Expression patterns of flowering genes during flower induction and determination in sweet orange (Citrus sinensis L. Osbeck)

In recent years, several genes putatively involved in the regulation of floral induction have been identified in C. sinensis. However, the expression patterns of these genes in response to different treatments known to alter floral induction have not been investigated. As the first level of regulation for the expression of a given phenotype, characterizing transcript levels of C. sinensis flowering genes will be useful for developing models that could enable discovery of mechanisms regulating floral induction in C. sinensis and other species of subtropical origin. This study investigated patterns of transcript accumulation of putative flowering signal genes CsFT and CsSL1, and the floral identity genes CsAP1 and CsLFY in response to several drought, low temperature and gibberellin treatments known to alter floral induction in C. sinensis. Results supported a role for CsFT as an integrator of flowering signals initiated by low temperatures and water deficit whereas CsSL1 was responsive only to signals initiated by low temperatures. Accumulation of CsFT transcripts was proportional to the duration of floral-inductive water deficit and to levels of floral-inductive temperatures. Water deficit reduced CsAP1 and CsLFY transcript accumulation while trees were under water deficit but induced higher levels of CsAP1 and CsLFY transcripts after irrigation was resumed than in well-irrigated trees. The patterns of transcript accumulation of CsAP1 and CsLFY supported a role of these two genes as markers of floral initiation. Based on the patterns of accumulation CsAP1 and CsLFY transcripts, floral determination occurs right after floral induction and initiation of growth is required for their up-regulation. Accumulation patterns of CsAP1 and CsLFY transcripts corresponded to the basipetal gradient of flowering observed in C. sinensis shoots and the initiation of multiple flowering cohorts. Gibberellins and the presence of fruit both had a negative effect on the accumulation of CsFT transcripts and exogenous gibberellins also reduce the accumulation of CsAP1 and CsLFY transcripts in buds. Accumulation of CsFT transcripts changed diurnally, responded quickly to environmental stimuli, and required alternation of light and dark cycles in order to sustain increasing levels of CsFT transcripts accumulation. Results provide initial information about the regulation of flowering in C. sinensis at the transcript level and could be helpful to design models of how flowering is induced and regulated in C. sinensis, other citrus cultivars and other subtropical species.