Identification of single nucleotide polymorphisms and the functional characterization of two apple Kip-related proteins, MdKRP4 and MdKRP5

Fruit size in apple is determined through the combination of cell production and expansion. Cell cycle genes identified in apple include two that may negatively regulate cell production. These genes are cyclin dependent kinase inhibitors, referred to as KRPs (Kip-Related Proteins). Characterization of these genes is needed to determine how they influence fruit growth and size. Two approaches included identification of polymorphisms within the coding region of these KRPs in a population of Malus domestica varying in fruit size, and transformation of Arabidopsis thaliana with MdKRP4 and MdKRP5. One polymorphism identified in MdKRP4 resulted in an amino acid substitution that correlated with small fruit size. Preliminary phenotypic observations of the transformants indicate smaller, serrated leaves and altered floral morphology, similar to plants overexpressing A. thaliana KRP2. These results indicate the KRPs have an important role in cell cycle regulation, potentially impacting cell production and fruit growth in apple.