant, show similar specific alterations in lignin structure due to the incorporation of unusual phenolics in the polymer. The specific structural traits of lignins, which account for higher (CAD deficiency) or lower (COMT deficiency) susceptibility to alkaline delignification processes have been delineated. Lignins of COMT depressed plants essentialy comprise guaiacyl (G) units together with substantial amounts of 5-hydroxyguaiacyl (5-OH-G) units. They display a markedly higher frequency of resistant interunit bonds including new structures (benzodioxanes) and a lower frequency of free phenolic lignin units. These changes make the lignin polymer less amenable ta alkaline delignification. Lignins of CAD deficient plants display a higher content of free phenolic groups, and incorporation of sinapaldehyde units, both associated with a greater solubility in dilute alkali at room temperature. The lignin characteristics of the CAD antisense poplars allow a reduction in the amount of alkali required for paper pulp making using the Kraft procedure.