Tuber induction in potato (Solanum tuberosum L.) is a complex, multilevel process, which integrates environmental and internal signals to ensure optimal life strategy during the growing season (reviewed in [34, 37]). Environmental requirements for tuberization vary among potato subspecies and varieties. S. tuberosum ssp. andigena strictly requires short-day photoperiod for tuber formation, however, andigena plants with inactivated phytochrome B gene (involved in photoperiod sensing) tuberize even under continuous light . S. tuberosum ssp. tuberosum plants are less dependent on day-length. Generally, photoperiodic signal is integrated with other environmental factors, such as nitrogen availability, temperature and light intensity, as well as with the overall metabolic status of the plant, so that the plants can tuberize even under long-day photoperiods (reviewed in ).
Plants exposed to conditions that favor tuberization display significant metabolic and growth changes in both above- and underground organs. Soon after transfer to inducing conditions, the rate of photosynthetic assimilation, starch synthesis and sucrose export from leaves substantially increase . Underground stolons of induced plants stop elongating and begin to swell, eventually forming tubers . Active storage of carbohydrates in tubers leads to reduced vegetative growth, flowering and fruiting (reviewed in ).
In addition to stolons, practically any axillary bud on stems or stem cuttings can form a tuber, provided the plant has been induced to tuberize. Depending on the degree of induction, buried axillary buds on single-node cuttings cultured in vivo form tubers, either sessile ones or attached to stolon tips (reviewed in ). Single-node cuttings tuberize synchronously when cultured in vitro in darkness on a medium with reduced nitrogen content and optimal sucrose concentration [15, 48]. However, only cuttings taken from induced plants develop tubers directly. Cuttings from non-induced plants form primarily long shoots/stolons instead of tubers irrespective of the sucrose concentration in the medium .
Tuber initiation, either on intact plants or single-node cuttings, was demonstrated to be under the coordinated control of many plant hormones including gibberellins, cytokinins, abscisic acid, jasmonic acid and others. Although the evidence for involvement of some hormones remains controversial (e.g. ), the role of gibberellins as key negative regulators has been established as unequivocal [6–8, 34]. Exogenous application of gibberellins promoted stolon elongation and inhibited tuber formation, whereas a drop in gibberellin level preceded the first visible signs of swelling in stolon apices . Besides being dominant regulators of stolon-to-tuber transition, gibberellins also play a role in the photoperiodic control of tuberization. Reduced gibberellin levels were shown to accompany changes in morphology, metabolism and gene expression in leaves of plants induced to tuberize [27, 6, 1, 34]. However, while photoperiod sensing and gibberellin signaling are interconnected in many respects , they are considered to inhibit tuberization at least partially through independent pathways .
In the present work, we have characterized a novel mutant of potato (Solanum tuberosum L. ssp. tuberosum), which displays a strong tendency to spontaneous tuberization (ST) under both in-vitro and in-vivo conditions. The phenotype of the ST mutant has been analyzed in detail at the morphological, biochemical and molecular levels. Because the number of available potato mutants or genetically modified lines with altered tuberization is limited, the ST mutant provides a rare and useful tool for studying various aspects of tuber induction. Based on the characterization of this ST mutant, we propose a revised model of the regulatory roles of sucrose and gibberellins in the inductive mechanism of potato tuberization.