Functional Characterization and Identification of Single Nucleotide Polymorphisms in KRP4 and KRP5, Two Negative Regulators of Cell Production in Apple

Fruit growth in apple (Malus ×domestica) is achieved through a combination of cell production and expansion, processes which are in-turn regulated by multiple genes. Cell production is regulated by the progression of individual cells through the cell cycle. Multiple cell cycle genes closely associated with the regulation of cell production during different stages of flower and fruit development have been previously identified in apple. Two such genes are the cyclin dependent kinase inhibitors, referred to as KRPs (Kip-Related Proteins; KRP4 and KRP5). The proteins encoded by KRP4 and KRP5 are associated with reduced cell production activity. Functional characterization of these genes may help determine their roles in the control of fruit growth and size in apple. To achieve this objective, transgenic Arabidopsis thaliana plants expressing either the apple KRP4 or KRP5 were generated. Phenotypic observations of these plants revealed smaller, greatly serrated leaves, and altered floral morphology, similar to that noted in plants overexpressing the Arabidopsis KRP2 gene. These results suggest that the apple KRPs are negative regulators of cell production and thereby organ growth. Identification of polymorphisms in these genes may lead to the development of markers associated with the control of cell production and organ growth. To achieve this objective, the coding regions of KRP4 and KRP5 in multiple Malus ×domestica accessions were sequenced. Several polymorphisms identified in the coding region of KRP4 may result in amino acid differences that can potentially alter KRP4 function. Together, data from this study suggest that KRP4 and KRP5 negatively regulate cell production and fruit growth in apple.