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elongating stem internodes, AtMYB52 is expressed in developing protoxylem
and in the elongating interfascicular fibre cells, despite the fact that at this
stage, they do not yet have visible SW thickening. In non-elongating inter-
nodes, a high level of expression was evident in developing metaxylem cells
and interfascicular fibres ( Zhong et al., 2008 ). Dominant repression of
AtMYB52 causes a severe reduction in SW thickening in both interfascicular
fibres and xylary fibres in Arabidopsis inflorescence stems but not in vessels.
However, it should be noted that overexpression did not reveal any ectopic
deposition of SW and no changes were observed in the secondary wall thick-
ness of fibres and vessels ( Zhong et al.,2008 ). The transcription of AtMYB52 is
strongly activated by AtMYB46 and AtC3H14 whereas AtMYB7, AtMYB4
and AtMYB32 negatively regulate AtMYB52 transcription ( Ko et al.,2009 ).
Another repressor of the lignin branch of the phenylpropanoid pathway that
does not belong to subgroup 4 was recently characterized by Bhargava et al.
(2010) . They established that a loss-of-function mutation in PAP1 (PRODUC-
TION OF ANTHOCYANIN PIGMENT1) also called AtMYB75 (myb75-1)
results in increased cell wall thickness in xylary and interfascicular fibres within
the inflorescence stem. The total lignin content and S/G ratio of the lignin
monomers were also affected. Transcript profiles from the myb75-1 inflores-
cence stem revealed marked upregulation in the expression of a suite of genes
associated with lignin biosynthesis and cellulose deposition, as well as cell wall-
modifying proteins and genes involved in photosynthesis and carbon assimila-
tion ( Bhargava et al.,2010 ). These results contrast with earlier activation
tagging experiments in Arabidopsis in which overexpression of PAP1 activated
phenylpropanoid genes, resulting in an enhanced accumulation of lignin and
flavonoids ( Borevitz et al.,2000 ). The increased accumulation of lignin was
supposed to be due to an augmentation in the quantity of common hydroxy-
cinnamoyl-CoA esters shared by the biosynthetic pathways of both lignin and
anthocyanin. Tohge et al. (2005) performed a comprehensive analysis of the
metabolome and transcriptome of A. thaliana overexpressing the PAP1 gene
that suggests that PAP1 regulates flavonoid biosynthetic genes causing the
specific accumulation of cyanidin- and quercetin-type flavonoids in a relatively
specific manner. However, they did not check the lignin content nor did
histochemistry of the vascular tissues.
Taken together, these results raise the possibility that PAP1 may function as
both an activator and a repressor. Such dual functionality has been reported
for both mammalian and plant TFs ( Boyle and DesprĀ“s, 2010 ). Dual function
roles can be exerted in a ''context-dependent'' or ''signal-dependent'' fashion.
Interestingly, PAP1 physically interacts with another SW regulator, the
KNOX TF KNAT7. These regulatory proteins may form functional com-
plexes that contribute to the regulation of SW deposition in the Arabidopsis
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