Boron (B) deprivation induces various responses in plant cells, some of which can be observed very early. However, it has been unknown what kind of signal is generated by the stress. We found that B deprivation induced the expression of stress-responsive genes within 1h in suspension-cultured tobacco BY-2 cells. The induction was largely suppressed by withholding medium Ca(2) or by adding a Ca(2) channel blocker. Analysis using aequorin-expressing cells showed that B-deprived cells took up more Ca(2) than control cells. These results suggest that Ca(2) influx plays a role in B deprivation stress signaling.