Biology Reference
In-Depth Information
months (June-August), leaf relative water content decreased while abscissic
acid (ABA) and H
2
O
2
levels increased. Sub-cellular localization by CsCl
3
precipitation indicated that H
2
O
2
started to accumulate in leaf mesophyll
cells at the start of the summer drought period (June). In July, H
2
O
2
started
to accumulate in the cell walls of both xylem vessels and sclerenchyma.
Interestingly, high H
2
O
2
levels were also correlated with the de novo forma-
tion of sclerenchyma cells in the leaf cortex. Although H
2
O
2
is necessary for
peroxidase-mediated monolignol polymerization, an observed augmentation
in H
2
O
2
levels should not be automatically linked to increased lignification.
Drought stress, like other abiotic stress, stimulates reactive oxygen species
(ROS) production leading to oxidative stress and it is therefore interesting to
verify ROS sub-cellular distribution. Analyses showing that levels of the
antioxidant ascorbic acid together with the absence of any observable organ-
elle damage also suggested that the high H
2
O
2
content was not associated with
intracellular oxidative stress. The observation showing that increases in cell
wall localized H
2
O
2
were associated with significant increases (up to 69%) in
leaf lignin content suggested that H
2
O
2
was needed for oxidative polymeriza-
tion. However, the maximum increase in lignin accumulation occurred inMay,
prior to the peak of H
2
O
2
production that was observed in July, suggesting
that ROS production might also be involved in stress-signalling pathways.
In trees, drought stress affects both xylem anatomy and lignification. In
poplar, drought stress reduced both fibre length and cross-sectional area in
early summer but not in late summer (
Arend and Fromm, 2007
). Similarly,
early summer drought stress also reduced vessel cross-sectional area. Overall,
total vessel area was not significantly affected because vessel number
increased. However, no cell wall analyses were performed and it is therefore
difficult to know whether the observed reduction in xylem cell expansion was
associated with modifications in the quantity and/or chemical composition of
different cell wall polymers including lignin. In Pinus radiata logs, frequent
drought stress has been shown to induce concentric shelling (
Donaldson,
2002
). Microscopic examination revealed the presence of numerous false
growth rings alternating with bands of poorly lignified tracheids. A closer
study of individual cells showed that both the middle lamella and outer S1
layer of the secondary cell wall were poorly lignified and most likely the cause
of the observed poor cohesion between individual cells. In certain cases, wall
lignification was extremely reduced and collapsed cells were observed. The
presence of collapsed, deformed cells is reminiscent of the irregular xylem
phenotype observed in the Arabidopsis irx4 mutant characterized by
extremely reduced cell wall lignin content due to a mutation in the CCR
gene (
Jones et al., 2001
). In contrast to the less lignified status of the S1 layer,
the S3 layer of drought-stressed tracheids was often more heavily lignified
