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coordinated and has involved the development of major transcriptional
regulation control mechanisms. The phenylpropanoid metabolism, source
of thousands of compounds, many of which are specific to particular plant
species, is involved in protection against biotic and abiotic stresses and
includes antipathogenic phytoalexins, antioxidants and UV absorbants, has
been extensively studied ( Dixon and Paiva, 1995; Weisshaar and Jenkins,
1998 ). In comparison, the lignin branch pathway has received much less
attention even though the associated genes are not only developmentally
regulated but also induced in response to many environmental stresses such
as wounding ( Lauvergeat et al., 2002 ), mechanical stress ( Paux et al., 2005 ),
UV light irradiation and pathogen attacks ( Lauvergeat et al., 2001 ).
Early promoter deletion studies identified specific regions in the promoters of
lignin biosynthetic genes responsible for wounding, UV irradiation and patho-
gen activations. This suggests that the AC elements are implicated in response
to environmental stresses ( Weisshaar and Jenkins, 1998 ). A large number of
studies focused on PAP1 and AtMYB4, which are both regulated by different
environmental stress factors. AtMYB4 belongs to subgroup 4 of the R2R3-
MYB family containing a C-terminal EAR motif signature ( Ohta et al.,2001 ).
Many EAR-motif-containing repressor proteins have been shown to play a key
role in modulating plant defence and stress responses ( Kazan, 2006 ). AtMYB4
expression is downregulated by exposure to UV-B light, indicating that dere-
pression is an important mechanism for acclimation toUV-B in A. thaliana ( Jin
et al.,2000 ). Increased expression of AtMYB4 in Arabidopsis was also found in
response to cadmium exposure ( van de Mortel et al.,2008 ). ZmMYB31
( FornalĀ“ et al.,2010 )andZmMYB42 ( Sonbol et al.,2009 ) are both induced
by wounding. Interestingly, Bedon et al. (2010) described a monophyletic clade
(Sg4C) in conifers that expanded following the split between gymnosperm and
angiosperm lineages ( Fig. 2 ). Many of these genes have stress-responsive pro-
files to wounding, jasmonic acid (JA) treatment and exposure to cold. They are
believed to play a role in broad defence responses.
Schenke et al. (2011) simultaneously applied an abiotic stress (UV-B light)
known to induce the production of UV-protective flavonols in Arabidopsis
cell suspension cultures together with a biotic stress by adding the bacterial
elicitor flg22. In response to these combined two treatments, they observed
an inhibition of the flavonol biosynthesis genes whereas the production of
defence-related compounds such as phytoalexins and lignin, a structural
barrier thought to restrict pathogen spread, was induced. Since all these
compounds derive from the phenylpropanoid metabolism, the authors pro-
posed a cross talk between the two stresses involving antagonistic regulation
of two opposing MYB TFs, the positive regulator of the flavonol pathway
AtMYB12 on one side (UV-B-induced and flg22-suppressed) and the nega-
tive regulator AtMYB4 on the other side (UV-B- and flg22-induced). Further
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