EXPERIMENTS ON THE GROWTH AND INHIBITION OF ISOLATED PLANT PARTS. II. THE ACTION OF SEVERAL ENZYME INHIBITORS ON THE GROWTH OF THE AVENA COLEOPTILE AND ON PISUM INTERNODES

Abstract

that one of the enzymes controlling growth of the Avena coleoptile is probably of sulfhydryl nature, and that its apparent concentration decreases with increasing age of the coleoptile. The growth inhibition caused by the combination of iodoacetate with this enzyme (or co-enzyme) is prevented by several organic acids, and these acids also promote growth when administered alone. It was deduced that the oxidation of the organic acids, probably through a modified form of the Krebs cycle, in some way yields energy for growth through a process inhibited by iodoacetate, or possibly provides a system alternative to that inhibited by iodoacetate, and independent of it. Since iodoacetate is generally regarded as a reagent for sulfhydryl groups, although not strictly 1 Received for publication September 22, 1948. This work was made possible through a grant from the Committee on Growth of the National Research Council, acting for the American Cancer Society, to whom we are much indebted. Acknowledgment is made to Miss Virginia Reardon, Mrs. Jane Behnke Rogers and Mr. Paul Mangelsdorf, Jr., for technical assistance given during this work. specific for them, it was felt desirable to obtain further evidence for the role of a sulfhydryl enzyme in growth. The present paper therefore reports experiments with arsenite and with some phenylmercury compounds, which are considered to be specific for sulfhydryl enzymes. In this connection the effects of certain sulfhydryl compounds and also that of arginine have been studied. To ensure that the growth phenomena studied are not peculiar to the coleoptile, parallel experiments have been carried out on etiolated pea stems. In general the results show that there is little about the oat coleoptile which cannot be generalized to a dicotyledonous stem. MATERIALS AND METHIODS.-The growth of Avena coleoptiles was measured as reported in the previous paper. The sections were held on combs, in most experiments arranged so as to break the surface of the solution. For comparison, a few experiments were carried out with the combs lying on the bottom of the Petri dish, so that the sections were about 1 mm. below the surface ("submerged sections"). All solutions for coleoptile sections contained 1 per cent sucrose and 1 mg. per 1. indoleacetic acid. Where pea stems were used, these were from 7-day old Alaska plants raised in alkaliwashed maple sawdust, or in water, in the dark. The apical 15 or 20 mm. from the third internode was taken just before the next internode had begun extending, and floated on the surface of the solutions studied. Since sucrose has little effect on the growth of pea sections, it was omitted from these solutions. Slit pea curvatures were carried out in the usual way (Went, 1934; Thimann and Schneider, 1938) with six stems (twelve curvatures) in each Petri dish, the curvatures being allowed to develop in the dark room for about 30 hr., and measured by both "stem reference" and "inflection" methods. The curvatures and the straight growth measurements were thus made on exactly comparable material. All plants were raised, and experiments carried out, at 250C. and 85 per cent relative humidity, with red light for the manipulations and darkness at other times. Solutions for test were brought to pH 6.0 in all cases. The arsenite was prepared by dissolving As203 in KOH and neutralizing with H2SO4. RESULTS.-The effect of arsenite on coleoptile sections.-Arsenite is a powerful inhibitor of growth. Its action is exerted at considerably lower concentrations than iodoacetate. Fig. 1, which is from one of many experiments, shows the time course of growth of coleoptile sections with and without arsenite. Inhibition becomes apparent at