PhysiologyPatterns of enzyme activities and gene expressions in sucrose metabolism in relation to sugar accumulation and composition in the aril of Litchi chinensis Sonn.
Introduction
The amount and types of sugars in fresh fruit directly influence its quality and flavor. The sink strength and regulation of carbohydrate metabolism in fruits have been investigated extensively based upon the functions of invertase, sucrose synthase (SS), and sucrose phosphate synthase (SPS) (Ho, 1988, Miron and Schaffer, 1991, Geiger et al., 1996, Choudhury et al., 2009). In tomato, these enzymes participate in at least four futile cycles of sucrose/hexose interchange that govern fruit sugar content and composition (Ngugen-Quoc and Foyer, 2001).
Sucrose unloading and utilization depend on its cleavage into hexoses, which is catalyzed by SS and/or invertase (Ruan et al., 2010). SS has a more important function in sugar accumulation than invertase. In transgenic tomato, the sucrose unloading rate into fruit increases in response to increasing SS activity, while plants with similar SS activities but very different invertase activities show highly similar patterns of sucrose unloading (N’tchobo et al., 1999). Three different types of invertase isoenzymes exist: soluble acid invertase (SAI), which is found in vacuoles, cell wall acid invertase (CWAI), which is bound to the cell wall, and neutral invertase (NI), which is found in cytosol. The amount of hexoses available is determined by vacuolar acid invertase activity in potato tuber (Zrenner et al., 1996), tomato fruit (Ohyama et al., 1995), and grape (Davies and Robinson, 1996). The key enzyme for sucrose formation is SPS. Increased SPS activity occurs in sucrose-accumulating tomato genotypes (Dali et al., 1991), muskmelon (Lingle and Dunlap, 1987), citrus (Komatsu et al., 1999), and banana (Choudhury et al., 2009). The genes of sucrose metabolism enzymes have been isolated and their expression patterns have been investigated in relation to sugar accumulation in citrus (Komatsu et al., 1999, Komatsu et al., 2002), peach (Vizzotto et al., 1996), and banana (Choudhury et al., 2009).
Litchi (Litchi chinensis Sonn.), a member of the Sapindaceae family, is an important evergreen fruit crop widely cultivated in the warm subtropical areas across the world. The edible portion of the fruit is the aril. Litchi arils accumulate sugars amounting to 15–20% of the fresh mass, and the total amount of sugars accumulated varies among cultivars (Wang et al., 2006). Sucrose, fructose, and glucose have been identified as the principal sugars in litchi, although the sugar composition varies considerably among cultivars (Paull et al., 1984, Wang et al., 2006). While the sugar content and composition in several litchi cultivars have been previously studied during maturation, knowledge about the mechanism of sugar accumulation and composition based on an analysis of the transcript and protein accumulation patterns of sucrose metabolism enzymes is extremely limited. In the present study, four main litchi cultivars with different sugar contents and compositions were used to investigate gene expression profiles and changes in enzyme activities in sucrose metabolism during aril development. We classified 42 cultivars into three types based on their sugar composition. Five cultivars of each type were selected to monitor the key enzymes in determining hexose/sucrose ratio in the litchi aril.
Section snippets
Plant materials
The experiment was conducted at the experimental orchard of the South China Agricultural University, Guangzhou, China. Litchi (Litchi chinensis Sonn.) trees received standard horticultural practices and disease and insect control. Five fruits from different canopy positions were pooled as one replicate, and three replicates were set for every cultivar. The samples were taken to the laboratory immediately after harvest, and arils were separated and frozen immediately in liquid nitrogen and
Aril sugar contents and composition
Litchi fruit can be described as a round, heart-shaped, or ovoid drupes with arils as the edible part. Litchi fruit development takes between 70 and 100 d after anthesis, depending on the cultivar and location (Subhadrabandhu and Stern, 2005). The average fruit weight ranges from 16 g to 35 g and flesh recovery is 50–70%. Aril development begins around 21–35 d after anthesis and the growth stage lasts about 49–56 d. The aril is white to off-white, translucent, slightly acidic, juicy, sweet, and
Enzyme activities and gene expressions in sucrose metabolism in relation to sugar composition
The sugar composition in the aril of mature litchi varied widely among different cultivars (Fig. 1). Sucrose, glucose, and fructose are predominant sugars in the litchi aril, and different hexose/sucrose ratios were observed among different litchi cultivars (Paull et al., 1984, Wang et al., 2006). The tested cultivars were divided into three groups based on differences in hexose/sucrose ratios, according to a previous report (Wang et al., 2006): sucrose-prevalent type (hexose/sucrose < 1),
Conclusion
Sucrose, fructose, and glucose are the principal sugars in the litchi aril. Litchi cultivars can be divided into three types, namely, the sucrose-prevalent type (hexose/sucrose < 1), the intermediate type (1 < hexose/sucrose < 2), and the hexose-prevalent type (hexose/sucrose > 2). The hexose/sucrose ratio is highly related to SAI and SS activities. SAI and SS are possibly regulated at the transcriptional levels. SPS activity is an unlikely determinant of hexose/sucrose ratio in litchi. These results
Acknowledgements
The project was supported by Special Fund for Agro-scientific Research in the Public Interest (Project no. 20090344-5); China Litchi Industry Technology Research System (Project no. NYCYTX-32), Ministry of Agriculture, China; and Key Laboratory of Innovation and Utilization for Germplasm Resources in Horticultural Crops in Southern China of Guangdong Higher Education Institutes, South China Agricultural University (no. KBL11008).
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These authors contributed equally to this work.