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Abstract
Apple (Malus × domestica) is a flavorful, edible fruit with great commercial significance. The fruit consists of two fleshy tissues, pith and cortex, that display differential growth. Early growth of the fruit cortex involves intensive cell production while later stages are mediated largely by cell expansion. In this work, the spatial and temporal regulation of fruit growth was evaluated particularly during early fruit development (EFD). Transcriptome analyses during EFD indicated that early de-repression of growth was associated with enhanced cell division related changes in gene expression. Phytohormone profiling during this period indicated that ABA, ethylene and JAs together function as components of a growth-restricting module during bloom and shortly thereafter. The roles of two transcription factors in regulating EFD were evaluated. Spatio- temporal expression patterns of MdTCP2b (TEOSINTE BRANCHED 1; CYCLOIDEA; PROLIFERATING CELL FACTOR) were consistent with negative regulation of cell production, and determination of the timing of exit from cell production- to cell expansion-mediated growth. The heterologous expression of MdTCP2b in Arabidopsis promoted flowering, reduced leaf size and promoted positive curvature of the leaves, consistent with a role as a negative regulator of organ growth. MdTCP2b partially functions by binding to the promoter of a cell cycle inhibitor KIP RELATED PROTEIN, KRP4 and promoting its expression. MdTCP2b can be post- transcriptionally regulated spatially and temporally regulated by miR319. GROWTHREGULATING FACTOR GRFs (GROWTH REGULATING FACTORs) are positive regulators of cell production and growth. Transcript abundance of MdGRF11 and a GRF INETRATING FACTOR (MdGIF3) are consistent with positive regulation of cell production during EFD. MdGRF11 is post-transcriptionally regulated by miR396. MdGIF3, a transcriptional co-activator, was identified as a partner protein of MdGRF11 through yeast two-hybrid assays (Y2H), suggesting that this complex functions together to promote cell production during EFD.