Rhamnogalacturonan-II Dimerisation Reinforces Salt Resistance in Sugar Beet.

Salinity stress predominantly affects negatively charged cell wall polymers, for example, pectin. Excess Na+ ions interact physically and affect growth in stress-sensitive plants. However, the salinity resistance of sugar beet cell walls remains unclear. To get a better understanding of cell wall assembly, we investigated arabinogalactan-proteins (AGPs), extensins and pectic polysaccharides (homogalacturonan, rhamnogalacturonan-I and rhamnogalacturonan-II), in relation to underlying physiological mechanisms and growth expansion with low and adequate boron (B) under salinity. Findings revealed that salt stress affects AGPs and reduces cross-linking of RG-II, resulting in the softening of the sugar beet plant's cell wall. Adequate B compensates for plant growth by improving water flow into the cell, as indicated by the transpiration rate and stomatal conductance. In particular, the higher reduction of the Na+/Ca2+ ratio in the young leaves and apoplastic fluids and higher RG-I content and dimeric RG-II pectin (a key component of cell wall integrity) offered by adequate B, hint at protection against cell wall defects. However, no influence of B was detected for AGPs and extensins. This suggests that adequate B rescues cell wall integrity, thereby conferring strengthening and acid growth.