Seasonal fluctuations in day length regulate important aspects of plant development such as the flowering transition or, in potato (Solanum tuberosum), the formation of tubers. Day length is sensed by the leaves, which produce a mobile signal transported to the shoot apex or underground stems to induce a flowering transition or, respectively, a tuberization transition. Work in Arabidopsis, tomato and rice (Oryza sativa) identified the mobile FLOWERING LOCUS T (FT) protein as a main component of the long-range 'florigen', or flowering hormone, signal. Here we show that expression of the Hd3a gene, the FT orthologue in rice, induces strict short-day potato types to tuberize in long days. Tuber induction is graft transmissible and the Hd3a-GFP protein is detected in the stolons of grafted plants, transport of the fusion protein thus correlating with tuber formation. We provide evidence showing that the potato floral and tuberization transitions are controlled by two different FT-like paralogues (StSP3D and StSP6A) that respond to independent environmental cues, and show that an autorelay mechanism involving CONSTANS modulates expression of the tuberization-control StSP6A gene.

The CONSTANS-FT pathway defines a core module for reproductive transition in both long-day (LD) and short-day (SD) plants. Changes in the transcriptional function of the CONSTANS (CO) protein have been proposed to mediate differential SD activation of FLOWERING LOCUS T (FT) orthologs in SD plants. Potato Andigena genotypes have an obligate SD requirement for tuber formation, and this photoperiodic response correlates with activation of the FT StSP6A gene in leaves. The potato StCOL1 factor represses expression of this mobile tuberization signal, but the control mechanism is poorly understood. Here, we analyzed StCOL1 diurnal oscillation and protein accumulation at different photoperiods and light wavelengths. We observed that the potato StCOL1 gene peaked at dawn and that, in contrast to the Arabidopsis AtCO homolog, the light receptor phyB is necessary for protein stabilization in the light. Reduced StCOL1 levels in RNAi lines strongly correlated with downregulated expression of an additional potato FT family member, StSP5G. Co-regulated StCOL1 and StSP5G expression suggested that StCOL1 activates this target directly rather than controlling StSP6A expression. By hybridization of a universal protein-binding microarray, we established that StCOL1 binds a TGTGGT element, and we found that immunoprecipitated StCOL1 protein fractions were enriched in StSP5G promoter fragments bearing this element. We show that StSP5G represses tuberization in LD conditions and that this FT-like homolog suppresses StSP6A gene expression. Rewiring StCOL1 transcriptional function from direct activation of the StSP6A inducer signal to the control of an FT-like repressor thus mediates the strict SD requirement of Andigena plants for tuberization.

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The control of carbon allocation, storage and usage is critical for plant growth and development and is exploited for both crop food production and CO 2 capture. Potato tubers are natural carbon reserves in the form of starch that have evolved to allow propagation and survival over winter. They form from stolons, below ground, where they are protected from cold temperatures and animal foraging. We show that BRANCHED1b (BRC1b) acts as a tuberisation repressor in aerial axillary buds, which prevents buds from competing in sink strength with stolons. BRC1b loss of function leads to ectopic production of aerial tubers and reduced underground tuberisation. In buds, BRC1b promotes dormancy, ABA signalling and downregulation of plasmodesmata gene expression. This limits sucrose unloading and access of the tuberigen factor SP6A to axillary buds. Moreover, BRC1b directly interacts with SP6A and blocks its tuber-forming activity in aerial nodes. Altogether these actions help promote tuberisation underground.A central question in plant biology is how carbon partitioning is regulated in response to changing environmental conditions, and how this control is integrated with developmental programs. Plant meristems, groups of undifferentiated, actively dividing cells, are sink organs that use sucrose for respiration, growth and development. Under low sucrose availability, they remain quiescent or dormant. Axillary meristems, shoot meristems located in the leaf axils, can give rise to aerial branches with orthotropic growth that produce leaves, inflorescences and flowers. They can also generate stolons, specialised shoots, homologous to branches, that grow horizontally on the soil surface or below ground and have the potential to generate new clonal plants, thus being strategic for vegetative propagation. Branches and stolons differ in their developmental responses to external stimuli, such as day length (see below).Solanum tuberosum (potato) is one of the best-studied stolon-forming species, as their underground stolons develop energy-rich, starch-accumulating tubers, which constitute a large fraction of the dietary intake for many people worldwide. In the potato landrace ssp. andigena, the stolons become strong sinks when winter approaches, and form tubers at their sub-apical region. Tubers overwinter buried in soil, where they are protected from cold and animal foraging, to generate entirely new plants in spring.Tuberisation is controlled by the FLOWERING LOCUS T (FT)-like SELF-PRUNING 6A (SP6A) gene. In potato ssp. andigena, activation of this gene is fully dependent on short days (SD). In long days (LD), the CONSTANS-like1 (COL1) transcription factor represses SP6A expression through the activation of another FT-like gene, SP5G 1,2 . In SD, lack of SP5G allows SP6A activation, and the SP6A protein is then phloem-transported from leaves to stolons. At the stolon apices, SP6A form a Tuberigen Activation Complex (TAC) that acts to promote tuber formation 3,4 .Unlike stolons, neither branches nor branch primordia, namely axillar...