A Cytokinin-Activating Enzyme Promotes Tuber Formation in Tomato

BACKGROUND Dedicated storage organs in the form of tubers are evolutionary novelties that share a common function but originate in diverse species from different organs. Tubers in potato, Solanum tuberosum, are derived from the swollen tips of specialized basal lateral juvenile shoots, called stolons. Lateral buds of tomato, Solanum lycopersicum, a potato sibling species, only form regular shoots. The evo-devo mechanisms restricting tuber formation to basal juvenile axillary meristems of potato while completely inhibiting it in tomato meristems are not currently understood. RESULTS Ectopic expression of tomato LONELY GUY (LOG1), a cytokinin (CK) biosynthesis gene, imparts potential to the outgrowing juvenile tomato buds to generate, de novo, aerial minitubers (TMTs). TMTs are morphologically, developmentally, and metabolically homologous to aerial potato tubers and display a unique transcriptome with altered hormonal signaling networks. The new hormonal balance stimulates ectopic branching of dormant axillary meristems and loss of apical dominance without disruption of polar auxin transport and obviates the need for specific branching genes. miR156, a master regulator of juvenility, extends tuber-forming potential to distal axillary buds in both wild-type potato and tomato primed by LOG1 signaling. CONCLUSIONS Ubiquitous activation of TLOG1 uncovered a developmentally suppressed tuber-forming potential within tomato axillary meristems. Other meristems in other plants may also carry hidden, suppressed organogenesis potentials. The unlocking of this potential by the activity of a single gene represents a prime example of an evolutionary novelty in the making and suggests that CKs may function as universal regulators of storage-organ formation in plants.