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Abstract

The onset of flowering is a crucial developmental transition in the life cycle of higher plants. Among the factors controlling the floral transition, temperature is of key importance. One gene that is known to contribute to the thermal regulation of flowering is the MADS-domain transcription factor FLOWERING LOCUS M (FLM). The FLM transcript is expressed throughout the plant and subject to alternative splicing. Here we show that two splice variants of FLM, FLMbeta and FLMdelta, which alternatively include either the 2 nd or the 3 rd exon, change in their relative abundance in response to changes in ambient temperature. Interestingly, the proteins encoded by the two FLM splice forms have opposite effects on flowering time. Whereas constitutive expression of FLMbeta delayed flowering, which is in line with the published function of FLM as a floral repressor, 35S:FLMdelta lines flowered significantly earlier than Col-0 control plants. BiFC and yeast two-hybrid analyses showed that the two FLM isoforms are able to form both homo- and heterodimers, suggesting that FLMdelta might exert its dominant effect on flowering by interfering with FLMbeta function. To identify FLM target genes, ChIP-seq was performed at 16°C, when FLMbeta is the dominant splice variant, on a genomic GFP-tagged FLM rescue line. FLM was found to directly interact with regulatory elements in the promoters of several flowering time and flower patterning genes, including SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1, APETALA1 and 2, and SEPALLATA3. In order to investigate the molecular mechanism that is responsible for the opposite effect that FLMbeta and FLMdelta have on flowering, spliceform-specific ChIP-seq experiments are currently being under way. Our results demonstrate that alternative splicing can give raise to proteins with markedly different activities and highlight the potential of alternative splicing as a mediator of plasticity in gene function.