The Cell Wall as a Barrier Against Water Loss and Plant Pathogens

Abstract

The ability to survive a range of stresses is crucial to the survival of plants. Structural modifications in the cell wall through pectin cross linkages may be key to mitigating damage caused by stress. Pectin reduces cell wall permeability and increases rigidity through calcium ion crosslinks to carboxylate ions in galacturonic acid residues in homogalacturonan, and boron crosslinks to apiosyl residues in rhamnogalacturonan II side chains. The objective of this research was to understand the influence of calcium and boron in vitro, and how changes in viscosity and rigidity may translate to resistance to dehydration and fungal pathogens in Allium spp. and Arabidopsis pectin methylesterase/boron mutant genotypes. Allium spp. served as an ideal model to study dehydration stress as the cells are large and a single layer of epidermal cells can be easily separated. Arabidopsis was useful given the availability of mutant genotypes. CaCl2 and H3BO3 were both found to significantly (p<0.05) increase the viscosity of pure pectin standards, in addition to reducing percent water loss in the same standards. However, the impact of these compounds on improving dehydration stress resistance in the plant species of interest was less clear. The efficacy of calcium in enhancing dehydration stress resistance in Allium spp. was highly variable, in certain instances improving resistance and in others decreasing it. Nevertheless, calcium increased the force required to shear Allium fistulosum by ~63.39 N g-1, suggesting it did cause structural modifications. Furthermore, Allium fistulosum (drought resistant) lost significantly less water (p<0.05) over 16-18hr compared to Allium cepa (drought sensitive) and had a lower limit of damage based on protoplasmic streaming, suggesting a link between resistance to dehydration stress and freezing stress. There also appears to be a link between boron and resistance to both dehydration stress and fungal pathogen stress. A boron transporter mutant (bor1) showed a greater susceptibility to dehydration stress (p>0.05) and Colletotrichum higginsianum infection (p<0.05). Because of the mechanism of infection, the rapid rate of Colletotrichum higginsianum infection in bor1 is indicative of a weak cell wall. While the response to stress is highly complex, collectively this thesis indicates calcium, boron, pectin and the cell wall in general may play important but relatively under researched roles in plant resistance to both abiotic and biotic stress.