Effects of gibberellin treatment during flowering induction period on global gene expression and the transcription of flowering-control genes in Citrus buds.

Gibberellins (GAs) affect flowering in a species-dependent manner: in long-day and biennial plants they promote flowering, whereas in other plants, including fruit trees, they inhibit it. The mechanism by which GAs promote flowering in Arabidopsis is not fully understood, although there is increasing evidence that they may act through more than one pathway. In citrus, GA treatment during the flowering induction period reduces the number of flowers; the mechanism of flowering inhibition is not clear; the hormone may act directly in the bud to determine its fate toward vegetative growth, generate a mobile signal, or both. However, bud metabolic and regulatory pathways are expected to be altered upon GA treatment. We investigated the effect of GA treatments on global gene expression in the bud during the induction period, and on the expression of key flowering genes. Overall, about 2000 unigenes showed altered expression, with about 300 showing at least a two-fold change. Changes in flavonoids and trehalose metabolic pathways were validated, and among other altered pathways, such as cell-wall components, were discussed in light of GA's inhibition of flowering. Among flowering-control genes, GA treatment resulted in reduced mRNA levels of FT, AP1 and a few flower-organ-identity genes. mRNA levels of FLC-like and SOC1 were not altered by the treatment, whereas LEAFY mRNA was induced in GA-treated buds. Surprisingly, FT expression was higher in buds than leaves. Overall, our results shed light on changes taking place in the bud during flowering induction in response to GA treatment.