Biology Reference
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elements and trans-acting factors might be involved in the regulation of
promoter activity (
Rahantamalala et al., 2010
).
Recent chapters also highlighted the fact that interpretation of the results
obtained through functional characterization of TFs is method dependent. For
instance, it is important to understand that the dominant repression strategy,
often used to overcome the problems of TF functional redundancy and lack of
mutant phenotype, can also be misinterpreted, particularly for transcriptional
repressors. This was clearly demonstrated with the repressor KNAT7 (
Li et al.,
2011, 2012
) which was initially described as an activator based on dominant
repression results (
Zhong et al.,2008
). Some TFs seem to behave either as
activators or repressors on the same target genes depending on the methods
used and/or on the TF expression level. For instance, Arabidopsis tagging lines
overexpressing PAP1/MYB75 produce more lignin (
Borevitz et al.,2000
),
whereas loss-of-function mutants also have more lignin and thicker cell walls
(
Bhargava et al.,2010
). These apparent discrepancies between mutants and
overexpressors suggest a possible dual function role that could be dependent on
the threshold level of PAP1. Indeed, the importance of the threshold level of a
given TFwas emphasized by
Hussey et al. (2011)
who observed opposite effects
of SND2 on SW between lines overexpressing SND2 at different levels. TFs
exhibiting dual functions often physically interact with partner proteins giving
rise to functional activating or repressing complexes. PAP1 was shown to be
involved in at least two types of combinatorial complexes: it is part of WD40-
dependent ternary complexes regulating anthocynanin biosynthesis (reviewed
in
Ramsay and Glover, 2005
), and also interacts with KNAT7, a negative
regulator of the SW formation (
Bhargava et al.,2010
). PAP1 seems to occupy a
critical position at the crossroad between the lignin, flavonoid and polysaccha-
ride pathways; its involvement in combinatorial complexes provides the plant
with a high flexibility to complex environmental and/or developmental cues.
Although major progress has been made thanks to the model plant
A. thaliana, the growing number of TF characterization made in other plants
(reviewed here) highlights major species- or lineage-specificities and strength-
ens the importance of deciphering SW regulatory networks in target species.
To this end, the availability of a growing number of genome sequences
will allow the discovery of new genes involved in the regulation of SW.
For example, the recent release of the Eucalyptus grandis genome allowed
us—via a combination of phylogenetic and transcriptomic analysis—to
identify MYB groups specific to woody plants that contain genes preferen-
tially expressed in the vascular cambium and xylem (Soler and Grima-
Pettenati, unpublished results). The identification and functional characteri-
zation of such new players will provide novel insights into the complex
regulatory networks controlling the formation of the lignified SW in plants of
