Biology Reference
In-Depth Information
phenylpropanoid genes in transgenic tobacco plants did not reveal significant
changes in transcript abundance of genes involved in the general phenylpro-
panoid pathway (PAL, C4H), but it revealed significant increases in the
expression of genes known to be involved in the monolignol-specific portion
of the pathway. Indeed, genes involved in conversion of esters to aldehydes,
and aldehydes to alcohols (side-chain modification), that is encoding hydro-
xycinnamoyl-CoA transferase (HCT), CCR and CAD, were about fivefold
upregulated whereas the genes involved in the differential pathways leading
to the monolignol monomers (ring modification), that is encoding C3H,
F5H, CCoAOMT and COMT were up to 40-fold upregulated. Moreover,
in apparent contrast to the strong intensity of the phloroglucinol staining of
transgenic xylem, the Klason lignin content expressed as a percentage of cell
wall residue was very slightly increased (
Goicoechea et al., 2005
). One likely
interpretation is that the dramatic increase in SW resulted not only from the
increase of lignin but also from the concomitant increase of SW polysacchar-
ides. This hypothesis was confirmed recently by experimental evidence
showing firstly that overexpression of EgMYB2 in Arabidopsis protoplasts
is able to activate the expression of the biosynthetic genes of cellulose, xylan
and lignin (
Zhong and Ye, 2009
) and secondly, that EgMYB2 is able to
complement the myb46-myb83 double mutant restoring secondary wall
thickening in vessels and concurrently normal plant growth (
Zhong et al.,
2010a
). Thus, the wood-associated EgMYB2 is a functional ortholog of
AtMYB46 (
Zhong et al., 2007
) and of its later characterized homolog
AtMYB83 (
McCarthy et al., 2009
) and is therefore a transcriptional regula-
tor of all three major secondary wall components. Indeed,
Zhong et al. (2007)
showed that AtMYB46 is expressed predominantly in fibres and vessels in
Arabidopsis stems. Dominant repression of AtMYB46 caused a drastic re-
duction in the thickening of SW fibres and vessels whereas overexpression
resulted in an activation of the biosynthetic pathways of cellulose, xylan, and
lignin and concomitantly led to ectopic deposition of SW in cells that are
normally non-sclerenchymatous (
Zhong et al., 2007
). The authors also
demonstrated using EMSA and ChIP experiments that AtMYB46 is a direct
target of SECONDARY WALL-ASSOCIATED NAC DOMAIN PRO-
TEIN1 (SND1 also called NST3 or ANAC012), which is itself a key tran-
scriptional activator regulating the developmental programme of secondary
wall biosynthesis (see below). These results were major breakthroughs
demonstrating that MYB proteins can also behave as master regulators in
the NAC-mediated transcriptional network. Indeed, SND1 and AtMYB46
function at the top of the transcriptional network leading to the regulation of
lignin biosynthesis together with cellulose and xylan. The role of AtMYB46
as a master switch for SW formation in Arabidopsis was further confirmed by
