Tissue-specific High Expression of TFL1 Correlates with Citrus Juvenility

Early flowering is a desirable trait for many horticultural crops including citrus, which are woody trees that can have a juvenile phase from several to over 10 years. Research on floral organ development has identified genes critical to the regulation of juvenility/maturity. TFL1 is a negative regulator where disruption of its expression resulted in a shorter vegetative growth phase in Arabidopsis (Shannon and Meeks-Wagner, 1991. A Mutation in the Arabidopsis TFL1 Gene Affects Inflorescence Meristem Development. Plant Cell 3:877-892). In this study, we sequenced TFL1 orthologs in the citrus gene pool from a wide array of species including Citrus reticulata (mandarin), C. grandis (pumello), C. medica (citron), C. micrantha (papeda), C. paradisi (grapefruit), C. sinensis (sweet orange), C. limon (lemon), C. latifolia (Persian lime), Microcitrus australasica and Eremocitrus glauca. This helps to understand the evolution of citrus flowering as influenced by TFL1. The consensus sequences will be used to design gene silencing and editing targets for engineering early flowering citrus. RT-qPCR expression measurements indicated that 1-year old Hamlin sweet orange had much higher levels of TFL1 than the mature trees, and this pattern was observed at the shoot apex tissues. On the other hand, mature Hamlin trees at the ages of 3, 5 and 15 years did not show differences from each other. Fluctuations in expression of TFL1 and other flowering genes were also studied from January to June in both juvenile and mature Hamlin trees.