Unravelling the basis of the dual role of TFL1 in reproductive development

December 07, 2023

Reproductive development in plants involves a transition from the vegetative phase during which leaves are continuously produced at the shoot apex, to the reproductive phase marked by the production of inflorescence branches and flowers. Scientists at the Max-Planck Institute for Plant Breeding Research in Cologne have used morphological characterization coupled with protein expression patterns and gene expression profiling to investigate how a regulatory protein called TERMINAL FLOWER 1 carries out two distinct functions at the shoot apex during flowering in the model species Arabidopsis thaliana.

Figure 1. The shoot apical meristems of tfl1 mutant plants enlarge more rapidly than those of wild type. Top and side views of the shoot apical meristem of wild-type (Col-0, above) and tfl1 mutant plants (below) after growth in 8 long days (LDs). The areas of the cells in the tfl1 mutant apex increase and the meristem enlarges before that of wild type, because tfl1 flowers earlier. — **Figure 1. The shoot apical meristems of *tfl1* mutant plants enlarge more rapidly than those of wild type.** Top and side views of the shoot apical meristem of wild-type (Col-0, above) and *tfl1* mutant plants (below) after growth in 8 long days (LDs). The areas of the cells in the *tfl1* mutant apex increase and the meristem enlarges before that of wild type, because *tfl1* flowers earlier.

© M. Cerise

**Figure 1. The shoot apical meristems of *tfl1* mutant plants enlarge more rapidly than those of wild type.** Top and side views of the shoot apical meristem of wild-type (Col-0, above) and *tfl1* mutant plants (below) after growth in 8 long days (LDs). The areas of the cells in the *tfl1* mutant apex increase and the meristem enlarges before that of wild type, because *tfl1* flowers earlier.

© M. Cerise

TERMINAL FLOWER 1 (TFL1) is a repressor of gene transcription that is produced at the Arabidopsis plant apical meristem, where it carries out two major functions of preventing flowering during vegetative development, and subsequently maintaining the apical meristem in an active state so that flowers and branches are continually produced in the inflorescence. The aim of the study was to understand how TFL1 carries out these two distinct functions at the molecular-genetic level.

Firstly, Martina Cerise and colleagues from the laboratory of George Coupland characterized changes in morphology of the shoot apical meristem of tfl1 mutants over time and confirmed that it flowered earlier than wild type (Figure 1). Then, they monitored the spatial and temporal localization a fluorescent version of the TFL1 protein by confocal microscopy and demonstrated that it has a dynamic distribution pattern at the shoot apex. During the vegetative phase, TFL1 is located beneath the meristem, but at floral transition, its localization becomes diffuse, before accumulating to the tip of the meristem during inflorescence development (Figure 2).

Figure 2. The pattern of TFL1 distribution at the shoot apex is dynamic and changes according to the age of the plant. The expression of the TFL1 protein (in blue) is below the meristem at the vegetative phase (after growth for 7 long days), before becoming diffuse after 13 long days and then localizing to the shoot apex during the reproductive phase (25 long days). — **Figure 2.** **The pattern of** **TFL1 distribution at the shoot apex is dynamic and changes according to the age of the plant.** The expression of the TFL1 protein (in blue) is below the meristem at the vegetative phase (after growth for 7 long days), before becoming diffuse after 13 long days and then localizing to the shoot apex during the reproductive phase (25 long days).

© M. Cerise

**Figure 2.** **The pattern of** **TFL1 distribution at the shoot apex is dynamic and changes according to the age of the plant.** The expression of the TFL1 protein (in blue) is below the meristem at the vegetative phase (after growth for 7 long days), before becoming diffuse after 13 long days and then localizing to the shoot apex during the reproductive phase (25 long days).

© M. Cerise

Next, the group used RNA sequencing to identify genes that were differentially expressed at the shoot apex of mutants lacking TFL1 in comparison with plants that expressed TFL1, at different time points from the vegetative phase until when the apex started to produce flowers. They identified that TFL1 mainly acts on two different clusters of genes, whose functions reflect the dual functions of TFL1, and they suggest that TFL1 contributes to two modes of gene regulation. The first acts during the vegetative phase and involves the repression of transcription of genes that promote flowering, thereby preventing plants from flowering too early. The second function of TFL1 is to stably and continuously block the expression of genes that promote the development of floral organs. Therefore, TFL1 prevents the meristem from turning into a flower and maintains its indeterminacy, so that it can continuously produce flowers and inflorescence branches.

Finally, the authors used a genetic approach to show that repression of the transcription of SEPALLATA4 gene, which encodes a transcription factor downstream of TFL1, is important for the dual functions of TFL1 in modulating flowering time and meristem maintenance.

Overall, the study shows how a detailed characterization of the dynamic TFL1 protein expression over time could be correlated with gene expression patterns to understand the basis of the roles of TFL1 in regulating flowering time and shoot indeterminacy.

Unravelling the basis of the dual role of TFL1 in reproductive development

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