Expression of class I knotted1-like homeobox genes in the storage roots of sweetpotato (Ipomoea batatas)

As a first step in clarifying the involvement of class I knotted1-like homeobox (KNOXI) genes in the storage root development of sweetpotato (Ipomoeo batatas), we isolated three KNOXI genes, named lbkn1, lbkn2 and lbkn3, expressed in the storage roots. Phylogenetic analysis showed that lbkn1 was homologous to the SHOOT MERISTEMLESS (STM) gene of Arabidopsis, white lbkn2 and lbkn3 were homologous to the BREVIPEDICELLUS (BP) gene. Of these, expression of lbkn1 and lbkn2 were upregulated in developing and mature storage roots compared with fibrous roots. lbkn1 and lbkn2 showed different expression patterns in the storage roots. lbkn1 was preferentially expressed at the proximal end and around the primary vascular cambium, while lbkn2 expression was highest in the thickest part and tower in both the proximal and distal ends. In contrast to lbkn1 and lbkn2, expression of lbkn3 in roots was not consistent among sweetpotato cultivars. The distribution of endogenous trans-zeatin riboside (t-ZR) in sweetpotato roots showed a similarity to the expression pattern of KNOXI genes, supporting the idea that KNOXI genes