Gibberellin A1 is required for stem elongation in spinach

The effects of the growth retardants 2'-isopropyl-4'-(trimethylammonium chloride)-5'-methylphenyl piperidine-1-carboxylate (AMO-1618) and calcium 3,5-dioxo-4-propionylcyclohexanecarboxylate (BX-112) on stem elongation were investigated in the rosette plant spinach (Spinacia oleracea L.) under long-day (LD) conditions. Stem growth induced by a LD treatment was prevented by both retardants. The inhibition caused by AMO-1618 was reversed by gibberellin A1 (GA1) and GA20, whereas the effects of BX-112 were reversed by GA1 only. Six GAs (GA53, GA44, GA19, GA20, GA1, and GA8) were quantified by gas chromatography-selected ion monitoring using internal standards. Plants treated with BX-112 had reduced levels of GA1 and GA8 and accumulated GA53, GA44, GA19, and GA20. The relative levels of four additional GAs (3-epi-GA1, GA29, GA60, and GA81) were compared by ion intensities only. Relative to GA81, the level of GA29 was decreased by BX-112, whereas the levels of GA60 and 3-epi-GA1 were increased. Transfer of spinach from short-day conditions to LD conditions caused an increase in all identified GAs of the early 13-hydroxylation pathway with GA20, GA1, and GA8 showing the largest increases. These findings support the position that, of the GAs belonging to the early 13-hydroxylation pathway, GA1 is the primary GA active per se for stem elongation in spinach. The increase in endogenous GA1 in plants in LD conditions is most likely the primary factor for stem elongation.