Regulation of gibberellin biosynthesis and stem elongation by low temperature in Eustoma grandiflorum

Heat-induced rosetted Eustoma grandiflorum requires low temperature for induction of stem elongation and flowering. Although heat-induced rosetting is associated with a reduction of gibberellin A₁ (GA₁) content, how thermo-induction affects GA biosynthesis is unclear. Thus, we examined levels of GA, precursors including that of ent-kaurene which is the first committed step in GA biosynthesis. We used uniconazole, an ent-kaurene oxidase inhibitor to estimate the ent-kaurene biosynthesis activity.The accumulation level of ent-kaurene in stems of the cold-treated seedlings was approximately 1.8 times that of the non-cold-treated seedlings, whereas no difference was observed in the leaves. No change was observed in endogenous levels of GA₁ and GA₂₀ in stems of the heat-induced rosetted plants during the cold treatment, whereas their levels increased with stem elongation after transfer to warm conditions. In contrast to the levels of GA₁ and GA₂₀, endogenous levels of ent-kaurene, ent-kaurenoic acid, GA₅₃, GA₄₄ and GA₁₉ in the stems markedly increased at the end of cold treatment. These results indicate that ent-kaurene biosynthesis and its metabolism early in the GA biosynthetic pathway are stimulated by low temperature and, later, the stimulation leads to an increment of endogenous levels of GA₁ which is essential for stem elongation of the heat-induced rosetted E. grandiflorum.