TONG Xiao-lei, WANG Zheng-yang, MA Bai-quan, ZHANG Chun-xia, ZHU Ling-cheng, MA Fengwang, Ll Ming-jun

1 State Key Laboratory of Crop Stress Biology for Arid Areas (SKLCSBAA)/College of Horticulture, Northwest A&F University,Yangling 712100, P.R.China

2 College of Forestry, Northwest A&F University, Yangling 712100, P.R.China

1. lntroduction

Sucrose is a major photosynthetic product in leaves, which is transported to the sink organs in most plant species (Lutfiyyaet al. 2007; Chenet al. 2012). In sink tissues, two key enzymes, invertase and sucrose synthase, are required to participate in the sucrose cleavage reaction (Geigenbergeret al. 1993; Kleczkowskiet al. 2010). Invertase catalyzes the irreversible hydrolysis of sucrose to fructose and glucose (Hiroseet al. 2008). Sucrose synthase (SUS) is a cytosolic and reversible enzyme that catalyzes sucrose degradation and supplies ADP- and UDP-glucose for starch and cell wall polysaccharide synthesis (Yagiet al. 2003).Both invertase and sucrose synthase enzymes play crucial roles in plant growth, development, stress responses, and carbon partitioning in sink tissues (Sturm and Tang 1999;Koch 2004). SUS is also important for cellulose synthesis,and SUS suppression might impair cell wall integrity by reducing the UDP-glucose supply, which is the essential substrate for the synthesis of cellulose and non-cellulose cell wall compounds (Albrecht and Mustroph 2003; Fujiiet al.2010). High SUS activity was closely associated with sink strength during the sink-source transition, and SUS activity determines sink strength in potato tubers (Zrenneret al.1995; Strum and Tang 1999). Furthermore, SUS regulates several developmental processes, such as flowering induction (Ohtoet al. 2001), seed development (Iraqiet al.2001), cell division (Gaudinet al. 2000), and storage product accumulation (Rooket al. 2001).

The gene encoding SUS was isolated primarily from starch-storing and sucrose- or hexose-storing plants (Sturm and Tang 1999). SUS belongs to a small multi-gene family in both monocot and dicot species that contains four subunits with a molecular weight of approximately 83–100 kD. Based on the phylogenetic relationship and structural analyses,SUS genes were divided into three major groups: SUS I,SUS II, and SUS III (Zhanget al. 2011, 2013). Variation in the number of SUS family members has been discovered in different plant species; such as,Arabidopsis,Lotus japonicus,Amygdalus persicaandHevea brasiliensistree all encode six distinct active SUS genes (Baudet al. 2004;Horstet al. 2007; Xiao 2014; Zhanget al. 2015), while the poplar genome encodes seven SUS genes (Anet al. 2014).

Apple is an economically important fruit crop worldwide.As a predominant photosynthesis product (70–80% of the total photosynthetic product in leaves), sorbitol is transported to the sink organs in apple (Yamakiet al.1992), although sorbitol only represents 3–8% of the total soluble sugar in mature fruit (Yamakiet al. 1986). In the sink, sorbitol is predominantly converted to fructose before entering central metabolism. However, the role of sucrose metabolic enzymes in the regulation of sink strength in apple remains unclear. In this study, we report the isolation and characterization of SUS family members in apple based on the Genome Database for Rosaceae (GDR,http:/www.rosaceae.org) and TheArabidopsisInformation Resource (TAIR, http://www.arabidopsis.org/). Additionally,the expression level ofMdSUSgenes was detected in different apple tissues and transgenic lines in which sorbitol concentration was significantly decreased at the same time sucrose concentration was elevated. Our results would provide basic information for further functional analysis of the MdSUS genes and help elucidate whether the two metabolic systems (one for sucrose and one for sorbitol) are coordinated in response to sorbitol and sucrose utilization in sink tissues.

2. Materials and methods

2.1. Plant materials

The 9-year-old Gala apple tree was used in this study at the Horticultural Experimental Station of Northwest A&F University, Yangling, China. Young shoot tips and mature leaves were collected and used for RNA extraction. The samples were immediately frozen in liquid nitrogen and stored at -75°C. The fruits were randomly collected at 16, 34, 55, 75, 98, and 122 days after blooming (DAB),respectively, and each developmental stage had three replicates of 10 fruits. Fruit samples were mechanically peeled and cored. Pulps were cut into small sections,immediately frozen in liquid nitrogen, and stored at –75°C for RNA extraction. All the materials used in the study were the same as those in Weiet al. (2014).

2.2. Characterization and phylogenetic analysis of SUS genes in the apple genome

The amino acid sequences of SUS genes inArabidopsiswere used as a query sequence to identify homologous genes of the apple reference genome sequence (Velascoet al.2010) using BLAST (https://blast.ncbi.nlm.nih.gov/Blast.cgi), which uncovered 11 apple transcript IDs from the SUS family collected after removing transcripts withE>e-10. Amino acid sequences were aligned using CLUSTAL X (http://www.clustal.org/), and molecular evolutionary genetics analysis(MEGA) ver. 6.06 (http://www.megasoftware.net/) was used to construct a phylogenetic tree based on the neighbor joining (NJ) method, the parameters are as follows: the model is JTT+G, the missing is set to ‘complete deletion’and the check parameter Bootstrap is 1 000. The specific conserved domains in the SUS family were searched on InterProScan (http://www.ebi.ac.uk/Tools/InterProScan/),and the conserved domains were predicted by Pfam(http://pfam.xfam.org/search/sequence). The number of amino acids, molecular weight, predicted theoretical isoelectric point (pI), aliphatic index, and grand average of hydropathicity (GRAVY) were calculated using ProtParam tool (http://www.expasy.org/tools/protparam.html). Multiple EM for Motif Elictiation (MEME, http://meme.nbcr.net/meme/cgi-bin/meme.cgi) was used to identify conserved motifs shared by the MdSUS proteins. The program Clustal W2 was used to calculate the similarity/identity of amino acid/nucleotide sequences.

2.3. Quantitative real-time RT-PCR (qRT-PCR)

Total RNA was isolated using the cetyltrimethyl ammonium bromide (CTAB) method (Changet al. 1993). RNA samples were treated with DNase I (Invitrogen, USA) to remove genomic DNA contamination. The quality of isolated total RNA was assessed on a 1.5% agarose gel and then adjusted to 200 ng μL–1using a NanoDrop Lite spectrophotometer(ND2000, USA). One mg of total RNA per sample was used for cDNA synthesis using SYBR?PrimeScript? RT-PCR Kit II(TaKaRa, Kyoto, Japan) according to the manufacturer’s instructions. All of the cDNA samples were verified by semi-quantitative RT-PCR, diluted 1:10 with RNase-free water, and then stored at –20°C. qRT-PCR was performed in a reaction mixture (total volume 20 μL) containing 10 μL SYBR?PremixEx TaqII (TaKaRa, Dalian, China), 0.8 μL gene specific primers (10 μmol L?1), 2.0 μL cDNA template(50 ng μL?1), and 6.4 μL sterile distilled water. Amplification was performed with an iQ5.0 instrument (Bio-Rad, USA).The amplification program was one cycle at 95°C for 3 min, followed by 40 cycles at 95°C for 20 s, 55°C for 20 s, and then 72°C for 20 s. All of the analyses had three biological replicates.MdActinwas used as the internal control gene. All premiers are showing in Appendix A.Melting curve analysis was performed at the end of 40 cycles to ensure proper amplification of the target fragments.

2.4. Assay of SUS enzyme activity

SUS was extracted and assayed according to Liet al.(2012). Briefly, 0.5 g sample was homogenized in 2-mL 200 mmol L-1Hepes-KOH buffer (pH 8.0) containing 5 mmol L-1MgCl2, 2 mmol L-1ethylenediaminetetraacetic acid (EDTA), 2.5 mmol L-1dithiothreitol (DTT), 2 mmol L-1benzamidine, 0.1 mmol L-1leupeptin, 0.1% BSA, 2%glycerol, 1% Triton X-100, and 4% polyvinylpyrolidone(PVPP). The extract was centrifuged at 16 000×g for 20 min at 4°C and immediately desalted in a Sephadex G25 PD-10 column; then, SUS activity was determined. The enzyme extract (20 μL) was incubated at 27°C for 30 min in a final volume of 100 μL assay medium containing 20 mmol L-1Hepes-KOH (pH 7.0), 100 mmol L-1sucrose and 4 mmol L-1UDP. The reaction was stopped by boiling in water for 3 min, and blanks contained the same assay mixture with denatured extract. The uridine diphosphate glucose(UDPG) content produced in the assay was measured spectrophotometrically following the reduction of NAD+coupled to UDPG dehydrogenase activity in a reaction mixture (1.0 mL) containing 5 mmol L-1MgCl2, 2 mmol L-1NAD+, 0.02 U UDPG dehydrogenase, and 100 μL of the reaction mixture for SUS in 200 mmol L-1glycine (pH 8.9).The mixture was incubated at 27°C for 30 min, and NADH production was determined at 340 nm. SUS activity was calculated in terms of UDPG production per hour under the assay conditions.

3. Results

3.1. Genes encoding SUS in the apple genome

The amino acid sequences of SUS genes inArabidopsiswere used as queries to identify the homologous gene of the reference genome sequence of apple (Velascoet al.2010). After filtering to remove redundant sequences and excluding short and/or low identity sequences, 11 SUS genes were identified on chromosomes 2, 9, 10, 11, 13, 14, 15,and 17 (Fig. 1). Phylogenetic analysis indicated that these SUS genes were grouped into three gene families: SUS I,SUS II and SUS III (Fig. 2). The SUS I and SUS III families each contained four members;MdSUS1.1,MdSUS1.2,MdSUS1.3, andMdSUS1.4; andMdSUS3.1,MdSUS3.2,MdSUS3.3, andMdSUS3.4, respectively. The SUS II family contained three homologous genesMdSUS2.1,MdSUS2.2andMdSUS2.3. The full length coding and amino acid SUS sequences ranged from 1 611 to 2 652 bp and 536 to 883 bp,respectively. The amino acid sequence similarity betweenMdSUSand the corresponding genes inArabidopsisranged from 55.45 to 81.58%, and two conserved domains were identified as the sucrose synthase domain and the glycosyltransferase domain, which were located at the N-and C-terminal ends, respectively (Table 1).

Fig. 1 Chromosomal localization of the sucrose synthase genes in apple (MdSUSs). Their accession numbers have been retrieved from the Genome Database for Rosaceae (GDR, http://www.rosaceae.org/).

Fig. 2 Phylogentic tree derived from amino acid sequences of the sucrose synthase genes in apple (MdSUS), Arabidopsis thaliana(AtSUS), Prunus persica L. (PpSUS) and Oryza sativa L. (OsSUS). The numbers near the branches indicate bootstrap values calculated from 1 000 replicate analyses.

3.2. Gene structure and protein sequence analysis of MdSUS genes

The Clustal W2 Program (http://www.clustal.org/) was used to align multiple sequences.MdSUS2.2shared much higher similarity of amino acid and nucleotide sequence with its homologous genes,MdSUS2.3andMdSUS2.1, and the scores reached 96.83 and 77.24%, respectively at the amino acid level and reached 98.20 and 74.98%, respectively at the nucleotide level (Appendix B). Specifically,MdSUS1.1andMdSUS1.2were more closely related than other genes(95.77 and 99.21% at the amino acid level and nucleotide level, respectively).

The structures of theMdSUSgenes were evaluated by comparing cDNA sequences with their corresponding genome sequences. The number of exons and introns inMdSUSgenes ranged from 8 to 16 and 7 to 15, respectively.MdSUS2.2is characterized by 8 exons; however,MdSUS2.1contains the most exons (16 exons). Among theseMdSUSgenes, substantial conservation of exon/intron organization could be observed, which was similar to the high similarity observed by aligning the 11 amino acid sequences (Fig. 3, Table 1).Furthermore, SUS genes that belonged to the same group were investigated, and they showed similar structural characteristics in terms of exon number, size, and length distribution. The open reading frame (ORF)size ofMdSUSgenes ranged from 2 316(MdSUS3.3) to 2 700 bp (MdSUS3.1) with an average of 2 454 bp (Table 1).

MdSUS proteins were composed of 536–883 amino acids, and their aliphatic index ranged from 85.09 to 95.18. The predicted molecular mass of the 11MdSUSproteins ranged from 85 364.75 to 99 251.77 Da with an average of 89 539.63 Da, whereas the predicted theoretical pI ranged from 5.75(MdSUS2.1) to 7.17 (MdSUS3.3), which was similar to SUS proteins in most plants(Komatsuet al. 2002; Haradaet al. 2005;Ruanet al. 2008). The GRAVY index indicated that all of theMdSUSproteins were hydrophilic (Table 1). The MEME(http://meme.nbcr.net/meme/cgi-bin/meme.cgi) server was used to analyze conserved motif distribution among the 11MdSUSproteins. All 11MdSUSfamily members not only shared at least 10 common conserved motif compositions but also had a consistent order of motif arrangement (motifs 4, 7, 12,3, 2, 13, 5, 8, 1, 6) (Fig. 3-B). In addition,motifs 3, 2 and 13 in each protein sequence were closely connected (Figs. 3-B and 4).

?

3.3. Expression profile of MdSUS genes and enzymatic activity in different apple tissues

qRT-PCR was used to evaluate theMdSUSexpression profiles in different apple tissues, including shoot tips, mature leaves,young and mature fruits. As shown in Fig. 5, expression was detected in all four tissues, although theMdSUSgenes showed different expression patterns.The six genes,MdSUS2.2,MdSUS2.3,MdSUS3.2,MdSUS3.3,MdSUS3.4andMdSUS3.1, exhibited high expression in shoot tips, whereas the three genes,MdSUS1.1,MdSUS1.2, andMdSUS1.3,were consistently expressed at high levels in young fruits. The two genes,MdSUS2.1andMdSUS1.4,showed the highest expression levels in mature fruits. After evaluating the expression ofMdSUSgenes in different tissues, the SUS enzymatic activity was determined. Of all tissues examined, SUS enzyme activity in shoot tips was the highest, and was the lowest in the mature leaves (Fig. 5-D).

Fig. 3 Schematic gene structure of 11 sucrose synthase genes in apple (MdSUS). A, structure of MdSUS genes. Introns and exons are represented by black lines and yellow boxes, respectively. B, analysis with MEME (Multiple EM for Motif Elictiation) to investigate 15 conserved motifs of MdSUS proteins. Each colored box represents conservative motif.

Fig. 4 Multiple sequence alignment of proteins synthetized by sucrose synthase genes in apple (MdSUS). Identical (100%)and conservative (81–91%) of amino acid are shaded in deep blue and pink, respectively. The conserved motifs were marked by red lines.

3.4. Expression profile of MdSUS genes and enzymatic activity at different fruit developmental stages

The expression profile ofMdSUSgenes in developing fruit were examined (Fig. 6). Five of the 11MdSUSgenes, that was,MdSUS2.2,MdSUS2.3,MdSUS3.1,MdSUS3.2, andMdSUS3.4, showed extremely low expression during all fruit development stages, whereas three genes (MdSUS1.1,MdSUS1.2, andMdSUS1.4) were highly expressed at various stages of apple fruit development. Five genes,MdSUS1.3,MdSUS2.2,MdSUS2.3,MdSUS3.3andMdSUS3.1, showed reduced expression during apple fruit developmental stages, althoughMdSUS2.1showed increased expression during apple fruit development.The SUS enzyme activity at different fruit developmental stages as well as in different tissues were examined. The results showed that SUS enzyme activity gradually declined during fruit development. Then, the relevance between the expression ofMdSUSgenes and their enzymatic activity were analyzed. As shown in Table 2, there was a significant correlation with SUS enzyme activity and the expression ofMdSUS2.2,MdSUS2.3,MdSUS3.1, andMdSUS3.3during the fruit development process.

4. Discussion

Recently, more SUS family genes have been identified in different plant species, includingArabidopsis, rice, tobacco,cotton, and peach, using comparative genome approaches(Baudet al. 2004; Hiroseet al. 2008; Chenet al. 2012; Zouet al. 2013; Zhanget al. 2015). Applying available research from these model plants and using database searches, at least 11 SUS genes in apple were found. According to the distinct molecular signatures and expression patterns of SUS genes, we deduced that theMdSUSgenes might play different roles in sucrose metabolism in different tissues and at different developmental stages in the same tissue.

Fig. 5 Expression and enzyme activity profile of the sucrose synthase genes in apple (MdSUS). A, MdSUS I. B, MdSUS II.C, MdSUS III. D, sucrose synthase (SUS) activity. Bars mean standard error.

Table 2 Correlation coefficient between relative expression abundance of the sucrose synthase genes of apple (MdSUS) and sucrose synthase (SUS) enzyme activity during fruit development of apple

4.1. MdSUS genes showed similar structures in one group but differed between groups

Generally, phylogenic and structure analysis should be performed in this multi-gene family to predict possible genetic and evolutionary relationships and possible functions. SUS proteins have been divided into at least three groups, SUS I, SUSA, and New Group/NG, on the basis of the phylogenetic tree (Zhanget al. 2011, 2013).Our results support the idea that each of the three groups have at least one gene (the three groups in this study were rename as SUSI, SUSII and SUSIII, respectively). As shown in Fig. 2, the SUS II group is evolutionarily older than SUS I and III groups.

The length and position of introns and exons provide vital information regarding the evolutionary relationships of genes (Hu and Liu 2011). Our schematic structural analysis indicates thatMdSUSgenes have 10–14 introns(Table 1 and Fig. 4), which is consistent with Zhanget al.(2015) resusts, that is, SUS members encode 11–14 introns in their coding regions in peach. Similarly, the number of introns in the coding sequence ranged from 11 to 14 for cotton SUS genes (Chenet al. 2012). Moreover,PtrSUS in Populus tremulaalso have 9–14 introns (Zhanget al.2011). The above observations indicate that these similar characteristics in SUS family genes might contribute to their functional similarity within the same group.

4.2. The MdSUS genes exhibit distinct but partially overlapping expression patterns

To characterize the role of eachMdSUSmember, we first measured transcription levels for the 11MdSUSgenes in various plant organs, including shoot tips, mature leaves,young and mature fruits. The results showed that the 11MdSUSgenes exhibited distinct but partially overlapping expression patterns.MdSUS3.2,MdSUS3.3,MdSUS3.4andMdSUS3.1were closely clustered into group III, together withAtSUS5/6fromArabidopsisand withOsSUS5/6from rice, which are primarily expressed in shoot tips (Hiroseet al. 2008; Zhanget al. 2011). Therefore, these four genes might be key regulators of sucrose metabolism in shoot tips. No gene expressed exclusively in one tissue, and no tissues showed the expression of only one gene. These results suggest thatMdSUSgene functions are diversified and complex.

Sugar is a very important aspect of fruit quality that mostly relies on the accumulation of soluble sugar in fruit,followed by glucose and fructose. The transcript level of the 11MdSUSgenes in the fruit developmental process was assessed (Fig. 6). Most of theMdSUSgenes decreased during apple fruit development. In our study,MdSUS2.1andMdSUS1.1/1.2were mainly expressed in young and mature fruits, indicating that these genes might play key roles in fruit development, which is the most economically valuable part of the apple plant. Additionally,MdSUS1.3expression was at a lower level in apple fruit at all stages(Fig. 5-B). In contrast,MdSUS2.1is most highly expressed during fruit development, so it might play a key role in the process of fruit development, especially in the sucrose metabolism of mature fruit. In addition, MdSUS enzymatic activity was consistent withMdSUS1.3,MdSUS2.2,MdSUS2.3,MdSUS3.1andMdSUS3.3expression during fruit development.

Fig. 6 Expression pro file of the sucrose synthase genes in apple (MdSUS) at different fruit developmental stages. A, MdSUS I.B, MdSUS II. C, MdSUS III. D, sucrose synthase (SUS) activity. DAB, days after blooming. Bars mean standard error.

5. Conclusion

This study represents a primary investigation of SUS genes in apple, including analysis of structural characteristics,expressions in four different tissues and fruit developmental stages. This study can be used to understand the potential physiological roles of SUS genes in sucrose metabolism during apple fruit development as well as the structurefunction relationship among members of the apple SUS gene family.

Acknowledgements

This work was supported in part by the National Natural Science Foundation of China (31372038) and the Natural Basic Research Plan in Shaanxi Province of China(2015JQ3082). The authors would like to thank Prof.Cheng Lailiang from the School of Integrative Plant Science,Cornell University, USA for supplying the shoot tips cDNA of transgenic apple with anti-sense A6PR, and Mr. Fu Xuanchang from the College of Horticulture, Northwest A&F University, China for maintaining the plants.

Appendicesassociated with this paper can be available on http://www.ChinaAgriSci.com/V2/En/appendix.htm

Albrecht G, Mustroph A. 2003. Localization of sucrose synthase in wheat roots: Increasedin situactivity of sucrose synthase correlates with cell wall thickening by cellulose deposition under hypoxia.Planta, 217, 252–260.

An X, Chen Z, Wang J, Ye M, Ji L, Wang J, Liao W, Ma H. 2014.Identification and characterization of thePopulussucrose synthase gene family.Gene, 539, 58–67.

Baud S, Vaultier M N, Rochat C. 2004. Structure and expression profile of the sucrose synthase multi-gene family inArabidopsis.Journal of Experimental Botany, 55, 397–409.

Chang S, Puryear J, Cairney J. 1993. A simple and efficient method for isolating RNA from pine trees.Plant Molecular Biology Reporter, 11, 113–116.

Chen A Q, He S, Li F F, Li Z, Ding M Q, Liu Q P, Rong J K.2012. Analyses of the sucrose synthase gene family in cotton: Structure, phylogeny and expression patterns.BMC Plant Biology, 12, 85.

Fujii S, Hayashi T, Mizuno K. 2010. Sucrose synthase is an integral component of the cellulose synthesis machinery.Plant Cell Physiology, 51, 294–301.

Gaudin V, Lunness P A, Fobert P R, Towers M, Riou-Khamlichi C, Murray J A, Coen E, Doonan J H. 2000. The expression ofDcyclingenes defines distinct developmental zones in snapdragon apical meristems and is locally regulated by theCycloideagene.Plant Physiology, 122, 1137–1148.

Geigenberger P, Stitt M. 1993. Sucrose synthase catalyzes a readily reversible reactionin vivoin developing potato tubers and other plant tissues.Planta, 189, 329–339.

Harada T, Satoh S, Yoshioka T, Ishizawa K. 2005. Expression of sucrose synthase genes involved in enhanced elongation of pondweed (Potamogeton distinctus) turions under anoxia.Annals of Botany, 96, 683–692.

Hirose T, Scofield G N, Terao T. 2008. An expression analysis profile for the entire sucrose synthase gene family in rice.Plant Science, 174, 534–543.

Horst I, Welham T, Kelly S, Kaneko T, Sato S, Tabata S,Parniske M, Wang T L. 2007. Tilling mutants ofLotus japonicusreveal that nitrogen assimilation and fixation can occur in the absence of nodule-enhanced sucrose synthase.Plant Physiology, 144, 806–820.

Hu L F, Liu S Q. 2011. Genome-wide identification and phylogenetic analysis of theERFgene family in cucumbers.Genetics and Molecular Biology, 34, 624–633.

Iraqi D, Tremblay F M. 2001. Analysis of carbohydrate metabolism enzymes and cellular contents of sugars and proteins during spruce somatic embryogenesis suggests a regulatory role of exogenous sucrose in embryo development.Journal of Experimental Botany, 52,2301–2311.

Kleczkowski L A, Kunz S, Wilczynska M. 2010. Mechanisms of UDP-Glucose synthesis in plants.Critical Reviews in Plant Sciences, 29, 191–203.

Koch K E. 2004. Sucrose metabolism: Regulatory mechanisms and pivotal roles in sugar sensing and plant development.Current Opinion in Plant Biology, 7, 235–246.

Komatsu A, Moriguchi T, Koyama K, Omura M, Akihama T.2002. Analysis of sucrose synthase genes in citrus suggests different roles and phylogenetic relationships.Journal of Experimental Botany, 53, 61–71.

Li M, Feng F, Cheng L. 2012. Expression patterns of genes involved in sugar metabolism and accumulation during apple fruit development.PLoS ONE, 7, e33055.

Lutfiyya L L, Xu N F, D’Ordine R L, Morrell J A, Miller P W,Duff S M G. 2007. Phylogenetic and expression analysis of sucrose phosphate synthase isozymes in plants.Journal of Plant Physiology, 164, 923–933.

Ohto M, Onai K, Furukawa Y, Aoki E, Araki T, Nakamura K.2001. Effects of sugar on vegetative development and floral transition inArabidopsis.Plant Physiology, 127, 252–261.Rook F, Corke F, Card R, Munz G, Smith C, Bevan M W. 2001.Impaired sucrose-induction mutants reveal the modulation of sugar-induced starch biosynthetic gene expression by abscisic acid signalling.The Plant Journal, 26, 421–433.

Ruan Y L, Llewellyn D J, Liu Q, Xu S M, Wu L M, Wang L,Furbank R T. 2008. Expression of sucrose synthase in the developing endosperm is essential for early seed development in cotton.Functional Plant Biology, 35,382–393.

Sturm A, Tang G Q. 1999. The sucrose-cleaving enzymes of plants are crucial for development, growth and carbon partitioning.Trends in Plant Science, 4, 401–407.

Velasco R, Zharkikh A, Affourtit J, Dhingra A, Bhatnagar S K. 2010. The genome of the domesticated apple(Malus×domesticaBorkh).Nature Genetics, 42, 833–839.

Wei X, Liu F, Chen C, Ma F, Li M. 2014. TheMalus domesticasugar transporter gene family: Identifications based on genome and expression profiling related to the accumulation of fruit sugars.Frontiers in Plant Science, 5, 569.

Xiao X, Tang C, Fang Y, Yang M, Zhou B, Qi J, Zhang Y. 2014.Structure and expression profile of the sucrose synthase gene family in the rubber tree: Indicative of roles in stress response and sucrose utilization in the laticifers.The FEBS Journal, 281, 291–305.

Yagi T, Baroja-Fernandez E, Yamamoto R. 2003. Cloning,expression and characterization of a mammalian Nudix hydrolase-like enzyme that cleaves the pyrophosphate bond of UDP-glucose.Biochemical Journal, 370, 409–415.

Yamaki S, Ino M. 1992. Alteration of cellular compartmentation and membrane-permeability to sugars in immature and mature apple fruit.Journal of the American Society for Horticultural Science, 117, 951–954.

Yamaki S, Ishikawa K. 1986. Roles of four sorbitol related enzymes and invertase in the seasonal. Alleration of sugar metabolism in apple tissue.Journal of the American Society for Horticultural Science, 111, 134–137.

Zhang C H, Yu M L Ma R J, Shen Z J, Zhang B B, Nicholas K K. 2015. Structure, expression profile, and evolution of the sucrose synthase gene family in peach (Prunus persica).Acta Physiology Plant, 10, 37–81.

Zhang D Q, Xu B H, Yang X H, Zhang Z Y, Li B L. 2011.The sucrose synthase gene family inPopulus: Structure,expression, and evolution.Tree Genetics Genomes, 7,443–456.

Zhang J S, Arro J, Chen Y Q, Ming R. 2013. Haplotype analysis of sucrose synthase gene family in threeSaccharumspecies.BMC Genomics, 14, 314.

Zou C, Lu C, Shang H, Jing X, Cheng H, Zhang Y, Song G.2013. Genome-wide analysis of the Sus gene family in cotton.Journal of Integrative Plant Biology, 55, 643–653.

Zrenner R, Salanoubat M, Willmitzer L, Sonnewald U. 1995.Evidence of the crucial role of sucrose synthase for sink strength using transgenic potato plants (Solanumtuberosuml).The Plant Journal, 7, 97–107.