Cell wall pectic β-1,4-galactan contributes to increased plant freezing tolerance induced by cold acclimation

Subzero temperatures are often lethal to plants. Many temperate plants have a mechanism called cold acclimation that allows them to sense a drop in temperature and prepare for freezing stress through accumulation of soluble sugars and cryoprotective proteins. Plant cells are surrounded by a cell wall and as ice formation primarily occurs in the extracellular matrix (the apoplast), cell wall properties are important for plant freezing tolerance. Although previous studies have shown that the amounts of constituent sugars of the cell wall, in particular those present in pectic polysaccharides, are altered by cold acclimation, the significance of this change during cold acclimation has not been clarified. We found that {beta}-1,4-galactan, which forms neutral side chains of pectin, accumulates in the cell walls of Arabidopsis and various freezing tolerant vegetables during cold acclimation. The gals1 gals2 gals3 triple mutant, which has reduced {beta}-1,4-galactan in the cell wall, exhibited reduced freezing tolerance compared with wild-type Arabidopsis during initial stages of cold acclimation. Expression of genes involved in the galactan biosynthesis pathway such as GALACTAN SYNTHASES and UDP-glucose 4-epimerases were induced during cold acclimation in Arabidopsis explaining galactan accumulation. Cold acclimation resulted in a decrease in extensibility and an increase in rigidity of the cell wall in the wild type, whereas these changes were not observed in the gals1 gals2 gals3 triple mutant. These results indicate that the accumulation of pectic {beta}-1,4-galactan contributes to acquired freezing tolerance by cold acclimation via changes in cell wall mechanical properties.