The MADS-box transcription factor SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1 (SOC1) integrates environmental and endogenous signals to promote flowering in Arabidopsis. However, the role of SOC1 homologs in regulating flowering time in fruit trees remains unclear. To better understand the molecular mechanism of flowering regulation in loquat (Eriobotrya japonica Lindl.), two SOC1 homologs (EjSOC1-1 and EjSOC1-2) were identified and characterized in this work. Sequence analysis showed that EjSOC1-1 and EjSOC1-2 have conserved MADS-box and K-box domains. EjSOC1-1 and EjSOC1-2 were clearly expressed in vegetative organs, and high expression was detected in flower buds. As observed in paraffin-embedded sections, expression of the downstream flowering genes EjAP1s and EjLFYs started to increase at the end of June, a time when flower bud differentiation occurs. Additionally, high expression of EjSOC1-1 and EjSOC1-2 began 10 days earlier than that of EjAP1s and EjLFYs in shoot apical meristem (SAM). EjSOC1-1 and EjSOC1-2 were inhibited by short-day (SD) conditions and exogenous GA3, and flower bud differentiation did not occur after these treatments. EjSOC1-1 and EjSOC1-2 were found to be localized to the nucleus. Moreover, ectopic overexpression of EjSOC1-1 and EjSOC1-2 in wild-type Arabidopsis promoted early flowering, and overexpression of both was able to rescue the late flowering phenotype of the soc1-2 mutant. In conclusion, the results suggest that cultivated loquat flower bud differentiation in southern China begins in late June to early July and that EjSOC1-1 and EjSOC1-2 participate in the induction of flower initiation. These findings provide new insight into the artificial regulation of flowering time in fruit trees.

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TERMINAL FLOWER1 (TFL1), a key factor belonging to the phosphatidyl ethanolaminebinding protein (PEBP) family, controls flowering time and inflorescence architecture in some plants. However, the role of TFL1 in loquat remains unknown. In this study, we cloned two TFL1-like genes (EjTFL1-1 and EjTFL1-2) with conserved deduced amino acid sequences from cultivated loquat (Eriobotrya japonica Lindl.). First, we determined that flower bud differentiation occurs at the end of June and early July, and then comprehensively analyzed the temporal and spatial expression patterns of these EjTFL1s during loquat growth and development. We observed the contrasting expression trends for EjTFL1s and EjAP1s (APETALA 1) in shoot apices, and EjTFL1s were mainly expressed in young tissues. In addition, short-day and exogenous GA 3 treatments promoted the expression of EjTFL1s, and no flower bud differentiation was observed after these treatments in loquat. Moreover, EjTFL1s were localized to the cytoplasm and nucleus, and both interacted with another flowering transcription factor, EjFD, in the nucleus, and EjTFL1s-EjFD complex significantly repressed the promoter activity of EjAP1-1. The two EjTFL1s were overexpressed in wild-type Arabidopsis thaliana Col-0, which delayed flowering time, promoted stem elongation, increased the number of branches, and also affected flower and silique phenotypes. In conclusion, our results suggested that EjTFL1-1 and EjTFL1-2 do not show the same pattern of expression whereas both are able of inhibiting flower bud differentiation and promoting vegetative growth in loquat by integrating GA 3 and photoperiod signals. These findings provide useful clues for analyzing the flowering regulatory network of loquat and provide meaningful references for flowering regulation research of other woody fruit trees.

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