TERMINAL FLOWER-1/CENTRORADIALIS inhibits tuberisation via protein interaction with the tuberigen activation complex

Potato tuber formation is a secondary developmental programme by which cells in the subapical stolon region divide and radially expand to further differentiate into starch-accumulating parenchyma. Although some details of the molecular pathway that signals tuberisation are known, important gaps in our knowledge persist. Here, the role of a member of the TERMINAL FLOWER 1/CENTRORADIALIS gene family (termed StCEN) in the negative control of tuberisation is demonstrated for what is thought to be the first time. It is shown that reduced expression of StCEN accelerates tuber formation whereas transgenic lines overexpressing this gene display delayed tuberisation and reduced tuber yield. Protein-protein interaction studies (yeast two-hybrid and bimolecular fluorescence complementation) demonstrate that StCEN binds components of the recently described tuberigen activation complex. Using transient transactivation assays, we show that the StSP6A tuberisation signal is an activation target of the tuberigen activation complex, and that co-expression of StCEN blocks activation of the StSP6A gene by StFD-Like-1. Transcriptomic analysis of transgenic lines misexpressing StCEN identifies early transcriptional events in tuber formation. These results demonstrate that StCEN suppresses tuberisation by directly antagonising the function of StSP6A in stolons, identifying StCEN as a breeding marker to improve tuber initiation and yield through the selection of genotypes with reduced StCEN expression.

Grafting induces flowering time and tuber formation changes in Brassica species involving FT signalling

Brassica species are widely cultivated and important biennial and annual crops. The transition from vegetative to reproductive development in Brassica species is critical in agriculture and horticulture. Grafting is a useful tool for improving agricultural production and investigating the movement of long-range signals. Here we established a hypocotyl micrografting system in B. rapa crops and successfully grafted the rootstock of turnip onto many different scion genotypes. Grafting with turnip rootstock prolonged vegetative growth, delayed flowering and improved seed yield in rapeseed. The late-flowering turnip rootstock could delay flowering of the scion of the early-flowering turnip accession. The BrrFLC1 (FLOWERING LOCUS C1 in B. rapa) transcript levels and H3K4me3 levels at the BrrFLC1 locus were up-regulated and subsequently suppressed the downstream FT (FLOWERING LOCUS T) signals in leaves of the scion to delay flowering. Vernalization treatment can efficiently promote flowering time in turnip. The non-vernalised turnip flowered early after grafting onto the rootstock of the vernalised turnip, which was accompanied by high levels of FT homologue expression in leaves of the scion. Hypocotyl excision experiments revealed that the process of tuber formation was suppressed by removing the hypocotyl tissue, which in turn repressed the expression of tuberization-related genes. Our findings suggest that the rootstock generates mobile signals that are transported from the rootstock to the scion to fine-tune FT signalling and modulate flowering time.