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

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30 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 Concluding Remarks Figure 1 shows the main route of lignin biosynthesis in plants and the enzymes involved in each metabolic step (adapted from Rastogi and Dwivedi, 2008). For most of the genes coding for these enzymes, transgenic plants have been produced using genesilencing techniques (Anterola and Lewis, 2002). The aim of these works was to reduce lignin content or to change the structure of lignin, goals which have economical and environmental implications: cheaper and more process-amenable trees for pulp and paper manufacture with less pollution, more readily digestible forage for livestock and improved feedstock for fuel/chemical production (Anterola and Lewis, 2002). The potential applications of changing or silencing the expression of these genes were comprehensively discussed by Anterola and Lewis (2002). Some questions discussed by these authors were, “why do vascular plants consistently produce various types of cell walls with lignins from either two or three monolignols, and how does this occur in vivo? Why are these moieties also differentially deposited into specific regions of developing cell wall types?” Despite the complexity of the lignin biosynthesis network, Anterola and Lewis (2002) used accumulated knowledge obtained with mutants or transgenic plants (in which levels of particular enzymes of the pathway have been modified in some way) to show that monolignol biosynthesis is under fine metabolic and transcriptional coordination. For several of these genes, repressed expression led to diverse and undesirable impacts on plant physiology due to the multiple products that originate from the phenylpropanoid pathway. Therefore, the PAL, C4H, C3H and 4CL genes (see Figure 1) are not good targets for genetic manipulation aiming to reduce lignin, while CCoAOMT,
31 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 COMT and CAD genes can be used to reduce and change lignin composition (Rastogi and Dwivedi, 2008). The biosynthesis of monolignols in phenylpropanoid metabolism is a metabolic grid; that is, different routes may be used to synthesize a compound. This gives plants the plasticity to produce lignin when a step is inhibited for whatever reason such as when a specific gene is silenced. In addition, enzymes related to lignin biosynthesis usually derive from multigene families; depending on the stressing situation (biotic or abiotic), different genes for different isozymes can be activated. For the sake of complexity, it is also well known that the relative proportions of the monolignols may vary due to environmental influence (Boudet, 1998). It has been suggested that plants are versatile in their ability to tolerate substantial changes in lignin structure and incorporate phenolic compounds other than the three known monolignols (Ralph et al., 2004). Recently, it was shown that acylated monolignols could also serve as lignin precursors (Lu and Ralph, 2003). This can make lignin more recalcitrant to disruption during pulping in the paper industry, for example (Lapierre et al., 1999). However, it does not appear that “non-traditional” phenolics, other than the monolignols, are incorporated into lignin (Anterola and Lewis, 2002). There are several examples indicating that lignin found in nature may comprise modified molecules (Lu et al., 2004; Morreel et al., 2004). Phenolic amides may also be part of the cell wall, as proven by over-expression of the enzyme responsible for their biosynthesis, which caused an increase in their incorporation into the cell wall matrix (Hagel and Facchini, 2005). Recent research also showed that acetylation of syringyl units seems to be an exclusive characteristic of angiosperms, as the process was observed among 11 angiosperm species but not in 2 gymnosperm species (Del Rio et al., 2007).
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Abiotic and biotic stresses and changes in the lignin ... - ResearchGate

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