Multiple Signaling Pathways Control Tuber Induction in Potato

Potatoes (tubers of Solanum tuberosum) are grown and eaten in more countries than any other crop, and in the global economy they are the fourth most important crop after the three cereals maize, rice, and wheat. Therefore, research into potato tuber initiation and development, which enables our understanding and possible manipula- tion of these processes, is of great relevance. In addition to improving the yield and quality of potato harvests and increasing resistance to pathogen infection, research is also directed at improving the nutritional content of the tuber, and "pharming" which is removing the starch in the potato tuber and instead producing organic compounds such as proteins that are too expensive or cannot be produced in bacterial or yeast culture systems. Research on potatoes has many advantages in that they are easily transformable and therefore amenable to genetic manipulation, and can be propagated rapidly both in tissue culture and through cuttings. Also, microtubers can be induced to form in tissue culture and are used in experi- mental systems in some laboratories. Other laboratories have used stem cuttings as small models of the whole plant. Last but not least, the potato is very closely related to the tomato, for which there is a good genetic map. The main drawback to the use of potatoes in research is the fact that most potato species are polyploid, which means that classical genetic experiments cannot be performed. What is a potato tuber? It is not formed from a root, as is often supposed, but from an underground stem called a stolon. In conditions that are noninductive for tuberization, e.g. LD, the stolons often grow upward and emerge out of the soil to form a new shoot (Fig. 1). In tuber-inducing conditions, e.g. SD, however, the stolons grow under- ground until the tip of the stolon swells to form the tuber. The swelling is caused when the stolon ceases to elongate and cells in the pith and cortex enlarge and divide trans- versely. Later, cells in the perimedullary region enlarge and divide in random orientations to form the bulk of the tissue of the mature tuber. If the plant is put back into noninducing conditions after a tuber has been formed, the plant loses its induced state, and after a lag of up to 2 weeks stolon growth may resume from the tuber. Stolon forma- tion occurs in both tuber-inducing and noninducing con- ditions; however, the angle and amount of stolon growth has been correlated with the strength of the inductive signal. Very strong induction results in "sessile" tuber formation with no prior stolon growth (Fig. 2; Van den Berg et al., 1996). Tubers can actually form on other parts of the plant above ground, normally from axillary nodes on the stem, and in specific circumstances they can even form from flowers (Ewing and Struik, 1992). These aerial tubers are usually formed only on injured or diseased plants, where translocation of assimilates below ground has been pre- vented, or in plants grown in very strong inducing conditions. This Update cannot possibly summarize all of the knowl- edge available about potato tuberization, much of which can be found elsewhere (Li, 1985; Ewing and Struik, 1992, and refs. therein). Therefore, it will principally focus on the role of the environment and possible hormonal signals involved in the induction of tuberization rather than on the postinduction processes such as starch and storage protein accumulation that occur during tuber formation.