LAUSR

Drought and low temperature are two key environmental factors that induce adult citrus flowering. However, the underlying regulation mechanism is poorly understood. The bZIP transcription factor FD is a key component of the florigen activation complex (FAC) which is composed of FLOWERING LOCUS T (FT), FD, and 14-3-3 proteins. In this study, isolated and characterized of CiFD in citrus found that there was alternative splicing (AS) of CiFD, forming two different proteins (CiFDα and CiFDβ). Further investigation found that their expression patterns were similar in different tissues of citrus, but the subcellular localization and transcriptional activity were different. Overexpression of CiFD DNA sequence (CiFD-DNA), CiFDα or CiFDβ in tobacco and citrus showed early flowering, and CiFD-DNA transgenic plants were the earliest, followed by CiFDβ and CiFDα. Interestingly, CiFDα and CiFDβ were induced by low temperature and drought, respectively. Further analysis showed that CiFDα can form FAC complex with CiFT, Ci14-3-3, and then bind to citrus APETALA1 (CiAP1) promoter and promotes its expression. However, CiFDβ can directly bind to CiAP1 promoter independent of CiFT and Ci14-3-3. These results showed that CiFDβ can form a more direct and simplified pathway which independent of FAC complex to regulate drought-induced flowering through AS. In addition, a bHLH transcription factor (CibHLH96) binds to CiFD promoter and promotes the expression of CiFD under drought condition. Transgenic analysis found that CibHLH96 can promote flowering in transgenic tobacco. These results suggest that CiFD is involved in drought- and low-temperature-induced citrus flowering through different regulatory patterns. This article is protected by copyright. All rights reserved.