The control of carbon allocation, storage and usage is critical for plant growth and development and is exploited for both crop food production and CO2 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 adverse environmental conditions and animal foraging. We show that BRANCHED1b (BRC1b) acts as a tuberization 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 tuberization. In aerial axillary buds, BRC1b promotes dormancy, abscisic acid responses and a reduced number of plasmodesmata. This limits sucrose accumulation and access of the tuberigen protein SP6A. BRC1b also directly interacts with SP6A and blocks its tuber-inducing activity in aerial nodes. Altogether, these actions help promote tuberization underground.
The work of P.C. was funded by BIO2014-57011-R (MINECO), BIO2017-84363-R (Spanish Ministry of Science and Innovation) (MCIN/AEI/10.13039/501100011033/) and FESF investing in your future. The work of S.P. was funded by BIO2015-73019-EXP (Spanish Ministry of Science and Innovation) (MCIN/AEI/10.13039/501100011033/), ERA-NET COSMIC EIG CONCERT-Japan (PCIN-2017-032) (Spanish Ministry of Science and Innovation) and European Union H2020 ‘ADAPT’ project. The work of R.T.-P. and J.C.O. was funded by CSIC-202020E079 (Spanish National Research Council). The work of V.W. was funded by BMBF (031B0191), DFG (SPP1530: WA3639/1-2, 2-1) and Max-Planck-Society. M.N. had an Excellence Severo Ochoa contract (MINECO, SEV-2013-0347). The CNB is a Severo Ochoa Center of Excellence (MINECO award SEV 2017-0712).