Lignins and Abiotic Stresses - Botanical Research - page 247

The phenylpropanoid pathway is supplied with substrates derived from
the shikimate pathway. Under stress, the phenylpropanoid pathway could be
stimulated by rerouting of substrates to this pathway or by increasing carbon
supply via the shikimate pathway. Observations showing a coordinated
activation of the shikimate and phenylpropanoid pathways under abiotic
stress support this idea ( Betz et al., 2009a,b; Caban´ et al., 2004; Scheible
et al., 2004 ).
The response of the phenylpropanoid pathway also appears to be regu-
lated at the transcriptional level. As shown above a number of stresses induce
significant differences in the accumulation of different phenylpropanoid gene
transcript levels. Although transcription factors specifically involved in the
regulation of the phenylpropanoid response to stress have not yet been
functionally characterized, recent analyses have suggested that a battery of
different transcription factors are involved in abiotic stress responses ( Ahuja
et al., 2010; Saibo et al., 2009; Shin, 2011 ). It also seems possible that
transcription factors involved in developmental lignin biosynthesis may
also be regulated by abiotic stresses. A recent review of lignin transcriptional
regulation networks ( Zhao and Dixon, 2011 ) highlighted the fact that the
repressor MYB factor AtMYB4 responds to wounding and UV ( Jin et al.,
2000 ). Interestingly, another study revealed a significant upregulation of a
NAC domain transcription factor in poplar leaves in response to elevated
CO 2 ( Cseke et al., 2009 ). NAC domain transcription factors have been
implicated as transcriptional switches that regulate secondary wall synthesis
including lignin synthesis and may then control carbon deposition into
secondary cell wall thickening in the poplar leaves ( Zhao and Dixon, 2011 ).
A better understanding of the regulation of stress lignin biosynthesis is also
complicated by the fact that most enzymes of the phenylpropanoid pathway
are encoded by multigene families. Only certain family members are involved
in the synthesis of constitutive lignins whereas others may be specifically
induced in response to both abiotic stresses ( Molas et al., 2006; Soltani et al.,
2006 ) and biotic stresses ( Bi et al., 2011 ). These observations suggest that
lignin synthesis in response to abiotic stresses is regulated differentially from
constitutive lignin synthesis.
Finally, lignin is only one of the major components of the secondary cell
wall, together with cellulose and hemicellulose. Lignin biosynthesis must
then be coordinated with cellulose and hemicellulose synthesis. Consistent
with this, some transcription factors which regulate developmental lignin
biosynthesis are also activators of the entire process of secondary wall
formation ( Zhao and Dixon, 2011 ). It is likely that lignification in response
to abiotic stresses is also coordinated with the synthesis and deposition of
other cell wall polymers.