Biology Reference
In-Depth Information
investigation of such a cross talk might help to understand how plants
integrate different stress signals in order to fine-tune their defence reactions.
PAP1, which is a positive regulator of the flavonoid pathway (
Borevitz
et al., 2000; Tohge et al., 2005
) and—depending on the authors—either a
positive (
Borevitz et al., 2000
) or a negative regulator (
Bhargava et al., 2010
)
of the lignin pathway, is known to be upregulated by different environmental
stress factors as well as by nutrient availability. The transcript levels of PAP1
and its close homolog PAP2/MYB90 are strongly induced during nitrogen
(N) and/or phosphorus (P) limitation and quickly repressed after nitrate
(NO
3
) addition to nitrogen-depleted Arabidopsis seedlings (
Scheible et al.,
2004
). Nitrogen fertilization has also been described to influence the lignin
content of stems in poplar (
Novaes et al., 2010; Pitre et al., 2010
). However,
more in-depth studies are needed to identify which TFs are involved in the
N-responsive regulation of lignification of secondary growth tissues.
As indicated above, some WRKY genes involved in defence responses
such as VvWRKY2, which is involved in grapevine resistance against
fungal pathogens, were shown to regulate lignification in response to biotic
stresses (
Guillaumie et al.,2010
). Finally, a direct and striking link between
the regulation of the lignified SW and the responses to biotic stress was
brought to light very recently by the work of
Ram´rez et al. (2011)
.They
showed that AtMYB46 is pivotal for mediating disease susceptibility to the
fungal pathogen Botrytis cinerea. AtMYB46bindstoanewcis-element
locatedinthe5
0
promoter region of the pathogen-induced Ep5C gene,
which encodes a type III cell wall-bound peroxidase and modulates the
magnitude of Ep5C gene induction following pathogenic attacks. They
demonstrated that different myb46 KO mutants exhibit increased disease
resistance to B. cinerea, a phenotype that is accompanied by selective
transcriptional reprogramming of a set of genes encoding cell wall proteins
and enzymes, of which extracellular type III peroxidases are notable. These
results suggest that AtMYB46 is a bifunctional TF acting as an activator
during vascular development but becoming a conditional repressor when
involved in the regulation of genes important for disease resistance to
B. cinerea (
Ram
´
rez et al., 2011
).
IX. HORMONAL CONTROL AND REGULATION
OF LIGNIN BIOSYNTHESIS
Some plant hormones such as auxin, cytokinin, gibberellin and brassinoster-
oids (BRs), have long been known to be involved in the induction of vascular
cell differentiation (
Kubo et al., 2005
). A class III homeodomain leucine
