Gene expression in the sugar metabolism between ‘Guanxi’ pummelo and its early-ripening mutant ‘Liuyuezao’

Abstract

‘Liuyuezao’ (LYZ) is a new early-season pummelo (Citrus maxima [Burm.] Merr.) cultivar developed from a spontaneous bud sport on a ‘Guanxi’ (GX) pummelo tree. LYZ and GX pummelos were used as the experimental materials to study the accumulation of soluble sugars and expression of related genes during fruit development. Large differences were discovered in the soluble sugar contents and accumulation between LYZ and GX. Fructose and glucose contents decreased gradually in GX but a unimodal trend in LYZ during fruit development. The accumulation patterns of sucrose were similar between the two pummelo cultivars. INV6, SUS2, SUS3, FRK1, FRK2, FRK3, HXK2 and HXK3 were the main genes with relatively high expression levels, and significant differences were observed in the expressions of INV6, SUS3, FRK3, and HXK2 between LYZ and GX in most fruit developmental stages. Almost all the invertases (INVs), sucrose synthases (SUSs), fructokinases (FRKs) and hexokinases (HXKs) were negatively correlated with glucose in LYZ but positively correlated with glucose and fructose in GX during fruit development. Compared to GX, higher expressions of INV6 and SUS3 may promote the hexose accumulation in LYZ fruit, then elevated hexose may upregulate the expression levels of FRK3 and HXK2 during fruit development. Overall, INV6, SUS3, FRK3 and HXK2 were proposed as prominent candidate genes for soluble sugar accumulation in pummelo fruits. This study provides a network of key candidate genes for further understanding the physiological and biochemical mechanisms of sugar accumulation in pummelo fruits.