Grafting of tomato mutants onto potato rootstocks: An approach to study leaf-derived signaling on tuberization

Photoperiod controls many plant developmental responses, including tuber formation in potato (Solanum tuberosum L.) plants. Photoperiodic stimuli are received by phytocromes in the leaves and must be conveyed to the underground portion of the plant for the tubers to develop, but the nature of the signal responsible for this is hitherto unknown. Plant hormones are known to have a role in tuber formation, through a series of complex interactions between them and with other substances. Here, some accessions from the large collection of hormone and photomorphogenic mutants in tomato (Lycopersicon esculentum Mill.) were used to study the process of tuberization through grafting onto potato rootstocks. The chosen photomorphogenic mutants were aurea (au, chromophore deficient), far red insensitive (fri, PHYA deficient), temporary red insensitive (tri, PHYB1 deficient) and high pigment (hp, exaggerated phytochrome response), as well as the hormone mutants gibberellin deficient-1 (gib-1), dwarf (d, brassinosteroid deficient), diageotropica (dgt, auxin insensitive), notabilis (not, ABA deficient), procera (pro, gibberellin hypersensitive). Tuber number, tuber and shoot dry weight and sprouting were quantified as a measure of the tuber induction capability of each genotype. Tomato scions were always less effective to promote tuberization than the potato scions. Among photomorphogenic mutants, the highest tuberization was achieved with the chromophore deficient (au). The tuber induction capability was (in decreasing order) d, gib-1, dgt, not and pro for hormone mutants. A clear-cut negative correlation (r = −0.98) was observed between dry tuber weight and dry shoot weight. Sprouting also varied to a large extent, the most sprouting-inducer was the gibberellin deficient scion. These results lead us to suggest that source-sink relationship, which is affected by both hormones and photomorphogenesis, has a pivotal role in tuber formation and that tomato scions fail to produce a substance(s) involved in the convertion of the stolon into the strong sink that forms the tuber.