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

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18 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 Eucalyptus nitens, which rarely exhibits a G-layer, was bent to 45 o . These plants displayed a TW with high cellulose content, reduced microfibril angle, and less Klason lignin (Qiu et al., 2008). Laeta procera TW has a peculiar polylaminate secondary wall, formed by thin lignified layers with small microfibril angles alternating with layers that are still thin but exhibit greater lignification (Ruelle et al., 2007). Most of the work on TW has focused on anatomical and biochemical characterization of the wood, failing to consider the physiological and molecular mechanisms involved. These studies refer only to changes in quantity or quality (syringyl and guaiacyl) of lignin and do not explore which factors are responsible for these changes in the lignifications process, whether hormones, proteins or genes. Auxin was thought to be the main hormone involved in inducing the formation of reaction wood (Déjardin et al., 2004). For years, experiments with exogenous supplies of auxin and auxin transport inhibitors showed that TW can be induced by a deficiency of auxin (Little and Savidge, 1987). However, reaction wood (from both tension and compression) can be formed without obvious changes in the balance of auxin. This finding indicated the importance of investigating other factors, such as mechanisms of auxin perception or even other signalling molecules (Hellgren et al., 2004). Ethylene appeared as a strong candidate since it was observed that this hormone is distributed asymmetrically in the TW and that expression of a 1-aminocyclopropane- 1-carboxylate oxidase gene (PttACO1) controls the process (Andersson-Gunnerås et al., 2003). In addition, there is a connection between ethylene and lignification, as observed in the phenotype of Arabidopsis mutants, where a mutation in a chitinase gene causes the ectopic expression of CCoAOMT, ectopic deposition of lignin and the overproduction of ethylene (Zhong et al., 2002).
19 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 In addition to ethylene, gibberellins also seem to be related to the formation of reaction wood. It was observed that the application of gibberellins to the stems of some angiosperms induces the formation of wood with characteristics similar to tension wood: fibres have inner layers similar to a G-layer, with high cellulose content, small microfibril angles and low lignin content (Funada et al., 2008). In addition to studies of plant hormones, important research has characterized the proteins involved in reaction wood formation. SDS-PAGE comparisons of the pattern of protein accumulation in TW and normal wood showed the presence of at least five proteins induced during the formation of TW (Baba et al., 2000). The extraction conditions used in the protocols led the authors to suggest that these five proteins were cell wall- or membrane-bound proteins. Additionally, a study on peroxidase activity showed that the increase in the S/G ratio, generally observed in TW, could be partly attributed to the increased activity of a syringaldazine peroxidase ionically linked to the cell wall (Aoyama et al., 2001). The molecular mechanisms involved in the formation reaction wood and their relationship to metabolomics data have been investigated. However, due to the complexity of differential accumulation of lignin in TW, very little is known about the genes involved in the process. ESTs obtained from different wood tissues, among them TW and opposite wood from the poplar (Populus tremula x P. alba), indicated the presence of some genes specific to these tissues (Déjardin et al., 2004). In tissues under tension, ESTs related to arabinogalactan protein-like (AGP), fructokinase and sucrose synthase were abundant. In contrast, the opposite wood exhibited greater CCoA-OMT expression, which is consistent with the high levels of lignin synthesis observed in this wood.
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