Abstract
Boron (B) as borate cross-links pectic polysaccharides at rhamnogalacturonan II regions, thereby contributes to building the cell wall structure. We previously showed that B deprivation immediately induces Ca2+ influx and upregulation of stress-responsive genes in suspension-culture tobacco BY-2 cells. As a possible mechanism of such quick responses, we hypothesize that the cell wall with insufficient borate cross-linking cannot resist the turgor and allows the plasma membrane to stretch, thereby triggers Ca2+ influx through mechanosensitive ion channels.
Adding 0.4 M mannitol to reduce the membrane stretch suppressed the B deprivation-responsive gene expression. Higher dose Ca2+ (30 mM), which is expected to mitigate a structural disturbance of cell wall through enhanced cross-linking of polygalacturonate, also suppressed the upregulation of several genes. These results are consistent with our hypothesis that a weakened cell wall structure mediates the early responses of tobacco cells to B deprivation.
