Comparative gibberellin relationships in tall and dwarf peas (Pisum sativum L.) Public Deposited

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  • Experiments were performed to determine whether a direct correlation exists between the growth rates of tall and dwarf peas grown either in the light or in the dark and endogenous gibberellin (GA) content. Three separate but related experimental approaches were utilized: (1) the determination of possible qualitative and /or quantitative differences in the GA's present in etiolated and light-grown tall and dwarf peas through extraction and bioassay techniques; (2) the detemination of possible variations in sensitivity of tall and dwarf peas to Amo-1618, a known inhibitor of GA biosynthesis; and (3) the development of a cell-free system from peas capable of synthesizing one or more GA's or tetracyclic intermediates in the pathway of GA biosynthesis. The results of the GA extraction experiments were somewhat variable, but they indicated that etiolated tall peas contained more presumptive GA₅ than did etiolated dwarf peas and that light-grown tall and dwarf peas contained approximately equal levels of presumptive GA₅. However, results of one experiment suggested that dwarf pea seedlings grown in the light actually contained more presumptive GA₅ than did tall pea seedlings grown under the same environmental conditions. Due to the variability between experiments it was concluded that results obtained from extraction and bioassay techniques may not quantitatively reflect endogenous GA relationships. Other results which suggested that dwarf pea seedlings synthesize less GA than tall pea seedlings when both are grown in the dark were obtained by a seed treatment with Amo-1618. The subsequent growth of seedlings of the dwarf variety was inhibited more on a percentage basis than seedlings of the tall variety. However, when the shoot tips of six-day-old etiolated tall and dwarf seedlings were treated with various dosages of Amo-1618, no difference in the sensitivity was observed for the two varieties. Thus it appeared that there was no difference in the rate of GA biosynthesis between established etiolated seedlings of tall and dwarf peas. Experiments indentical to those conducted with etiolated plants utilizing Amo-1618 were performed on light -grown peas. The results of these experiments showed that while seedlings of the tall variety were quite sensitive to the growth retardant, seedlings of the dwarf variety were hardly affected. Therefore these experiments with light -grown plants were inconclusive in determining whether the differences exhibited in the growth rates of light-grown tall and dwarf peas are due to differences in endogenous GA biosynthesis. A hypothesis has been proposed to explain the phenomenon of photoinhibition of stem elongation in tall and dwarf pea seedlings. According to this hypothesis, GA₁ is the limiting factor in growth and GA₅ is its immediate precursor. In complete darkness the conversion of GA₅ to GA₁ is uninhibited, but when seedlings are exposed to white or red light an inhibitor is produced which blocks the conversion of GA₅ to GA₁. Seedlings of the dwarf variety contain higher concentrations of this inhibitor than do seedlings of the tall variety, and this inhibitor is at least partially responsible for the difference in growth rates between tall and dwarf peas which are grown in the light. The biosynthesis of (-)-kaurene from 2-¹⁴C-mevalonate in cell-free extracts of pea seeds was investigated. (-)-Kaurene was identified as a product of the reactions by: (1) comparison with authentic (-)-kaurene on thin-layer and gas-liquid chromatography and (2) oxidation of the presumptive ¹⁴C-kaurene and (-)-kaurene with osmium tetroxide to form the common derivative kaurane-16, 17-diol. The enzyme system which synthesized (-)-kaurene required the presence of ATP, and Mg²⁺ or Mn²⁺ with Mn²⁺ being the better divalent cation activator. The apparent rates of (-)-kaurene biosynthesis were the same in extracts prepared from developing seeds of tall and dwarf peas. The growth retardants Amo-1618 and CCC were found to inhibit (-)-kaurene biosynthesis. CCC was a much less effective inhibitor than Amo-1618, with approximately 1000-fold higher concentrations of CCC than Amo-1618 being required to cause similar percentages of inhibition.
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