Cell Growth and Cellulases: Regulation by Ethylene and Indole-3-acetic Acid in Shoots of Pisum sativum

Abstract

ETHYLENE is produced by most plant tissues and has been shown to have a regulatory role in plant growth^1–3. The highest production in vegetative tissues occurs in parts which are rapidly growing; parts which are also the regions containing the most auxin. Ethylene is, however, a potent inhibitor of shoot growth. In actively growing parts, concentrations in air as low as 1 p.p.m. will reduce or arrest cell extension and, following this, there may be lateral cell expansion resulting in a swelling of the affected tissue. These effects are visible in etiolated pea seedlings within 24 h. Swelling of stem tissue is also a typical response to treatment with an auxin. Ethylene production can be stimulated several-fold by supplying an auxin, and so it has been suggested that many of the morphological and physiological changes brought about by auxins are the result of an induced production of ethylene^4–7. Thus when pea seedlings are decapitated immediately below the apical hook and indole-3-acetic acid (IAA) applied to the cut surface, knob-like swellings develop⁸ which appear to be similar to those induced by ethylene in intact plants. Fan and Maclachlan⁹ have found that this IAA-induced swelling is accompanied by a marked increase in soluble cellulase activity. If, as Burg¹⁰ suggested, both lateral expansion and the increase in cellulase are the result of IAA-induced production of ethylene, then similar increases in cellulase would be expected in tissue where swelling is produced by treatment with ethylene alone. To investigate the relationship between auxin, ethylene and cellulase in these growth responses, we have studied the effects of both IAA and ethylene on soluble enzymes which degrade cellulose in etiolated pea seedlings. Although IAA stimulates the production of ethylene in the swollen apical tissues of decapitated peas by some 280 per cent, our results show that the effects of IAA cannot be attributed to an enhanced production of ethylene.