An acylcyclohexadione retardant inhibits gibberellin A1 metabolism, thereby nullifying phytochrome‐modulation of cowpea epicotyl explants

The regulation by phytochrome of stem elongation in light‐grown plants depends on gibberellins (GAs). To investigate whether this is mediated by a change in GA metabolism, the effect of the GA biosynthesis inhibitor LAB 198 999 (an acylcyclohexadione derivative) on the end‐of‐day far‐red (FR) response in cowpea (Vigna sinensis L.) epicotyl explants has been investigated. Growth of epicotyl explants of light‐grown seedlings was enhanced when treated with far‐red light before incubation in the dark (end‐of‐day FR effect). Low doses of LAB 198 999 (0.05 and 0.5 μg explant−1) reduced the effect of FR, whereas 5 to 50 μg explant−1 stimulated elongation of both red light (R)‐ and FR‐treated epicotyl explants while nullifying the differences between R and FR treatments. In paclobutrazol‐treated epicotyl explants, FR enhanced the response to applied GA1 and GA20, whereas LAB 198 999 increased the activity of GA1 and decreased that of GA20, [3H]Gibberellin A1, injected into the basal part of the epicotyl, was transported and metabolized mainly to [3H]GA8 in the apical 20 mm of the epicotyl. The conversion of [3H]GA1 to [3H]GA8 was dramatically reduced by both end‐of‐day FR treatments and LAB 198 999 applications. In addition, both treatments enhanced epicotyl elongation. It is proposed that the regulation of cowpea epicotyl growth by phytocrome is mediated, at least partially, by modifying GA1 degradation.