Biology Reference
In-Depth Information
In addition to peroxidases, laccases have also been implicated in the
oxidation of monolignols during lignin formation. In one study (
Liang
et al., 2006
), salt treatment was associated with reduced root elongation
and apical swelling in maize roots, together with a significant increase in
ZmLAC1 expression. The authors initially hypothesized that the reduction in
root elongation was associated with increased lignification and/or cross-
linking of cell wall phenolics mediated by the ZmLAC1 protein. However,
subsequent experiments showed that other treatments causing similar inhibi-
tion of root elongation did not modify ZmLAC1 gene expression. This latter
observation would argue against a role of lignification in the inhibition of
root elongation; however, since neither lignin biosynthesis nor phenolic
metabolism were investigated in this study, it is difficult to come to any
firm conclusions. Interestingly, salt stress, as well as drought stress, cold
stress and ABA treatment have all been shown to stimulate the production
of the microRNA miR393, miR397b and miR402 in Arabidopsis (
Martin
et al., 2010
). The miR397b is predicted to target a laccase gene previously
shown to reduce root growth under dehydration in a knock-out mutant (
Cai
et al., 2006
). More recently, miR397a/b have been predicted to target the
Arabidopsis laccase genes AtLAC4 and AtLAC17 that play a role in the
lignification of interfascicular fibres (
Berthet et al., 2011
). Taken together,
these observations demonstrate that salt stress, like other abiotic stress,
affects the expression of both peroxidase and laccase genes with a likely
concomitant affect on lignification.
Another link between salt stress and lignification was revealed by a study
on the roots of tomato plants (
Sanchez-Aguayo et al., 2004
). In this work,
the expression of three SAM genes (encoding SAMS) was compared in the
roots and leaves of tomato plants after 6 days salt stress (5 g L
1
). Salt
stress stimulated SAM1 gene expression in both roots and leaves, but had
no effect on the expression of SAM2 and SAM3. SAM1 was also more
highly expressed in root tissues under non-stress conditions. Western blot
analyses confirmed the increase of SAMS proteins in stressed plants.
S-Adenosyl-
L
-methionine is the main methyl group donor for numerous
transmethylation reactions in plants and lignin biosynthesis (involving
successive methylation to form coniferyl and sinapyl alcohol) is believed
to impose the largest demand for methyl groups in plants (
Hanson et al.,
1994
). The authors suggest that the observed increase in SAM1 gene
expression and accumulation of SAMS proteins is potentially related to
an increase in lignification. Subsequent immunolocalization indicated that
SAMS proteins accumulated in all cell types of roots, leaves and stems in
both control and non-stressed plants, but that some cell types appeared
to accumulate SAMS proteins to a greater extent. SAMS proteins were
