Investigating the role of the cell wall in cold acclimation and freezing tolerance in Arabidopsis thaliana

Abstract

Many plants are susceptible to freezing stress, including crops necessary for the production of staple foods. Climate change is predicted to increase the occurrence of early autumn and late spring frosts, striking at times when plants are especially vulnerable. Most perennial plants can undergo cold acclimation, wherein freezing tolerance is increased through exposure to low, non-freezing temperatures. Encapsulating every plant cell is a thin, dynamic layer of complex polysaccharides and proteins – the cell wall. It offers structural rigidity, extensibility, a means of water transport, and protection from pathogens. The cell wall undergoes significant structural, compositional and physical remodelling during cold acclimation; it is therefore increasingly thought to have a role in freezing tolerance, perhaps by acting as a physical barrier to ice propagation and providing mechanical reinforcement against freezing-induced dehydration, though this remains unconfirmed.

In the present study, the potential role of the cell wall in both cold acclimation and basal freezing tolerance was explored further. An increase in freezing tolerance in
cold-acclimated Arabidopsis was shown to be correlated with both reduced cell-wall porosity and enhanced cell-wall mechanical integrity. These properties are thought to limit the nucleation and spread of ice through plant tissues and protect against cellular dehydration and collapse during freezing, respectively. Comprehensive microarray polymer profiling revealed a number of compositional changes that
occur in the Arabidopsis cell wall during cold acclimation. Most notably, there was an accumulation of extensins, a family of important structural proteins, and a shift
towards the demethylesterified form of pectin, a reorganisation which can have considerable impacts on the physical properties of the cell wall by facilitating the
formation of cross-links between pectic backbones. Mutants deficient in either extensin or demethylesterified pectin were shown to have impaired freezing tolerance, cell-wall porosity and mechanical properties.

In wild-type cell walls, the pectic polysaccharide rhamnogalacturonan-II (RG-II)
exists predominantly as a dimer centred on a borate-ester cross-link. The freezingsensitive sfr8 mutant, which lacks RG-II dimerisation, was also found to have compromised wall porosity and mechanics. These defects were partially reversed when RG-II dimerisation was restored by boron supplementation, as was its freezing sensitivity, suggesting that RG-II cross-linking is critical to full freezing
tolerance. A number of other mutants with defective cell walls, including those with structurally- or compositionally-impaired cellulose, hemicellulose, or pectins, were screened for freezing sensitivity and aberrant physical properties. The data largely
support the idea that plant freezing tolerance is dependent on the porosity and mechanical properties of the cell wall, themselves dependent on the structure, composition, and organisation of its constituent polymers.