Patterns of accumulation of CsFT, CsAP1 and CsLFY transcripts in leaves and buds are related to flowering gradients and cohorts in sweet orange

In sweet orange trees (Citrus sinensis), most flowers and inflorescences form basipetally in one year-old shoots, with about 75% of newly formed inflorescences borne on the three most distal nodes. Furthermore, in humid subtropical climates, climatic events during the floral induction period often result in the induction of multiple cohorts of inflorescences in the following spring. In this work we investigated whether patterns of accumulation of floral promoting CsFT, or floral identity genes CsAP1 and CsLFY are related to formation of the basipetal flowering gradient and cohorts. Levels of CsFT were not different in leaves located at different positions within the shoot, suggesting that there is no basipetal gradient in the generation of the putative flowering stimulus encoded by CsFT. Conversely, accumulation of CsAP1 and CsLFY occurred in a basipetal gradient in induced buds and accumulated in pattern consistent with the initiation of flowering cohorts. Our results suggest that the establishment of the flowering gradient and cohorts might be determined at the buds.