Abstract
Horticultural industries are increasingly crucial in providing livelihoods, food quality, profits, and economic growth. In many horticultural plants, extensive studies were conducted to study the roles of hormones, epigenetics, and genes in regulating the development of cell number, cell size, fruit size, fruit weight, and endo-reduplication primarily via a gene-mapping technique known as quantitative trait loci (QTL). In general, these plants encompassed those with full-genomes sequenced, such as the apple, tomato, strawberry, and bananas. However, apart from fully sequenced apple genomes, the genome sequences of many other plants, particularly highly profitable tropical fruits, such as mangoes, pineapples, durians, and coconuts are yet available. This chapter will describe the interplay of plant hormones in determining fruit cell number and cell size, which, in turn, affects the final fruit size in horticultural plants.