Abiotic and biotic stresses and changes in the lignin ... - ResearchGate

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The increased expression of fasciclin-like arabinogalactan proteins was also
observed in G-fibres in another poplar hybrid (Populus tremula x P. tremuloides)
(Andersson-Gunneras et al., 2006). In addition, several genes involved in lignin
biosynthesis exhibited reduced expression in the TW: PttPAL1, PttCCoAOMT1, 4CL,
HCT, C3H, CCR, F5H, COMT and CAD. A MYB transcription factor (PttMYB21a)
previously shown to repress biosynthesis of lignin displayed elevated expression levels.
Branches of artificially inclined Eucalyptus globulus showed that the expression
of a cellulose synthase gene (EgCesA) was correlated with the appearance of the G-
layer (Paux et al., 2005). The lignin biosynthesis genes for which expression changed
during TW formation included: 4CL, CCR, CAD, COMT, and CCoAOMT. The response
of these genes was varied and complex, with different patterns of expression for each
gene. The 4CL, CCR and CAD genes exhibited reduced expression during the first 6 h
of stress but the transcript levels were similar to those observed in control plants after a
week. According to the authors, this variation could not explain the reduced amount of
lignin often observed in tension wood; the lower amount of lignin, mainly G-type, was
probably due to the differential expression of CCoAOMT and COMT, genes that encode
methylation enzymes.
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Compression wood: Compression wood (CW) has small and rounded tracheids with
thicker cell walls, higher lignin content, less cellulose, large microfibril angles, and
typically lacks the S3 layer (McDougall, 2000; Donaldson et al., 2004; Yeh et al., 2005;
Yeh et al., 2006). Additionally, the lignin in this wood has high levels of p-
hydroxyphenylpropane units (Monties, 1989) and alterations in the chemical links
between lignin units, when compared with normal wood (Nimz et al., 1981).