Effects of auxin and abscisic acid on cytosolic calcium and pH in plant cells

Dark-grown corn coleoptiles and parsley hypocotyls and their roots were loaded with acetoxymethyl esterified forms of the Ca²⁺ indicator fluo-3, and the pH indicator 2',7'-bis(2-carboxyethyl)-5(and -6)-carboxyfluorescein. These tissues were treated with the plant growth regulator 2,4-dichlorophenoxyacetic acid (2,4-D), an auxin analogue, or abscisic acid (ABA), and the cytosolic pH (pH(cyt)) and cytosolic Ca²⁺ ([Ca²⁺](cyt)) changes were monitored by confocal scanning optical microscopy. Over a period of 4 min pH(cyt) decreased 0.1-0.2 pH unit and [Ca²⁺](cyt) increased from 280 to 380 nM in response to 2,4-D. ABA, on the other hand, induced cytosolic alkalinization of 0.05-0.1 pH unit with a concomitant increase in [Ca²⁺](cyt) from 240 to 320 nM over a 4-min period. Responses similar to these were observed in all the tissues tested. We suggest that pH(cyt) profoundly influences signaling by [Ca²⁺](cyt), possibly by regulating Ca²⁺-protein binding, and that the divergent effects of auxin and ABA on pH(cyt) underlie their mutual antagonism.