2 TRANSDUCTION PATHWAYS MEDIATE RAPID EFFECTS OF ABSCISIC-ACID IN COMMELINA GUARD-CELLS

Commelina guard cells can be rapidly closed by abscisic acid (ABA), an d it is thought that this signal is always transduced through increase s in cytosolic calcium. However, when Commelina plants were grown at 1 0 to 17 degrees C, most guard cells failed to exhibit any ABA-induced increase in cytosolic calcium even though all of these cells closed. A t growth temperatures of 25 degrees C or above, ABA-induced closure wa s always associated with an increase in cytosolic calcium. This sugges ts that there may be two transduction routes for ABA in guard cells; o nly one involves increases in cytosolic calcium. Activation of either pathway on its own appears to be sufficient to cause closure. Because the rates of ABA accumulation and transport in plants grown at differe nt temperatures are likely to be different, we synthesized and microin jected caged ABA directly into guard cells. ABA was released internall y by UV photolysis and subsequently caused stomatal closure. This resu lt suggests a possible intracellular locale for the hypothesized ABA r eceptor.