Mechanism of a Synergistic Effect of Kinetin on Auxin-induced Ethylene Production

Abstract

In hypocotyl segments of mung bean (Phaseolus mungo L.) seedlings, exogenously supplied indoleacetic acid was rapidly conjugated mainly into indoleacetylaspartic acid, which was inactive in inducing ethylene production. Kinetin is known to stimulate indoleacetic acid-induced ethylene production. The mechanism of kinetin action on indoleacetic acid-induced ethylene production by hypocotyl segments of mung bean seedlings was studied in relation to indoleacetic acid uptake and indoleacetic acid metabolism. Kinetin enhanced indoleacetic acid uptake during the initial 2-hour incubation and markedly suppressed the conversion of indoleacetic acid to indoleacetic acid conjugates throughout the whole 7-hour incubation. As a result, there was more free indoleacetic acid and less conjugated indoleacetic acid in the segments treated with kinetin than in those receiving no kinetin. A close relationship was demonstrated between the rate of ethylene production and the level of free indoleacetic acid, which was regulated by kinetin.It is well known that auxins stimulate ethylene production in a wide variety of plant tissues (1-5, 8, 9, 11, 14, 20), and many of the effects of auxin are now attributed to its effects on ethylene production (13,20). Induction of new enzymes is suggested to be involved in this process, based on the observation that inhibitors of protein and RNA synthesis retard auxininduced ethylene production and that the stimulation of ethylene production by auxin has a substantial lag period (1,3,14). Apparently, the ethylene-producing system induced by auxin is labile and is characterized by a rapid turnover rate (9,14).In excised segments of etiolated pea shoots, Kang et al. (9) and others (2,4,5) have shown that IAA-induced ethylene production parallels the free IAA level, which in turn inversely depends upon the rate of IAA conjugation and decarboxylation.Kinetin alone slightly stimulates ethylene production by etiolated seedlings of several species, but a remarkable synergistic effect of kinetin on IAA-induced ethylene production has been observed (4,8,10 This study was undertaken to determine whether kinetin plays a causal role in the uptake and metabolism of IAA. The present paper presents evidence showing that in mung bean hypocotyl segments kinetin suppresses the conversion of IAA into IAA conjugates, the main pathway of IAA metabolism. As a consequence, a higher free IAA level is sustained and is, in turn, responsible for higher ethylene production. MATERIALS AND METHODSPlant Materials and Chemicals. Seeds of mung bean (Phaseolus mungo L.) purchased from a local market were sorted and surface-sterilized with 0.02% sodium hypochlorite solution for 10 min. After thorough washing, the seeds were imbibed and aerated for 12 hr and were then grown in vermiculite for 4.5 days in darkness at 24 C. Under dim green light, 2 cm long segments were cut from hypocotyls at a point 1 cm below the hook. Twenty segments were incubated in 5 ml of incubation medium containing 50 mM KH,PO4 (pH 4.5) and 2% sucrose in...

Cytokinin-to-Auxin Ratios and Morphology of Shoots and Tissues Transformed by a Chimeric Isopentenyl Transferase Gene

“…Free IAA levels in these plants were reduced to an even greater extent than total IAA. These findings are contrary to numerous reports ofenhanced IAA levels resulting from exogenous applications of cytokinins (6,8,10,19) or from transformation of tobacco and carrot with the native ipt gene carried on a mutated Ti plasmid (7). We only observed a significant increase in free IAA in N. plumbaginifolia tissues transformed with the 35S-ipt gene or, to a lesser degree, with the native gene.…”

Interaction of Kinetin and Calcium in Relation to Their Effect on Stimulation of Ethylene Production

“…For instance, the auxin-induced ethylene production parallels the free auxin level (1, 10, 11). Kinetin has been shown to enhance the auxin-stimulated ethylene production by suppressing the auxin-conjugation system (11).…”

“…Seeds of mung bean (Phaseolus aureus Roxb) were grown in vermiculite for 3.5 days in darkness at 24 C. Segments 2 cm long were cut from hypocotyls at a point 1 cm below the hook, as previously described (11). Lots of 20 segments were incubated in 5 ml of a medium consisting of 50 mM potassium phosphate buffer (pH 6), 2% sucrose, and a given concentration of labeled and unlabeled kinetin or Ca2+, as indicated, in a 50-ml Erlenmeyer flask.…”

The rhd6 Mutation of Arabidopsis thaliana Alters Root-Hair Initiation through an Auxin- and Ethylene-Associated Process

“…A small effect on root-hair production was observed with kinetin, but a 100-fold increase in kinetin concentration only increased the relative rhd6 root-hair number from 30% of wild type to 50% of wild type (Table IV). Because cytokinin can induce the production of ethylene (Lau and Yang, 1973) and can act synergistically with auxin to increase ethylene levels (Kondo et al, 1975;Yu et al, 1981), it is possible that the kinetininduced suppression of the rhd6 phenotype is indirect.…”