Use of an Acylcyclohexanedione Growth Retardant, LAB 198 999, to Determine Whether Gibberellin A₂₀ Has Biological Activity per se in Dark-Grown Dwarf (le⁵⁸³⁹) Seedlings of Pisum sativum

Abstract

Dwarf (le5839) seedlings of Pisum sativum respond to gibberellin A20 (GA20) in the dark, although the same dosage of GA20 applied to light-grown le5839 seedlings elicits no growth response. The acylcyclohexanedione growth retardant, LAB 198 999, which is known to inhibit gibberellin oxidation and in particular 30-hydroxylation such as the conversion of GA20 to GA1, also inhibits the growth response of dark-grown dwarf (/e5839) seedlings to GA20.Thus, the biological activity of GA20 in the dark appears to be a consequence of its conversion to GA1, even though it is known from studies with light-grown seedlings that the le mutation reduces the conversion of GA20 to GA1.The biochemical basis of the Le/le gene difference in Pisum sativum L. was determined by Ingram et al. (5,6). Lightgrown shoots of the wild type converted GA203 to GA,, whereas the conversion of GA20 to GA1 was greatly reduced in shoots of the dwarf le genotype. Wild-type seedlings responded as well to GA20 as to GA1, whereas in dwarf le seedlings the response to GA20 was much less than that to GA1 (5, 7). It was concluded from this work that the activity of GA20 in light-grown stems is due to its conversion to GA,. GA, is purported to be the primary GA that controls internode growth in pea (5). The observation that the le allele is a leaky mutation accounts for the small response of lightgrown shoots to high doses of GA20 (5).The difference in internode elongation in light-and darkgrown pea seedlings has been described for several cultivars and genotypes (2,(8)(9)(10)14 There are two reports in the literature that the dwarf cv Progress No. 9 responded to GA20 in the dark, even though this GA elicited no growth response in red light when applied in the same manner and at the same dosages (1, 19). It was postulated that the conversion of GA20 to GA, is increased in the dwarf in the dark, although an alternative suggestion, namely that GA20 may have biological activity per se in the dark, was also made in both papers. The results were later rationalized in terms of enhanced sensitivity of dark-grown tissue to GA, (15), although the magnitude of this enhancement does not appear to be sufficient to explain the activity of low dosages of GA20 in the dark.In this study, the possibility that GA20 may have biological activity per se in dark-grown dwarf pea seedlings was investigated by comparing its activity in dark-and light-grown Le and le5839 seedlings in the presence of one of the new acylcyclohexanedione growth retardants (3, 11). These retardants have as their primary mode of action the inhibition of 3f-hydroxylation of GA20 to GA, (12

Effect of the Growth Retardant 3,5-Dioxo-4-butyryl-cyclohexane Carboxylic Acid Ethyl Ester, an Acylcyclohexanedione Compound, on Fruit Growth and Gibberellin Content of Pollinated and Unpollinated Ovaries in Pea

“…This inhibition is thought to be due to competitive interaction with 2-oxoglutarate because of its close structural similarity to the inhibitors (Griggs et al, 1991). ACHD compounds have been used in application experiments to test the activity per se of different GAs (Junttila et al, 1991;Kamiya et al, 1991;Martínez-García and García-Martínez, 1992;Sponsel and Reid 1992a;Zeevaart et al, 1993). The results have shown the importance of 3P-hydroxylation as an activation step and confirmed GA, as the primary active GA, particularly in stem elongation, as found previously with GA metabolic mutants (Phinney, 1984).…”

Gibberellin A1 is required for stem elongation in spinach.

“…(iii) The levels of GA1 and GA8 were lowered by BX-112 while the earlier precursors (GA53, GA44, GA19, and GA20) increased (Table 2). Thus, as was found in studies with rice (13,17,18), wheat (12), pea (19), and Salix (20), the main effect of BX-112 and other acylcyclohexanediones is inhibition of the 3f3-hydroxylation step of GA20 to GA1. This step is also blocked in dwarf mutants of several species: dl in maize (1), le in pea (10), dy in rice (11), and ga4 inArabidopsis (21).…”

Light regulation of gibberellin A1 content and expression of genes coding for GA 20‐oxidase and GA 3β‐hydroxylase in etiolated pea seedlings

“…Light‐induced de‐etiolation involves photomorphogenic changes of the above‐mentioned characteristics, particularly a reduction of the stem elongation rate. Results of early application of GA biosynthesis inhibitors indicate that GAs are also necessary for stem elongation of dark‐grown plants (Kende and Lang 1964; Reid 1983; Sponsel and Reid 1992). The role of GAs in dark‐grown plants, however, has been a matter of controversy (Reid 1988).…”