MdCKS1 Expression Involved in Fruit Size in Apple Cultivar ‘Fuji’ and ‘Ralls’

CKS (cyclin-dependent kinase subunit) is one of cell cycle genes, and cell cycle in apple (Malus × domestic Borkh.) fruit controlled cell number and fruit size. In order to explore CKS expression during the development of apple fruit size, the cultivar ‘Fuji’ and ‘Ralls’ whose fruit size had obvious difference were used for MdCKS1 gene cloning and expressing. In ‘Fuji’ and ‘Ralls’, the greatest growth rates of fruit size and weight occurred during 7–21 days after full bloom (DAFB), but in ‘Fuji’ during 21–35 DAFB the growth rates of fruit size and weight was still faster, and ‘Fuji’ remained much rapider growth rate of fruit size and weight than ‘Ralls’ within 7–35 DAFB, so the majority of cell production in ‘Ralls’ occurred between 7–21 DAFB and in ‘Fuji’ between 7–35 DAFB, and the biggest difference of fruit size and weight between ‘Fuji’ and ‘Ralls’ occurred at 35 DAFB. MdCKS1 sequence was identical with MdCKS1;1 (CO868566) and apple CKS1 (EB141951). MdCKS1 expression peak in ‘Fuji’ was two-fold higher than in ‘Ralls’; MdCKS1 expression peak in ‘Ralls’ occurred at 21 DAFB and in ‘Fuji’ at 35 DAFB which coincided respectively with rapid growth rate of early fruit. During 140–161 DAFB, ‘Fuji’ fruit had MdCKS1 expression peak again, but ‘Ralls’ not; ‘Fuji’ fruit size and weight had a larger extent of increase than ‘Ralls’, and the second significant difference of fruit size and weight between ‘Fuji’ and ‘Ralls’ occurred at 161 DAFB. In callus, MdCKS1 expression was much higher in ‘Fuji’ than in ‘Ralls’, up-regulated by auxin (NAA) or cytokinin (6-BA) or co-existence of them, and depressed by lower culture temperature. Altogether, ‘Fuji’ fruit had much rapider growth rate than ‘Ralls’ which resulted in significant difference of fruit size and weight between ‘Fuji’ and ‘Ralls’ respectively at 35 DAFB and 161 DAFB, and contributed to their fruit size difference; MdCKS1 expression in ‘Fuji’ fruit was much higher than in ‘Ralls’ both at early stage for fruit cell division and later stage for fruit cell expansion; MdCKS1 expression in callus was up-regulated by auxin or/and cytokinin. Hence, difference of fruit size between ‘Fuji’ and ‘Ralls’ was correlated with growth rate during fruit development, MdCKS1 expression correlated positively with growth rate and cell production of early fruit, and involved in fruit size which maybe was regulated by endogenous auxin or/and cytokine level of genotype.