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

**Ariana D. Forand 1, Y. Zou Finfrock 2,3, Miranda Lavier 3, Jarvis Stobbs 3, Li Qin 4, Sheng Wang 1,5, Chithra Karunakaran 3, Yangdou Wei 4, Supratim Ghosh <sup>6</sup> and Karen K. Tanino 1,\***


**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 *Allium* species was insignificant (*p* > 0.05). Nevertheless, synchrotron X-ray microscopy showed the localization of exogenously applied calcium to the apoplast in the epidermal cells of *Allium fistulosum*. Exogenous calcium application increased viscosity and resistance to shear force in *Allium fistulosum*, suggesting the formation of calcium cross-linkages ("egg-box" structures). Moreover, *Allium fistulosum* (freezing tolerant) was also more tolerant to dehydration stress compared to *Allium cepa* (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 *Arabidopsis* boron transport mutant, *bor1*, expressed greater water loss and, based on the lesion area of leaf tissue, a greater susceptibility to *Colletotrichum higginsianum* and *Botrytis cinerea*. 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.

**Keywords:** cell wall; pectin; calcium; boron; homogalacturonan; rhamnogalacturonan II; *Botrytis cinerea*; *Colletotrichum higginsianum*; dehydration; *Allium* and X-ray microscopy
