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Elucidating the mechanisms underlying the formation of storage roots is important for further improvement of sweetpotato. Although the process of storage root formation in sweetpotato has been extensively studied anatomically and physiologically, the genetic and molecular mechanisms remain largely unknown. Recently, extensive gene expression analysis has identified numerous genes differentially expressed between fibrous and storage roots. The results of these analyses agreed with previous anatomical and physiological observations in that genes related to starch and storage protein biosynthesis were up-regulated, and those related to lignin biosynthesis were down-regulated in the storage roots. Also, molecular biological studies and analyses using transgenic sweetpotato plants have suggested that an expansin protein and several transcription factor genes, such as a Dof-type zinc finger protein, MADS-box proteins, and KNOXI proteins, are involved in the characteristic physiological process of storage root formation. Furthermore, ongoing whole-genome sequencing of sweetpotato and the related Ipomoea species is expected to identify genetic differences related to storage root formation. Although these studies are valuable as a first step in elucidating the critical genes and genetic network that control the formation of storage roots, further physiological, genetic, and molecular biological studies are needed to reveal the entire molecular process. Discipline: Biotechnology Additional key words: tuberous root, development, gene *Corresponding author: e-mail mtanaka@affrc.go.jp Received 27 April 2015; accepted 12 January 2016. Introduction Sweetpotato (Ipomoea batatas) is an important food crop with global production totaling about 100 million tonnes (FAO 2013), and shows the highest daily energy yield per hectare among the major food crops available in developing countries (Woolfe 1992). Although sweetpotato originated in the tropical Americas, about 80% of its worldwide production now comes from Asian countries. Rich in vitamins and minerals, and also containing a large amount of starch, the storage roots of sweetpotato are an important source of nutrition, especially in developing countries (Woolfe 1992). In addition to table use, sweetpotato is utilized as an important raw material for food processing and industrial purposes, such as starch and pigment production. Given its outstanding calorie productivity and relatively high tolerance to unfavorable weather and cultivation conditions, sweetpotato is expected to become a candidate crop for feeding the world’s growing population, but its world average yield peaked in the 1990s and has not increased since (FAO 2013). To overcome the yield plateau for this important crop, the physiological and genetic basis of sweetpotato productivity should be clarified. The storage roots of sweetpotato are formed by a secondary thickening of adventitious roots formed on the underground nodes of transplanted stem cuttings. These storage roots accumulate photosynthetic products as a sink organ throughout the cultivation period of sweetpotato. This long duration of dry matter being accumulated in the storage roots is a major reason for the high and stable productivity of sweetpotato (Tsuno and Fujise 1965). Grafting experiments using various sweetpotato cultivars and wild-relative plants (I. trifida) have also suggested that the dry matter productivity of sweetpotato is highly dependent on the sink potential of developing storage roots (Hozyo & Park 1971, Hahn 1977, Nakatani et al. 1988). Furthermore, Kokubu (1973) suggested that the level of starch accumulated in storage roots is related to their anatomical structure.

Recent Progress in Molecular Studies on Storage Root Formation in Sweetpotato (<i>Ipomoea batatas</i>)