The study of apple, Malus x domestica, fruit cell division and expansion in vivo and in vitro

Abstract

Apple (Malus x domestica) fruit size is facilitated by cell division and cell expansion, which in turn is regulated by plant hormones such as auxins, gibberellins, and CKs. The work presented here examines the role of cell division and cell expansion in apple growth and determine which of the processes contributes most to final fruit size. Three apple cultivars selected for different size were used in this study. ‘Twenty Ounce’ (large-sized), ‘Royal Gala’ (medium-sized), and Crab apple (small-sized) apple cultivars were analysed over two consecutive seasons. Gene expression and cell area measurements were carried out each season. The expression of cell division markers MdCDKB2:2 is correlated with MdANT2 with both showing high expression patterns at early stages of timecourse and gradually decreasing towards the end of timecourse. Cell expansion markers MdEXP3, showed upregulated expression as the cells expanded, while MdARF106 was expressed in both cell division and cell expansion stages. Ripening related genes; MdACO1 and MdPG1, were highly expressed during the ripening stage. The cell areas of each cultivar were similar at both 0 day after pollination (DAP) and at ripening (120 DAP). Due to the seasonal limitation in apple studies, attempts to mimic apple cells growth in cell cultures were carried out. ‘Royal Gala’ cell culture was first established. Growth measurements of this culture showed that there was a 14 day growth period, during which the expression of the fruit expressed MdCDKB2:2 corresponding to the rapidly growing phase of the culture. Different hormone concentrations were screened in 96-, 24-, and 6-well plates using several different gradients of hormones (auxin, 2,4-D; CK, 6-BAP; Gibberellin, GA3) to assess optimum concentrations for cell expansion. Cell size was then measured and aligned to gene expression. Higher concentrations of 6-BAP or/and GA3, or ACC triggered cell expansion or ripening when combined with a lower concentration of 2,4-D, by increasing MdEXP3 and MdACO1 respectively, while a modest expression of MdCDKB2:2 indicated that cell division had decreased with these treatments. These expression studies suggest that the cell cultures appear to mimic the growing fruit thus provide new insights on the processes that take place during fruit development.