Apple Fruit Size Regulation by AP2 Domain Transcription Factors

Final fruit size in apple (Malus x domestica) is of great economic significance. Apple fruit development involves cell division during the first few weeks after bloom, followed by cell expansion. Final fruit size is greatly regulated by cell production which is in turn controlled by the activity of multiple genes. Several AP2-domain transcription factors, such as AINTEGUMENTA (ANT), control growth and organ size in Arabidopsis by regulating the duration of cell production and are potential candidates for fruit size regulation in apples. Several ESTs with similarity to Arabidopsis ANT and ANT-like (AIL) genes were identified from publicly available apple EST databases. Expression of three such genes, MdANT1, MdANT2 and MdAIL2, was analyzed using quantitative RT-PCR during fruit development in ‘Gala,’ across apple genotypes differing in fruit size potential (‘Gala’ and ‘Golden Smoothie’), and in thinned and un-thinned fruits. MdANT1, MdANT2 and MdAIL2 were highly expressed during early stages of fruit growth and exhibited a 4- to 9-fold decline in expression during later stages of fruit development. Decrease in MdANT1 and MdANT2 expression coincided with exit from cell production during fruit development in ‘Gala.’ Larger fruit size in ‘Golden Smoothie’ was mostly due to higher cell production than in ‘Gala.’ MdANT1 and MdANT2 were expressed at higher levels (2- to 10-fold) and for a longer duration in ‘Golden Smoothie,’ in comparison to ‘Gala.’ Manual thinning to one fruit per cluster increased fruit size in ‘Golden Smoothie’ by increasing cell production. This was associated with increased expression of MdANT1 (2-fold) and MdANT2 (5-fold). The above expression data indicate that MdANT1 and MdANT2 may be involved in regulating fruit growth and final size in apple by controlling cell production.