Growth hormones play a role in the

Growth hormones play a role in the formation and thickening growth of storage roots. Cytokinin and auxin [indole-3-acetic acid (IAA)] levels have been found to be high during the early stage of storage root formation in sweetpotato (Akita et al., 1962; Matsuo, 1983; Nakatani and Komeichi, 1992), suggesting that these hormones are involved in the onset and subsequent primary thickening growth of storage roots. The role of cytokinin in storage root formation is supported by the recent finding that the exogenous application of cytokinin induces storage root formation in the presence of high sucrose concentrations (Eguchi and Yoshida, 2008). Storage root yield is largely dependent on secondary thickening growth that occurs in the later stage of storage root development. It is positively correlated with concentrations of abscisic acid (ABA) and cytokinin, but not with IAA levels (Wang et al., 2006) which actually decrease gradually with secondary thickening growth (Akita et al., 1962; Nakatani and Komeichi, 1992). This leads to the hypothesis that cytokinin, auxin, and ABA possibly have different roles in the induction and/or thickening growth of storage roots, with auxin being involved in the initial formation of the storage root, ABA in the later secondary thickening growth, and cytokinin active during both the early and later stages. To date, however, the distinct role of each hormone has not been directly elucidated.

MADS-box genes encode transcription factors that play fundamental roles in diverse and important biological functions in animal, fungal, and plant development (Messenguy and Dubois, 2003; Kaufman et al., 2005). MADS-box proteins contain a highly conserved MADS-