Precocious flowering of Citrus seedlings and expression analysis of FT/TFL1 family genes and miR156/172 involved in vegetative phase transition

The long juvenile phase of fruit tree species inhibits their rapid breeding. Some Citrus seedlings show precocious flowering with a short juvenile phase, and provides good material for studying the molecular mechanism of phase transition and flowering in fruit trees. Recently, two microRNAs (miR156 and miR172) were reported to control the phase transition in woody plants. In Arabidopsis, the roles of flowering promoter FLOWERING LOCUS T (FT) and flowering repressors TERMINAL FLOWER1 (TFL1) have been well documented. However, little information is available on their homologues in fruit trees. Here, the expression studies were conducted to clarify whether these genes were involved in the precocious flowering of Citrus. We initially compared the flowering of 1-year-old seedlings derived from polyembryonic seeds of grapefruit (Citrus x paradise 'Marsh Seedless') and amanatsu (Citrus natsudaidai). Next, the effects of low temperature and GA3 treatment on flowering were investigated. Grapefruit seedlings showed various levels of precocious flowering, with the flowering rate ranging from 28 to 76% depending on the environmental condition and GA3 treatment. None of the amanatsu seedlings showed precocious flowering under any environmental condition. Chilling treatment (< 15 degrees C) during autumn (Sep. to Nov., in Japan) and subsequent GA3 treatment appeared to enhance the precocious flowering of grapefruit seedlings, though GA3 treatment alone sometimes inhibited precocious flowering. These results suggested that grapefruit has a precocious flowering ability, and the genetic control system for precocious flowering is conserved in grapefruit. The seasonal expression analyses of FT/TFL1 and miR156/172 suggested that the expressional changes of miR156/172 and FT were involved in the phase transition from the juvenile to adult stage, but these genes did not appear to be associated with the genetic control of precocious flowering.