ACID-INDUCED FILAMENT ELONGATION IN IPOMOEA NIL (CONVOLVULACEAE)

Abstract

It has been known for some time that morning glory filaments elongate in response to increases in concentration of gibberellins (Murakami, 1973) and decreases in ethylene production (Koning and Raab, in press), but many other aspects of their growth have remained unstudied. In the present work, the possible role of gibberellin-stimulated proton efflux in filament growth was examined. Although applied gibberellins stimulated extensive filament growth in vitro and the pH of the incubating medium became acidified during growth, gibberellin also induced growth in media buffered at alkaline pH values. Acidic buffers alone elicited only a very small amount ofgrowth. Fusicoccin, a potent stimulator of proton efflux, initially stimulated the rate of filament growth but elicited only a small increment of growth. In fact, continued presence of fusicoccin poisoned sustained gibberellin-induced growth. Vanadate ions, believed to inhibit proton efflux, had little effect upon gibberellin-induced growth except at extremely high concentrations. Based upon these results, it appears that the acid-induced component of growth stimulation by gibberellin is relatively minor in Ipomoea filaments. These results are quite different from those reported for filament elongation in Gaillardia (Koning, 1983a). RELATIVELY FEW STUDIES have examined how flower parts grow to achieve the species-specific array of flower parts necessary for successful pollination, but the fragmentary literature has been reviewed (Koning, 1983a, b, 1984). Filament elongation appears to be regulated by growth substances (Koning, 1983a, 1986). Such regulation could result from increasing concentrations of growth-promoting substances, decreasing concentrations of growth-inhibiting substances in the tissues, changing capability of the tissues to respond to non-limiting levels of endogenous growth substances, or some combination of these. The growth of flower parts can be (but is not always) a very rapid process, and it is already known that many rapid growth responses are accomplished by means of proton efflux into the cell wall milieu (Evans, 1974). Presumably this results in an optimal pH for hydrolytic enzymes present in the cell wall; these weaken its substructure and allow for turgor-driven cell expansion. Plant growth regulators increase the rate of proton efflux from plant cells, and the resulting acid-induced growth represents at least a small part of the mode of action for these I Received for publication 10 December 1985; revision accepted 28 May 1986. The support of the Rutgers Research Council, DNA Plant Technology Corp, and the Bureau of Biological Research is gratefully acknowledged. This project was also supported by a grant from the Charles and Johanna Busch Memorial Fund. The plant materials were identified with the generous assistance of Dan Austin. compounds. It has been suggested that plant growth regulators work solely by means of stimulated proton efflux (Hanson and Trewavas, 1982). The relative importance of acidinduced growth in the mode of action ofgrowth substances in accomplishing growth effects in flower parts is largely unstudied. Acid-induced growth, stimulated by auxin, can account for much, if not all, of the rapid filament elongation in Gaillardia (Koning, 1 983a). The present work examines the role of proton-induced filament growth in Ipomoea nil. The flowering process is probably understood more completely in Ipomoea nil and Ipomoea tricolor than in any other species. Flower induction (Imamura, 1967), differentiation (Bhar, 1970; Nishino, 1976), corolla expansion (Raab and Koning, 1985, in press), filament elongation (Koning and Raab, in press), corolla unfolding (Koning, 1986), and senescence (Kende and Baumgartner, 1974; Kende and Hanson, 1976; Konze et al., 1980) have been examined in some detail. Filament elongation appears to be regulated, at least in part, by gibberellin (Murakami, 1973; Koning and Raab, in press) and ethylene biosynthesis (Koning and Raab, in press). MATERIALS AND METHODS-Plant culturePlants of Ipomoea nil cv. Scarlett O'Hara (the seeds were donated by W. Atlee Burpee, Warminster, PA) were grown in the botany field plot at Rutgers Univ. during the summer and in the greenhouse during the winter. The plants were induced to flower under short-day con-