Physiological elevations in cytoplasmic free calcium by cold or ion injection result in transient closure of higher plant plasmodesmata

The concentration of cytoplasmic free calcium ([Ca2+](cyt)) required to clo se higher plant plasmodesmata was investigated using corn (Zea mays L. cv. Black Mexican Sweet) suspension-culture cells. Physiological elevations of [Ca2+](cyt) were applied by cold treatment, and ion injection was also used to increase [Ca2+](cyt) by diffusion (for small increases) or by iontophor esis (for larger increases). The impact of such treatments on [Ca2+](cyt) w as measured by ratiometric ion imaging. Intercellular communication during treatments was monitored using our recently developed electrophysiological technique that allows the electrical resistance of plasmodesmata and the pl asma membranes of a sister-cell pair to be measured. A 4-fold increase in t he calculated resistance of single plasmodesmata was observed in response t o cold treatment that caused a 2-fold increase in average [Ca2+](cyt) (from 107 to 210 nM). In response to iontophoresis of Ca2+, plasmodesmata were o bserved to go from "open" (low resistance) to "shut" (high resistance) and then back "open" within 10s. Our results thus indicate that higher plant pl asmodesmata respond quickly to physiological changes in [Ca2+](cyt).