## 4 Discussions
AMY and BAM play a prominent role in starch breakdown and participate in the regulation of plant growth, development, and stress responses [48]. PHS participates in phosphorylation degradation of starch [49]. In the present study, higher expression levels of _AMY6_, _BAM10_, _PHS2_, and _PHS3_ related to starch degradation pathways were detected than other genes and negative correlation with starch contents (Table S5), indicating that these genes probably play important roles in transferring starch into sugar in pitayas. Sugars and organic acids have strong influence on fruit pH, flavor, and taste [13]. Fructose, glucose, and sucrose were the main abundant sugar components reported in pitaya fruits [3; 4; 16]. In detail, the major sugar of 'WCHL' pitaya was glucose (59.29 mg/g FW) (Figure 3), consistent with the results in red-peel pitayas [3]. However, the dominant sugars of 'YCHL' pitaya were sucrose (42.64 mg/g FW), fructose (33.28 mg/g FW), and glucose (32.72 mg/g FW). Most sugars are highly produced at S7 except sucrose which is rich in S4 and S5 of pitayas (Figure 10). The key enzymes, including SPS, SuSy, Ivr, HXK, and FXK, showed various expression patterns for regulating sugar contents during fruit developmental stages [13; 16]. In our study, 18 candidate genes (three _SPSS_, three _SuSys_, four _Irs_, four _HXKs_, three _FXKs_, and one _PGJ_) involved in sugar metabolism  
Figure 10: Schematic representation of sugar and organic acid metabolites during fruit developmental stages in two yellow-peel pitayas. Phe: phenolic; Fla: flavonoid; AA: ascorbic acid; Glu: glucose; Suc: sucrose; Fru: fructose; Gal: galactose; Ino: inositol; Sor: sorbitol; Mal: malic; Cit: citric; CM: citromalic; Fum: fumaric. were obtained from yellow-peel Pitayas (Figure 8). _SPS2_, _Ivr20_, _FRK10_, and _HXK9_ had a positive correlation while _SitSy5_, _SuSy11_, _Ivr4_, _FRK5_, _FRK9_, _HXK1_, _HXK5_ and _HXK12_ were negatively correlated with sugar accumulation during fruit development of yellow-peel pitayas (Figure 8; Table S8), suggesting that these genes cooperatively regulate sugar synthesis of yellow-peel pitayas. Malic, citric, and citramalate acids were the three organic acids mainly accumulated at S2 to S5 in pitayas (Figure 10). Due to the degradation of citramalate acid in maturation stages, malic acid (15.06 mg/g FW) and citric acid (5.76 mg/g FW) were the most abundant organic acids in 'WCHL' and 'YCHL' pitayas, respectively. CMS has been reported as a key enzyme responsible for citramalate synthesis in _Escherichia coli_ and apple [50; 51]. In yellow-peel pitayas, the expression patterns of _CMS1_ and _CMS3_ had positive correlation with citramalic acid contents, indicating they probably responsible for citramalic acid biosynthesis of pitaya fruits. PEPC catalyzes the irreversible carboxylation of phospho-enolpyruvate (PEP) to form oxaloacetate which is the substrate for forming citrate [14; 52]. In this study, the accumulation of citric acids was significant positive correlation with the expression of _PEPC3_ and _PEPC4_ during pitaya fruit development. Furthermore, CS catalyzes the reaction of oxaloacetate and acetyl-CoA convert into citrate and coenzyme A, which is the first step in the TCA cycle [52]. However, no _CSs_ show correlated relationship with citrate in the two yellow-peel pitayas. This suggests that carbon fluxed into TCA cycle mainly via _PEPC3_ and _PEPC4_ rather than _CSs_, probably because rich acetyl-CoA fluxed into the irreversible step catalyzed by _CMSs_ for citramalate acid biosynthesis. MDH catalyzes the conversion between oxaloacetate and malate and FUM catalyzes the reversible interconversion between malate and fumarate [53; 54]. A higher content of malic acid in 'WCHL' pitaya were detected than that of 'YCHL' pitaya. Malic acid content had a negative correlation with _FUM1_ and _MDH1_. Compared with 'YCHL' pitaya, _MDH1_ was strongly down-regulated in 'WCHL' pitaya responsible for more malic acid in 'WCHL' pitaya than 'YCHL' pitaya.