Biology Reference
In-Depth Information
of phenylpropanoid and flavonoid biosynthetic genes also stands out in the
vascular expression of lignin genes (
Sivadon and Grima-Pettenati, 2004
and
references therein). Deep functional analyses of the promoters of the PAL
and 4CL genes revealed their modular organization and identified AC ele-
ments as being central in ensuring their coordinated expression in xylem
(a highly lignified tissue) and repression in phloem. Vascular expression of
PAL and 4CL involves a negative combinatorial interaction with a negative
regulatory element (possibly phloem-specific) suppressing the activity of a
cryptic cis-element for phloem expression. In the context of the minimal
PAL2 promoter, two AC elements (AC-I and -II) individually contribute
to the expression in phloem and to a low level of expression in xylem, whereas
their combined activities lead to a high expression in xylem (
Hatton et al.,
1995, 1996
). This type of negative control was proposed (
Leyva et al., 1992
)to
demonstrate the evolution of a non-specific vascular promoter that acquired an
element-suppressing expression in phloem and promoting expression in xylem.
In contrast, such a negative type of control does not seem to exist in the
promoters of genes specifically dedicated to lignin biosynthesis such as the
genes encoding cinnamoyl CoA reductase (CCR) or cinnamyl alcohol dehy-
drogenase (CAD) which catalyse the two last reductive steps of the monolignol
branch pathway. The Eucalyptus gunnii CCR and CAD2 promoters direct
similar expression patterns in xylem tissues in different plants suggesting that
the control of lignin gene expression is conserved between annual herbaceous
and woody perennials and that the lignin production is controlled, at least
in part, by the coordinated transcriptional regulation of these two genes
(
Baghdady et al.,2006;Feuilletet al., 1995; Lacombe et al., 2000; Lauvergeat
et al.,2002
). Using a combination of promoter deletion/mutation analysis,
electrophoretic mobility shift assay (EMSA) and/or in vivo footprinting, con-
served AC elements in EgCAD2 and EgCCR promoters were shown to be
crucial both for the formation of DNA-protein complexes and for the tran-
scriptional activation of EgCAD2 and EgCCR in vascular tissues (
Lacombe
et al., 2000; Rahantamalala et al.,2010
).
Given the importance of AC elements for specific vascular expression, an in
silico bioinformatic analysis of the promoters of all the lignin biosynthetic genes
in Arabidopsis was undertaken (
Raes et al.,2003
). The presence of AC elements
was observed for the majority of these genes. Exceptions include the promoters
of C4H, ferulate 5-hydroxylase (F5H) and caffeic acid-O-methyltransferase
(COMT), which do not have apparent AC elements. One hypothesis was
that these genes might contain more degenerative AC elements that may not
be picked up by the bioinformatic analysis. This seems to be the case for C4H
and COMT (
Zhou et al., 2009
) but not for Arabidopsis F5H. Indeed, earlier
work on F5H expression in Arabidopsis indicated that the tissue-specific and
