Molecular controls of tuberization

Tuber formation in potatoes (Solanum tuberosum L.) is a complex developmental process involving a number of important biological systems. Under conditions of a short-day photoperiod and cool temperature, a transmissible signal is activated that initiates cell division and expansion and a change in the orientation of cell growth in the subapical region of the stolon tip. In this signal transduction pathway, perception of the appropriate environmental cues occurs in leaves and is mediated by phytochrome and gibberellins (GA). Phytohormones also play a prominent role in regulating the morphological events of tuberization activated in the stolon apex. GA, cytokinins, and jasmonate-like compounds have all been implicated in regulating tuber development. High levels of GA are correlated with the inhibition of tuberization, whereas low levels are associated with induction. Transcription factors are proteins that bind to DNA to regulate gene activity and, in some cases, to mediate hormone levels. Several of these DNA-binding proteins are involved in regulating plant growth and meristem development in potato, including tuber formation. One type, designated POTM1, regulates cytokinin levels in potato meristems and controls branching of axillary shoots. Two other types that physically interact, the BEL and KNOX proteins, mediate vegetative development. Transgenic plants that overexpressed BEL and KNOX proteins exhibited enhanced tuber formation even under long-day conditions. KNOX overexpressers exhibited abnormal leaf architecture and dwarfism. These transgenic lines exhibited a decrease in the levels of GA and an increase in cytokinin levels. In addition, the BEL transgenic lines grew more rapidly than wild-type plants. Our results indicate that DNA-binding proteins of potato mediate tuberization by enhancing or repressing the activity of specific target genes.