Freezing stress is detrimental to plants, resulting in major crop losses in temperate regions. The plant cell wall is a dynamic network of proteins and polysaccharides including cellulose, hemicellulose and pectins. It is essential for plant survival, providing structural integrity, strength and protection against pathogens. As the cell wall is the site of ice formation, it has also been suggested that the wall could contribute towards protection of the plant against freezing damage. The cell wall undergoes remodelling during cold acclimation, but it is unclear what specific role this restructuring may play in freezing tolerance.
The sensitive to freezing8 (sfr8) mutant contains less cell wall fucose due do a mutation in the fucose biosynthetic gene MUR1. This was shown to result in a decrease in dimerisation of the cell wall pectic domain rhamnogalacturonan-II (RG-II), which in wild type plants is predominantly dimerised via a borate-ester cross-link. This decrease in dimerisation likely results in the observed freezing sensitivity of mur1 mutants, as supplementation of plants with boric acid was shown to restore freezing tolerance. Guard cell dynamics were also compromised in the sfr8 mutant, as stomata were found to be more restricted in their movements than wild type in response to ABA, CO2 and changes in humidity.
The freezing and guard cell phenotypes of sfr8 may be attributed to a decrease in the tensile modulus of the cell wall with reduced RG-II dimerisation. This makes the wall more vulnerable to deformation during freezing and prevents the guard cells from stiffening to allow an increase in stomatal aperture. RG-II dimerisation also mediates certain structural aspects of the cell wall that may facilitate supercooling by excluding ice nucleation and preventing ice growth. This research reveals the importance of RG-II dimerisation in cell wall dynamics and the impact cell-wall composition has on freezing and desiccation tolerance. These findings could lead to the identification of new targets for crop breeding
