EXPRESSION OF AN MESSENGER-RNA WITH SEQUENCE SIMILARITY TO PEA DEHYDRIN (PSDHN-1) IN GUARD-CELLS OF VICIA-FABA IN RESPONSE TO EXOGENOUS ABSCISIC-ACID

The function and location of guard cells uniquely subject them to stre ss. First, stomatal movements require large fluctuations in the concen tration of potassium salts. Second, guard cell inner walls are the fir st surfaces exposed to evaporation and apoplastic solutes may accumula te there as a result. We have therefore investigated whether guard cel ls exhibit atypical expression of dehydrin genes because dehydrins acc umulate in vegetative tissues in response to water stress. We have als o assayed far osmotin mRNA, which is up-regulated in leaves in respons e to various stresses. mRNA probes for several representative genes we re used with RNA extracts from control and water-stressed Vicia faba l eaflets. Correlatively, these probes were used with RNA extracts from ''isolated'' guard cells that had been incubated with combinations of abscisic acid, mannitol and Ca2+. (Isolated guard cells are epidermal strips sonicated to destroy cells other than guard cells.) Hybridizati on with the probe prepared for a dehydrin from Pisum sativum (Psdhn I) was detected in leaf extracts only if the leaf had been stressed. Sim ilarly, after 1- and 6-h incubations with abscisic acid, isolated guar d cells contained an mRNA that hybridized with the probe for Psdhn 1. Appearance of this abscisic acid-dependent mRNA required neither manni tol nor exogenous Ca2+. Regardless of the conditions or tissue, no hyb ridization was detected with the probe against osmotin, but our interp retation of this result is qualified. The simplest conclusion is that atypical expression of dehydrin is not the mechanism by which guard ce lls cope with their peculiar function and location.