Molecular and Biochemical Triggers of Potato Tuber Development

Abstract

In the present climate of functional elucidation of entire genomes by technologies comprising mass sequencing, insertional mutagenesis, and RNA expression profiling analysis, the weed Arabidopsis has rapidly established itself as the foremost plant species among plant scientists worldwide. One of the consequences of the focus on Arabidopsis is that an old theme “what is true for Escherichia coli is true for the elephant” is receiving increasing popularity in its modified version “what is true for Arabidopsis is true for all plants.” Although this is undoubtedly true for many aspects of plant development, there are still a substantial number of developmental processes that can only be studied in specific plant systems. The tuber life cycle of potato (Solanum tuberosum) plants represents an example of a developmental system that cannot be studied in model systems such as Arabidopsis. Potato is one of the most important crops worldwide: ranking fourth in annual production behind the cereal species rice (Oryza sativa), wheat (Triticum aestivum), and barley (Hordeum vulgare). Although in Europe and North America the consumption of potatoes is mainly in the form of processed foodstuffs such as fried potatoes and chips, in less-developed countries it represents an important staple food and is grown by many subsistence farmers. The main reasons for the increasing popularity of the potato in third-world countries are the high nutritional value of the tubers combined with the simplicity of its propagation by vegetative amplification. In addition to its clear importance for food and feed, the tuber also represents the starting material for the next generation of plants (so-called seed tubers). It is for this reason that processes related to tuber formation, storage, and sprouting have been studied intensively over many years; but because all potato varieties are true tetraploids and display a high degree of heterozygosity, genetics have played only a minor role in studying this process. However, because the potato is a member of the Solanaceae family, it was among the first crop plants to be accessible for transgenic approaches. Tuberizing stolon tips taken from a single plant. A number of distinct stages in swelling process are apparent, ranging from no swelling (1) to prominent subapical swelling (4). Note the progressive incorporation of the apical bud. Bar = 1 mm. Reprinted from Viola et al. (2001). Tuberizing stolon tips taken from a single plant. A number of distinct stages in swelling process are apparent, ranging from no swelling (1) to prominent subapical swelling (4). Note the progressive incorporation of the apical bud. Bar = 1 mm. Reprinted from Viola et al. (2001). Tuber development and related processes are difficult to study in the field and/or in soil-grown plants due to a low level of synchrony of the tuberization process under these conditions and to the obscuring effect of soil. To circumvent this problem, in vitro methods have been developed that allow synchronous tuber formation to occur with a high frequency (Appeldoorn et al., 1997; Coleman et al., 2001). These systems essentially consist of a single nodal stem explant that will result in the differentiation of the axillary bud into a tuber instead of a leafy shoot when placed on tuber-inducing medium (characterized by a high Suc content and in some instances supplemented with the antigibberellin CCC and a cytokinin) and incubated in darkness. At least at the levels of ultrastructure and the relative activities of enzymes of carbohydrate metabolism (Veramendi et al., 1999), this system has been shown to be comparable to soil-grown tubers. Although the usefulness of these in vitro systems is undisputed, it is important to keep in mind that by their nature they show little or no response to photoperiod, the factor that most influences tuberization. Tuber induction, initiation, enlargement, dormancy, and sprouting represent the typical life cycle of a potato tuber. In the following paragraphs we will follow this developmental scheme to outline our present understanding of the various processes with respect to biochemical and molecular triggers. Some emphasis will be given to transgenic approaches. Environmental factors and potential signaling molecules in the induction of tuberization. The qualifying term given in brackets identifies the condition favoring tuber induction. Environmental factors and potential signaling molecules in the induction of tuberization. The qualifying term given in brackets identifies the condition favoring tuber induction. With respect to the involvement of hormones, there are many reports in the literature describing the importance of gibberellic acid (GA), cytokinin, jasmonic acid and related compounds, or abscisic acid (ABA) for tuber induction. These data are in places somewhat contradictory, however, one clear and substantiated observation is that GA levels decline during tuber induction. Furthermore, in plants in whi