Plants
brace themselves for cold shock
We
already know that the cell wall is dynamically involved in many aspects of cell
physiology. Following the work of a research team from Warsaw,
Błonie and Rzeszów in Poland (Solecka et al., pp.
521–530), it has become apparent that changes in
cell wall biochemistry are implicated in low-temperature acclimation. In common
with many species, oilseed rape (Brassica napus) may be protected against freezing injury
by prior exposure to cold, but not freezing, conditions (chilling) over a
period of days or weeks. Thus, plants grown at 2 °C for 3
weeks showed some resistance to subsequent freezing temperatures as low as –10
°C. This frost tolerance was, however, rapidly lost if
the plants were transferred to 12 °C. Amongst the obvious changes associated with cold acclimation
was a significant reduction in leaf expansion, accompanied by increased tensile
stiffness. These changes were correlated with changes in cell wall metabolism.
Cell walls made up a greater proportion of the total leaf dry weight, with pectins constituting a greater proportion of the cell
walls. The latter feature would not necessarily be expected to increase cell
wall stiffness. However, pectin methylesterase
activity was increased by chilling in association with a much lower level of
pectin methylation, making the pectin polysaccharides
more available for cross-linking, thus contributing to wall stiffness. All
these changes except one were reversed when plants were ‘de-acclimated’ at 12 °C. These data certainly provide a clear indication
that changes in cell wall metabolism and, especially, in the synthesis and
modification of pectin are involved in cold acclimation. However, there remains
one puzzle: despite the decrease in cell stiffness, the one feature that was
not reversed by warming to 12 °C was
the increased specific activity of pectin methylesterase
(although it did decline as a proportion of cell wall weight). So, as the
authors rightly say, these interesting and significant results pave the way for
further important research.
