Molecular mechanism of TFL1 on the regulation of flowering time in Rosaceae

Abstract:The rose family(Rosaceae), consists of numerous horticultural plants that have significant economic value. The family includes strawberry, raspberry and rose in the Rosoideae subfamily; fruit trees of apple, pear and loquat in the Maloideae subfamily; and peach, plum and apricot in the Prunoideae subfamily. Consisting of herbaceous plants and woody fruits, the rosaceous plants present various modes of flowering, and the different flowering modes directly influence the flowering ornamental period and/or the fruit harvest period, which are both very important in the horticulture industry. Flowering is the vital physiological process for fruiting, and it is the mark for the transition from vegetative to reproductive growth, which is also known as floral transition. In Arabidopsis,a genus in the Brassicaceae family, FLOWERINGLOCUS T(FT) is a key florigen, which integrates the signals regulated by the internal and external environmental cues, and it works at the shoot apical meristem(SAM) to activate the downstream signals for floral transition. TERMINAL FLOWER1(TFL1) is a homologous gene of FT, which is originally reported in Arabidopsis, the function of TFL1 is opposite to FT, as it plays a role in delaying flowering time. TFL1 suppresses the expression of the downstream genes like LEAFY(LFY), flower meristem identity genes APETALA1(AP1) and CAULIFLOWER(CAL), and thereby consequently inhibits flowering. The tfl1 mutant shows determinate inflorescence and early flowering phenotype. FT can bind with another flowering transcriptionfactor FLOWERING LOCUS D(FD) and form a strong activator to promote the expression of downstream genes, however, when FD binds with TFL1, they form to become a powerful suppressor and inhibit flowering. In the process of vegetative growth, TFL1 strongly expresses in the central area of the inflorescence meristem, and suppresses the translation of AP1 and LFY, in order to keep the meristem at an undifferentiated status. In wild strawberry Fragaria vesca, Fv TFL1 highly expresses in the stem apex, young leaves,and vascular bundle in the vegetative stage. The 2bp deletion of Fv TFL1 causes the strawberry to be in a perpetual flowering pattern. In the apple, the Md TFL1 m RNA is detected from the shoot apex meristem in the vegetative growth stage during both the young and mature periods. In mature apple trees, Md TFL1 expresses in early spring bud primordium, and its transcription level increases gradually through the vegetative stage. In early summer, when the flower bud differentiation occurs, the expression of Md TFL1 cannot be detected, and in the shoot apex, the transcription of Md AP1, Md AFL(LFY homologue) and Md FT are up-regulated. The study of the genetic function of TFL1 shows that the transgenic plants get into the mature stage and flower very early after silencing the Md TFL1. A similar result is reported in the research of pear. In loquat, we have cloned two TFL1 homologous genes of Ej TFL1 and Ej TFL2, the expression pattern for them are similar as those of their homologous genes in the above rosaceous fruit trees. In addition,the expression of its downstream genes Ej AP1 and Ej LFY are up-regulated after Ej TFL1 is suppressed along with the emergence of flower bud differentiation. In a recent work, TFL1 homologous gene Ed FT from the Eriobotrya deflexa Nakai forma koshunensis was characterized, and it was able to promote Arabidopsis flowering. The juvenile period is a vital process that differs among the various rosaceous species. In herbaceous plants, like strawberry, they flower in the first growth season, and in woody fruit trees, like apple, pear and loquat, they have to maintain the juvenile state for several years before getting into the reproductive stage. In this study, using rosaceous woody fruit tree tissue culture seedlings, we found that TFL1 is able to regulate the juvenile period of the seedlings. Interesting results have been reported in both apple and pear in which after silencing TFL1, the length of the juvenile period of the young seedlings was shortened and they flowered immediately. To sum up, TFL1 is a key gene regulating flower bud differentiation,and it can keep the inflorescences in an indeterminate growth state, however, the understanding of the regulation mechanism of perennial plant flowering is still limited. In this paper, we reviewed the advances of the molecular mechanism of TFL1 homologous genes on the regulation of rosaceous plants flowering time and the effect of TFL1 on the transformation from the juvenile to mature period. We summarize the expression pattern and genetic function of TFL1 homologues in the processes of flowering transition and from the juvenile stage to adult stage in various rosaceous species. This work provides important information for further study of the molecular mechanism of TFL1 on the regulation of rosaceous plants flowering time and juvenile phase variation. In the future, more studies should be focused on the basic research of the floral transition of the rosaceous species, not only on the expression level of the target genes but also on the genetic functions. Moreover, further research will help to identify more key genes in the control of flowering time in horticultural plants besides TFL1 homologs, in order to provide effective service for the modification of the production traits of horticultural plants.