There is an increase in small fruit consumption especially
There is an increase in small fruit consumption especially berries in recently. Because small fruits are rich sources of both anthocyanins and phenolic compounds. These chemicals have been found in Vaccinium corymbosum in high levels. Research on small fruits is of vital importance for human nutrition and health (Milic et al., 2018; Okatan, 2018; Velioglu et al., 1998). V. corymbosum shows significant variation in berry size, however, cell layer-specific patterns of cell division, together with the sequences, structures and biological functions of cyclins and CDKs in the regulation of fruit size (or fruit mass) are much less exploited. Herein, we focused our attention on Southern highbush blueberry cultivars \'O\'Neal\' and \'Bluerain\' single fruit mass, cell-layer change, cyclins and CDKs gene structures, as well as its Nicotinic Acid receptor tendency during the flower bud enlargemental and fruit developmental process. These results help us to elucidate the complex cell division and cell expansion mechanisms, and cell-cycle regulatory mechanisms in V. corymbosum flower and fruit during ontogeny.
Materials and methods
Discussion Fruit size is an important factor influencing yield and consumer preference. The growth and development of fruit, especially most freshy fruit, is intensely accompanied by the cumulative increase of cell number and cell volume, which ultimately derives from cell division, accumulation and transformation of water and solutes (carbohydrates, sugars, and organic acids, etc.) (Fanwoua et al., 2013; Scofield et al., 2014; Xie et al., 2018). The fruit growth curve represents fruit growth pattern after blossoming, and is also a vital measurement parameter for field management and appropriate picking time. It had been reported that V. corymbosum fruit exhibited a double-sigmoid growth pattern, and three distinct biological processes were involved in the berry development: a rapid increase of pericarp size in stage I, rapid development of the embryo and endosperm tissues in stage II (or lag stage), followed a second rapid pericarp development in stage III (Xie et al., 2018). The majority of cells presented at ripening stage were formed at pre-anthesis (Collins et al., 1966; Godoy et al., 2008). Our results showed that V. corymbosum \'O\'Neal\' and \'Bluerain\' fruit followed the double-sigmoid growth pattern, and fruit mass increase mainly occurred from stage S4-S6 (Fig. 1). Ovary wall (pericarp) was the key tissue for blueberry fruit mass increase (Fig. 2), and approximately 70% of pericarp cell layers were produced before anthesis (Fig. 2, Fig. 3A), which indicated that cell division at the flower bud enlargement period was a much greater impact for final mass than that during the fruit development (Cano-Medrano and Darnell, 1997). It is noteworthy that an increase in the number of cell layers of ovary wall, axial placenta, ventral suture and vertical tissues was occurred during whole flower bud enlarge and fruit growth stage, and such increase had also been observed in other plant species, such as Solanoideae and Capsiceae (Pabon-Mora and Litt, 2011). However, the cell layers of axial placenta in the \'O\'Neal\' and \'Bluerain\' fruits were dramatically reduced during fruit expansion period, which was completely different with the process of Solanum lycopersicum fruit growth (Mazzucato et al., 1998), and the locule area also had smaller contribution than that in tomato fruit, indicated that different regulatory mechanisms were involved in V. corymbosum berry growth. In addition, cell layers of blueberry pericarp and vertical tissues were not significantly increased from stage V to S1, even somewhat decreased (Fig. 3A and D). Similar pattern had also been reported in Malus domestica and Solanum lycopersicum (Gillaspy et al., 1993; Malladi and Johnson, 2011; Renaudin et al., 2017), and was thought as a conserved mechanism to prevent fruit growth in the absence of seed set. After pollination and fertilization, the arrest of cell production was released and fruit growth initiated again.