Characteristics of the Induction of the Accumulation of Proline by Abscisic Acid and Isobutyric Acid in Detached Rice Leaves

Abstract

The effects of abscisic acid and isobutyric acid on levels of proline in detached rice leaveswere compared. The lowest concentration of abscisic acid that induces accumulation of prolinein detached rice leaves was much lower than that of isobutyric acid. The accumulation of porlineinduced by isobutyric acid was associated with acidification of the cell sap, whereas abscisic acidincreased levels of proline without decreasing the pH of the cell sap. Potassium chloride en-hanced the accumulation of proline that was induced by abscisic acid, but it did not stimulate thatinduced by isobutyric acid. Of particular interest is the finding that detached rice leaves treatedwith fusicoccin showed an increase in levels of proline and a decrease in the pH of the cell sap. Asynergistic increase in levels of proline was observed when fusicoccin and abscisic acid appliedsimultaneously. However, fusicoccin had no effect on the induction of the accumulation of pro-line by isobutyric acid. Benzyladenine inhibited the induction by abscisic acid of the accumula-tion of proline, but it did not inhibit the induction by isobutyric acid. It is concluded that themode of action of abscisic acid in inducing the accumulation of proline differs from that ofisobutyric acid. Our results also indicate that factors other than acidification of the cell sap are in-volved in regulation of the accumulation of proline.Key words: Abscisic acid — Isobutyric acid — Proline accumulation — Oryza sativa.Recent reports have suggested that a decrease in in-tracellular pH, measured as the pH of the cell sap, is involv-ed in stress-, ABA- and IBA-induced accumulation of pro-line in wheat coleoptiles and segments of barley leaves(Pesci and Beffagna 1984, 1985, Goring and Plescher1986). Venekamp (1989) showed that organic acids are thesource of drought-induced synthesis of proline in beanplants in the field. Previously, we reported that proline ac-cumulated in detached rice leaves during dark-induced se-nescence (Wang et al. 1982). More recently, we reportedthat secretion of protons from detached rice leaves plays aregulatory role in senescence (Chen et al. 1990). Secretionof protons decreases during senescence of detached riceleaves. The present investigation was conducted to com-pare the effects of ABA and IBA on the levels of proline indetached rice leaves and to test the hypothesis that ABA-and IBA-induced changes in levels of proline might be me-diated by changes in intracellular pH, as proposed by Pesciand Beffagna (1985).