There are compelling and long-standing reasons to identify novel ways to more effectively either utilize the lignin biopolymers or manipulate the amounts or forms of carbon allocated to the lignin-forming pathway, for example, to produce more desirable bioproducts in commercially cultivated plant species. Indeed, a number of biotechnological manipulations of both lignin contents and compositions in various plant species have already been carried out; that is, various transgenic/mutant lines have been successfully obtained using standard transformation procedures (see Anterola and Lewis, 2002, for examples and references therein). Generally, though, the effects of drastically reducing lignin contents in both woody and nonwoody vascular plants result in a significant impairment/weakening of the vascular apparatus, for example, collapsed vessels (for a discussion and examples, see Anterola and Lewis, 2002). Such defects potentially lead to severe drawbacks in growing biotechnologically modified plant lines commercially, as this can lead to, for example, premature lodging, weakening of plant stems, and dwarfing during growth/development.
To put the utility of employing mutant and/or genetically modified lines into sharper focus, the following examples should illustrate why this issue deserves attention. It is often overlooked that many lignin mutants have been described over a period spanning nearly a century, particularly the brown midrib mutants (see Anterola and Lewis, 2002). All had significant
CCR mutation in A. thaliana also resulted in a severely dwarfed phenotype and a delayed but coherent lignification program (Fig. 13.6A–C) (Laskar et al., 2006; Patten et al., 2005), whereas in tobacco, it resulted in a compromised vasculature and dwarfing as well (Piquemal et al., 1998). In an analogous manner, 4-coumarate CoA ligase (4CL), PAL, and C4H downregulation resulted in a significant loss of vascular integrity (see Anterola and Lewis, 2002) and/or other effects, such as dwarfing, due (mainly) to reduced lignin levels. Other concerns about deleterious effects on vascular integrity hold also for C3H downregulation (Patten et al., 2007).
These examples underscore the central question as to what extent lignin compositions/contents can actually be manipulated, without introducing structural defects prohibiting field applications of the resulting plant cultivars in, for example, bioethanol/biofuel/bioproduct generation. Another possible concern is that a weaker vascular apparatus may result in plants more susceptible to opportunistic pathogens. In short, it is becoming increasingly evident that a judicious balance must be maintained in growing vascular plants for commercial purposes and in reducing/modifying lignin contents/compositions. However, what flexibility exists in modifying lignin amounts/composition to avoid such adverse growth/developmental effects has not yet been determined.
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