Temporal cell wall changes during cold acclimation and deacclimation and their potential involvement in freezing tolerance and growth.

Plants adapt to freezing stress through cold acclimation, which is induced by nonfreezing low temperatures and accompanied by growth arrest. A later increase in temperature after cold acclimation leads to rapid loss of freezing tolerance and growth resumption, a process called deacclimation. Appropriate regulation of the trade-off between freezing tolerance and growth is necessary for efficient plant development in a changing environment. The cell wall, which mainly consists of polysaccharide polymers, is involved in both freezing tolerance and growth. Still, it is unclear how the balance between freezing tolerance and growth is affected during cold acclimation and deacclimation by the changes in cell wall structure and what role is played by its monosaccharide composition. Therefore, to elucidate the regulatory mechanisms controlling freezing tolerance and growth during cold acclimation and deacclimation, we investigated cell wall changes in detail by sequential fractionation and monosaccharide composition analysis in the model plant Arabidopsis thaliana, for which a plethora of information and mutant lines are available. We found that arabinogalactan proteins and pectic galactan changed in close coordination with changes in freezing tolerance and growth during cold acclimation and deacclimation. On the other hand, arabinan and xyloglucan did not return to nonacclimation levels after deacclimation but stabilized at cold acclimation levels. This indicates that deacclimation does not completely restore cell wall composition to the nonacclimated state but rather changes it to a specific novel composition that is probably a consequence of the loss of freezing tolerance and provides conditions for growth resumption.

Structural changes in cell wall pectic polymers contribute to freezing tolerance induced by cold acclimation in plants.

Subzero temperatures are often lethal to plants. Many temperate herbaceous plants have a cold acclimation mechanism that allows them to sense a drop in temperature and prepare for freezing stress through accumulation of soluble sugars and cryoprotective proteins. As ice formation primarily occurs in the apoplast (the cell wall space), cell wall functional 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 of pectic polysaccharides, are altered by cold acclimation, the significance of this change during cold acclimation has not been clarified. We found that ß-1,4-galactan, which forms neutral side chains of the acidic pectic rhamnogalacturonan-I, accumulates in the cell walls of Arabidopsis and various freezing-tolerant vegetables during cold acclimation. The gals1 gals2 gals3 triple mutant, which has reduced ß-1,4-galactan in the cell wall, exhibited impaired 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, was induced during cold acclimation in Arabidopsis, explaining the 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 ß-1,4-galactan contributes to acquired freezing tolerance by cold acclimation, likely via changes in cell wall mechanical properties.