Isolation and induced expression of a fructokinase gene from loquat

Fructose is essential for plant development as well as for fruit sugar composition and fruit quality. Fructose is one of the major sugars in the mature loquat ( Eriobotrya japonica Lindl.) fruit, which is popular because of its flavor and availability in early summer. The elucidation of the mechanism of fructose metabolism is of great importance for fruit quality improving. Fructose is primarily phosphorylated by fructokinase (FRK). In order to understand the fructose metabolism in the loquat fruit, a putative loquat FRK full-length cDNA designated as EjFRK was isolated in this study. The EjFRK encoding FRK possesses conserved regions inherent to plant FRKs. Transient expression of 35S: EjFRK -GFP fusion protein in onion epidermal cells showed that EjFRK was mainly expressed in the cytosol. The real-time RT-PCR analysis indicated that EjFRK was expressed in all loquat tissues. Monitoring the dynamic changes of EjFRK transcripts and FRK enzymatic activities demonstrated that EjFRK expression was at a relative high level during early fruit developmental stages and dropped to the lower level during maturation, similar with the changes in FRK activity, which was opposite to the fructose levels during fruit development. The results indicated that the high FRK enzymatic activity was not conducive to fructose accumulation in loquat fruit. The EjFRK transcript level in leaves of loquat seedlings was significantly enhanced after 6 h of treatment with 10 and 100 mM fructose or glucose, which indicates that EjFRK is modulated by fructose and glucose in vivo.