Effects of gibberellic acid and environmental factors on cytosolic calcium in wheat aleurone cells

Gibberellins (GAs) control a wide range of physiological functions in plants from germination to flowering. The cellular mechanisms by which gibberellic acid (GA₃) acts have been most extensively studied in the cereal aleurone. In this tissue, alterations in cellular calcium are known to be important for the primary response to GA, which is the production and secretion of hydrolytic enzymes. The extent to which cytosolic Ca²⁺ mediates the early events in GA action, however, is not known. In order to address this question, changes in cytosolic Ca²⁺ in wheat (Triticum aestivum L. cv. Inia) aleurone cells that occur rapidly after treatment with GA were characterized. In addition, GA-induced changes were compared with changes induced by three environmental stimuli that are known to modify the GA response: osmotic stress, salt (NaCl), and hypoxia. The Ca²⁺-sensitive dye fluo-3 was used to photometrically measure cytosolic Ca²⁺. It was found that GA₃ induced a steady-state increase in cytosolic Ca²⁺ of 100–500 nM. This increase was initiated within a few minutes of treatment with GA and was fully developed after 30–90 min. The changes in cytosolic Ca²⁺ that were induced by GA were distinct from those induced by mannitol, NaCl, or hypoxia. Mannitol caused a steady-state decrease whereas NaCl and hypoxia both increased cytosolic Ca²⁺. In the case of NaCl this increase was transient but for hypoxia the increase was prolonged as long as hypoxic conditions were maintained. Gibberellin-induced changes in cytosolic Ca²⁺ were not induced by the inactive GA, GA₈, nor did the GA-insensitive wheat mutant, D6899, respond to active GA₃ with altered cytosolic Ca²⁺. It is concluded that changes in cytosolic Ca²⁺ are an early and integral part of the GA response in aleurone cells. The data also indicate, however, that changes in Ca²⁺ are not sufficient, by themselves, to induce the GA response of aleurone cells.Abbreviations AM acetoxymethyl ester - GA gibberellin - GA₃ gibberellic acid - Mes 2-[N-morpholino]ethanesulfonic acid - PM plasma membraneThe author is very grateful to Dr. T-h. D. Ho for his gift of D6899 grain and to Dr. R. Hooley for supplying the inactive GA₈. This work was supported by National Science Foundation Grant DCB-9206692.