With a Little Help from My Cell Wall: Structural Modifications in Pectin May Play a Role to Overcome Both Dehydration Stress and Fungal Pathogens

Abstract

Cell wall structural modifications through pectin cross-linkages between calcium ions and/or boric acid may be key to mitigating dehydration stress and fungal pathogens. Water loss was profiled in a pure pectin system and in vivo. While calcium and boron reduced water loss in pure pectin standards, the impact on <i>Allium</i> species was insignificant (<i>p</i> > 0.05). Nevertheless, synchrotron X-ray microscopy showed the localization of exogenously applied calcium to the apoplast in the epidermal cells of <i>Allium fistulosum</i>. Exogenous calcium application increased viscosity and resistance to shear force in <i>Allium fistulosum</i>, suggesting the formation of calcium cross-linkages ("egg-box" structures). Moreover, <i>Allium fistulosum</i> (freezing tolerant) was also more tolerant to dehydration stress compared to <i>Allium cepa</i> (freezing sensitive). Furthermore, the addition of boric acid (H₃BO₃) to pure pectin reduced water loss and increased viscosity, which indicates the formation of RG-II dimers. The <i>Arabidopsis</i> boron transport mutant, <i>bor1</i>, expressed greater water loss and, based on the lesion area of leaf tissue, a greater susceptibility to <i>Colletotrichum higginsianum</i> and <i>Botrytis cinerea</i>. While pectin modifications in the cell wall are likely not the sole solution to dehydration and biotic stress resistance, they appear to play an important role against multiple stresses.