Salt, sorbitol and superoxide —
revelations about roots and ROS
The role of reactive oxygen species (ROS) in the life
of a plant is somewhat ambivalent. On one hand, active growth is associated
with ROS production and may indeed be dependent on those ROS, especially
superoxide, O2·– that is produced by apoplastic NADPH oxidase. On the
other hand, ROS may be very damaging and plant cells possess efficient
mechanisms to detoxify or remove them. Further, ROS are often produced when plants
are exposed to stress. What then happens in a growing organ that is exposed to stressful
conditions? To investigate this further, Bustos et al. at Códoba,
Argentina (p.
551–559) have exposed maize roots to hyper-osmotic stress, a situation in
which oxidative damage is known to occur in some plant species. Roots treated
for 3 h with either 150 mM NaCl or 300 mM sorbitol elongated
at half the rate of roots in control (Hoagland solution) conditions. Assays of
ROS showed that in control roots, the positions of the highest growth rates and
the highest O2·–coincided. However, in stressed roots the situation was more
complex. NaCl treatment led to reduced O2·–production, but the reduction mainly occurred closer to the tip
than the zone of growth. In sorbitol-treated roots, O2·–levels increased rather than decreased and again this change
took place much nearer the tip than the growth zone. No ROS-mediated damage was
reported. Roots maintained in the osmotica for 2 d
and then returned to higher water potentials showed a recovery in their growth
rates. This recovery was inhibited if roots were treated with the O2·–scavengers MnCl2 and Tiron, indicating a
dependence on O2·–. Further, the highest levels of O2·– now coincided once
again with the highest growth rates. The complexity of these responses suggests
that O2·–and possibly also other ROS have more than one role in root
growth, at least under hyper-osmotic conditions.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
