Exploring the role of boron and cell wall crosslinking in plant freezing tolerance and stomatal development

Abstract

Abiotic stresses such as freezing and drought can be extremely harmful to plants, resulting in substantial yield losses or even death. Freezing events can cause freezing-induced dehydration, therefore, many of the pathways involved in freezing tolerance are also involved in drought tolerance. The cell wall is a complex polysaccharide layer which has many essential roles in plant growth, defence and survival. Several studies have linked the cell wall to freezing tolerance. In addition, cell wall pectin rhamanogalacturonan-II (RGII) has recently been implicated in freezing tolerance.
The role of RGII dimerisation in freezing tolerance was investigated using mutants with altered dimerisation. mur1 mutants show reduced RGII dimerisation as a result of decreased levels of L-fucose while bor mutants show reduced dimerisation due to defective boron transport. Both RGII dimerisation mutants show reduced freezing tolerance indicating that dimerisation plays a role in freezing tolerance. This is supported by the fact that freezing tolerance was restored upon restoration of RGII dimerisation via supplementation.
Dimerisation mutants were seen to have increased desiccation and transpiration which improved with restoration of RGII dimerisation. Other factors may be involved in this response, however, RGII dimerisation mutants displayed an altered stomatal phenotype with a missing cuticular ledge. Again, supplementation with boron was able to restore a WT-like stomatal phenotype. Alterations to the stomatal morphology may be the cause of altered transpiration in these mutants as stomata are essential for control of gas exchange and mediation of water loss. However, further research is required to determine the exact role of RGII dimerisation in the desiccation response.
These findings highlight the importance of both the cell wall and RGII in drought and freezing tolerance. This research could identify new areas of interest for development of crop lines with improved freezing or drought resistant.