Sucrose-phosphate phosphatase from sugarcane reveals an ancestral tandem duplication

Background Sugarcane is capable to store large amounts of sucrose in the culm at maturity hence it became a major source of sucrose for the food and the renewable energy industries. Sucrose, the main disaccharide produced by photosynthesis, is mainly stored in the vacuole of the cells of non-photosynthetic tissues. Two pathways are known to release free sucrose in plant cells, one is de novo synthesis dependent on sucrose phosphate synthase (SPS) and sucrose phosphate phosphatase (S6PP) while the other is regulatory and dependent on sucrose synthase (SuSy) activity. The molecular understanding of genes that give rise to the expression of the enzyme sucrose phosphate phosphatase, responsible for the release of sucrose in the last synthetic step lag behind the regulatory SuSy gene. Results Sugarcane genome sequencing effort disclosed the existence of a tandem duplication and the present work further support that both S6PP.1 and S6PP_2D isoforms are actively transcribed in young sugarcane plants but significantly less at maturity. Two commercial hybrids (SP80–3280 and R570) and both Saccharum spontaneum (IN84–58) and S.officinarum (BADILLA) exhibit transcriptional activity at three-month-old plants of the tandem S6PP_2D in leaves, culm, meristem and root system with a cultivar-specific distribution. Moreover, this tandem duplication is shared with other grasses and is ancestral in the group. Conclusion Detection of a new isoform of S6PP resulting from the translation of 14 exon-containing transcript (S6PP_2D) will contribute to the knowledge of sucrose metabolism in plants. In addition, expression varies along plant development and between sugarcane cultivars and parental species.


Background
Sugarcane economic value as a world commodity is due to its high sugar yield, renewable energy and other biomolecules production [1]. The high sugar yield results from its ability to store large amounts of sucrose in the internodes of the culm as part of its developmental program. Sucrose is the most used form of sugar in plants and its partitioning depends on finetuning synthesis, storage and metabolic uses. Sucrose metabolism can be divided in three steps: synthesis, transport and accumulation. Sucrose synthesis occurs in leaf cells as part of photosynthesis, being the main metabolite obtained in this cellular process. Sucrose is synthesized in the cytosol and released by the action of the enzyme Sucrosephosphate phosphatase (EC 3.1.3.24, PF 08472), also known as sucrose-6 phosphate phosphohydrolase (S6PP). S6PP catalyzes the last reaction step of this metabolic pathway where sucrose-6-phosphate (Suc6P), product of the enzymatic reaction of sucrose phosphate synthase enzyme (EC 2.4.1.14) (SPS), is dephosphorylated and released as sucrose [2,3].
The release of free sucrose in the cytosol drives its transport to the phloem conducting vessels, being the most common form of carbohydrate translocated from source tissues to sink tissues and organs of the whole plant [4,5]. Finally, the produced sucrose is stored in the vacuoles of the parenchyma cells of all the nonphotosynthetic tissues. At this stage, accumulation, the sucrose enters the cells by either the apoplastic or symplastic route where the enzyme Sucrose synthase (EC 2.4.1.13) (SuSy), plays a regulatory role converting it into UDP-glucose according to the needs of the plant physiology through a reversible reaction. Alternatively, sucrose can also be activated by the SPS to form the intermediate Suc6P which becomes dephosphorylated by the action of the enzyme S6PP [6]. Dephosphorylation at sink tissues provides the energy necessary for the entry of sucrose into the vacuole, where it is finally accumulated in the form of glucose and fructose [7]. Different to this energetic function depicted in most land plants, some cyanobacteria synthesize sucrose as an adaptation to osmotic stress [8].
Modern varieties of sugarcane are interspecific hybrids (Saccharum spp.) selected after cross-breeding within Saccharum species, mainly between Saccharum officinarum L. and S. spontaneum L. [9]. Examples of these modern cultivars are R570 (bred in the French island of Reunion) and SP80-3280 (bred in São Paulo, Brazil) and both have their genomes partially sequenced [10,11]. A previous work released a collection of 314 sugarcane Bacterial Artificial Chromosomes (BACs) from R570 cultivar in which a tandem duplication of the gene S6PP was described [12]. This tandem duplication was further validated in the recent releases of genomes sequences [10,13]. The two S6PP genes have a similar gene structure with eight exons, five of which are conserved in size and contain the catalytic site of the protein. Being separated by an intergenic region of 630 bases between the STOP codon of S6PP.1 and the ATG codon from S6PP.2 ( Figure S1).
The S6PP enzyme is found in vascular (liver plants, gymnosperms and angiosperms) and non-vascular plants (green algae, mosses). In some vascular plants, the S6PP enzyme has been described as a homodimer of about 120 KDa. As examples: Rice S6PP has been described as dimeric of 100-120 KDa, with 50 KDa monomer [14,15]; the pea S6PP forms also a dimer of 120 KDa, with 55 KDa monomers [16]; whereas Arabidopsis S6PP dimer is of 90 KDa, with 52 KDa monomers [17]. Maize S6PP information is only available from denaturing gel and presents molecular mass of 47.2 KDa. Synechocystis and Anabaena, both cyanobacteria, present a smaller (27)(28) and monomeric S6PP enzyme [8,[18][19][20]. Despite this monomeric characterization in bacteria, especially belonging to the cyanobacteria clade, a two-domain containing protein SPS and S6PP has been identified in the proteobacteria Methylobacillus flagellatus, characterized to be a tetramer of 336 KDa with 84KDa monomers [21]. Regardless of these different S6PP arrangements, the basic unit (monomer) in all species share significant similarity at the amino acid sequence mostly in the catalytic domain [8,15].
The main aim of the present work is to characterize at molecular level the sugarcane S6PP to contribute to the sucrose metabolism knowledge in plants. We explored the existence of a new isoform, derived from the alternative splicing of the tandem duplication as a monomer with two catalytic domains. In addition to S6PP.1 and/or S6PP.2 genes transcripts, we detected a new isoform resulting from the translation of 14 exon-containing transcript (S6PP_2D). We demonstrate that both are actively transcribed in different sugarcane tissues, with differential expression along plant development stage and between sugarcane cultivars or parental species.

S6PP genomic organization of two commercial sugarcane cultivars
A comparative genomic approach addressed the presence of the previously described tandem duplication in two BAC in a collection of 21 sequenced genomic fragments from two sugarcane cultivars, SP80-3280 and R570 [11,12]. According to Table 1, the tandem duplication is present in all sequenced fragments irrespective of the cultivar. In addition, the genomic environment support that these genes are located at a single locus with structural genomic variants shared between the two cultivars as denoted by their neighboring genes and transposable elements insertions as shown in supplementary Figure S2. To improve our knowledge on the evolutionary history of these genomic fragments, a phylogenetic tree using at least 5000 bases per BAC was built from the alignment of these two tandem genes starting at the ATG from S6PP.1 up to the stop codon from S6PP.2 gene. Figure 1 depicts that differences between the genomic regions of these two hybrid cultivars do not group them distinctively. Ten of the SP80-3280 BACs genomic regions branch in one consistent sequence clade with the four R570 genomic regions, while the other seven genomic regions group separately. Grouping of both sugarcane cultivars in one clade is supported with high bootstrap which is indicative that these genomic regions are very similar with a few INDE LS and SNPs (data not shown).
NETWORK analysis allowed us to address the nucleotide diversity at population level. A fragment of 1539 nucleotides, which include the intergenic region between the two copies of S6PP, of all the aforementioned sequenced BACs were included in the original network alignment published by De Setta et al. [12]. Almost all the sequences grouped into four nodes, as shown in Fig. 2. All nodes are composed of domesticated varieties and SP80-3280 BACs, except node B. The R570 BACs are prevalent in the central node A, composed of 36 sequences, including S. officinarum (Badilla). A few S. officinarum sequences were clustered with SP80-3280 and hybrid cultivars used in Brazilian breeding programs (Fig. 2, node C). However, none of the S. spontaneum sequenced fragments were found with sugar accumulating varieties.
We investigated the promoter region of both S6PP.1 and S6PP.2 in our BAC collection and found that little nucleotide similarity is observed upstream the translation start-site ATG between the two genes. However, multiple nucleotide alignment within each of the promoter regions support high nucleotide identity. Analysis of the 620 nucleotides upstream of the S6PP.1 and S6PP.2 ATG, revealed that S6PP.1 has a defined and conserved TATA box as well as several overlapping motifs associated to ABA and light responsiveness, a few drought and auxin responsive elements ( Figure S3). All but three S6PP.2 upstream regions do not have TATA boxes at expected positions but all share binding sites to gibberellin, auxin and light responsive elements. SP803280_480_A02, SP803280_548_H17 and SP803280_ 264_P08 have a TATA box in the minus strand at position 271-277.

S6PP tandem duplication is ancestral to sugarcane
A survey in Phytozome13 database [22], revealed that the S6PP tandem duplication is not unique to sugarcane, is also present in at least two families (Poaceae and Ranunculaceae). We identified the tandem duplication in genomes of different cultivars from Sorghum bicolor and Panicum hallii, Panicum virgatum 5, Setaria italica v2.2, Setaria viridis v2.1, Miscanthus sinensis v7.1, but not in Zea mays, Brachypodium distachion nor Oryza sativa. The tandem duplicates are located in a fragment of~5000 bases in all analyzed Poaceae, whereas in the Ranunculaceae Aquilegia coerulea v3.1 (highlighted in gray, Table 2) the region containing the tandem duplicates is larger, of about 8000 bp.
In order to determine if the duplication occurred multiple times in Poaceae or if it was ancestral, a molecular phylogenetic tree was built using only the S6PP catalytic domain (Fig. 3, Figure S4). The S6PP_2D.1 and S6PP_ 2D.2 annotation was used to explore each specific domain in the two-domain S6PP protein. Two highly supported nodes, indicate that all S6PP.1 and all S6PP.2 Further to investigate the selection rate of the sugarcane homologs, dN/dS calculation [23] was performed on S6PP.1, S6PP.2, S6PP_2D, Miscanthus tandem orthologs located on chromosome 17 and maize ZmSPP1 (ZmPHJ40 08G139200) and ZmSPP2 (ZmPHJ40 10G092800). Reciprocal calculations support that these genes are equally under purifying selection with similar substitution rates most probably to maintain protein function. Nonetheless, the tandemly duplicated genes are more distantly related to each other than their corresponding Miscanthus tandem orthologs (Table S1). When maize ZmSPP1 and ZmSPP2 are used for dN/dS calculations, the tandemly duplicated genes are more similar to ZmSPP2 in agreement with the ML tree presented in Fig. 3.

Expression pattern of S6PP genes in sugar cane tissues
The expression pattern of S6PP.1 and S6PP_2D in sugarcane was verified with an experiment built with following rationale: sucrose is first produced in mature leaves as a result from photosynthesis; then it is translocated through the phloem to sink tissues (apical meristem and roots). We examined the expression pattern from two commercial hybrid varieties SP80-3280 and R570, as well as two progenitor species S. officinarum (Badilla) and S. spontaneum (IN84-58) in leaves, culm, meristem and roots as well as in different developmental time points (3, 6, and 9 months).
The heat map of log delta CT values (CT value of target gene -CT value of housekeeping) [24] presented ( Fig. 4) supports that the S6PP_2D isoform is expressed mainly at 3 months in most tissues examined, in A three-way ANOVA [25] analysis presented as a box plot in Fig. 5 support the interaction between cultivar, age and tissues on the expression of S6PP.1 and S6PP_ 2D genes. Significant differences in the expression are observed for Badilla and SP80-3280 tissues and age whereas it is less pronounced for R570 and IN84-58. The interaction between cultivar, age and tissue is supported statistically at p = 0.013 for S6PP and p = 0.007 for S6PP_2D.1. Both genes in Badilla and SP80-3280 genetic background are strongly influenced by factors such as age and tissues. In addition, all simple pairwise comparisons, between the tissues and ages per cultivar, were run with a Bonferroni adjustment applied (p.adjust = 0.05) [26]. The non-accumulating sugar IN84-58 cultivar presents the least variation in the expression pattern when all conditions are compared. Conversely, the commercial SP80-3280 varies most and presents the highest expression level.
Transcription of S6PP gene is expected in leaves, a photosynthesizing tissue capable of producing sucrose from the Suc6P, substrate of S6PP enzyme in the sucrose metabolic pathway. This free sucrose is translocated through the conductive vessels of the phloem for all non-photosynthetic tissues, including sink tissues, where it is accumulated after a series of interconversion reactions. The export of carbohydrates from photosynthesizing leaves (source) provides the substrate for the growth and maintenance of non-photosynthetic vegetative tissues (sink) [27] (Fig. 6). Sugars represent the main source of energy for all eukaryotic organisms, carbohydrates are essential for fundamental processes in plant growth and sugarcane tends to accumulate more sucrose near the last phase of the culture cycle, when it has a low growth rate.
According to the results presented in Fig. 4, S6PP genes are more expressed in tissues at 3 months of age and a decrease over time of S6PP_2D in source tissues. At early stages of the plant development when plant growth is accelerated, there is a greater requirement of sucrose [27]. Based on the expression of the S6PP transcripts, the results support that the gene is involved in the cyclic balance between synthesis, storage, use and re-synthesis to sustain the growth needs of the plant. Therefore, the transport and partition of sugars from phototrophic leaves (source) to heterotrophic organs (sink) through the phloem are the main parameters that control crop productivity [5].

Discussion
Sugarcane tends to accumulate more sucrose when it has a low growth rate [28], when it reaches maturity, between 10 and 14 months [29]. Early in vitro assays suggested that the catalytic activity of S6PP enzyme is downregulated to 40% in the presence of 50 mM of sucrose in sugar cane Pindar variety [30]. Further biochemical studies reported that S6PP from sugarcane (Saccharum spp. cv NCO310) and red beet were inhibited 40 and 59%, respectively in the presence of 100 mM sucrose [31]. In addition, S6PP from rice (Oryza sativa) presented 15% inhibition in 160 mM sucrose, and S6PP from Synechocystis sp. PCC 6803 is inhibited in 19 and 27% with 200 mM and 660 mM sucrose, respectively [15]. It is interesting to note that under natural conditions, commercial sugarcane varieties sucrose levels can reach high concentrations up to 650 mM [32]. A comparative approach of gene expression between mature and immature culms reported by Carson et al. [33] reveal that some of the enzymes involved in the sucrose metabolism are more abundant in immature tissues whereas are less expressed in mature tissues.
The results presented here support that the S6PP expression level varies between tissues and during plant development. According to [33], the accumulation of sucrose is dependent on the size and activity of the tissue. We anticipate that the results described here support that the ability to accumulate sucrose could be related by differences of S6PP gene expression levels between varieties. S. spontaneum (variety IN-8458), in general, showed the least variation of expression of both S6PP.1 and S6PP_2D, in tissues and across the developmental time analyzed but, this species is characterized by not accumulating large amounts of sucrose. However, S spontaneum brings increased tolerance to biotic and abiotic stresses and adaptability into breeding programs. Conversely, S. officinarum (Badilla), that presented the largest variation in levels of expression, is known to accumulate high levels of sucrose in the cell but has less resistance to diseases. Wang et al. [32] state that hybrid commercial varieties are capable of accumulating high concentrations of sucrose, up to 650 mM or 18% of their  fresh weight at maturity. Therefore, we speculate the reduction of the S6PP transcripts is due to the high accumulation of sucrose.

Conclusions
A comparative genomic approach between two commercial hybrids (R570 and SP80-3280) using sequenced BAC fragments, confirmed the presence of a tandem gene duplication in 21 genomic fragments, of which some are predicted to be transcribed as a single transcript, thus containing two S6PP domains. The phylogenetic analyses indicate that this tandem duplication is conserved in Poaceae thus, ancestral to Saccharum. Our RT-qPCR analyses indicate that the new isoform, S6PP_2D, is differentially expressed through developmental stage, depending also on the sugarcane cultivar studied. In summary, the present work describes the phylogeny and expression patterns of S6PP genes in sugarcane ancestral and commercial cultivars.

Comparative genomics
Twenty-one genomic fragments containing S6PP genes of the commercial hybrid sugarcane R570 and SP80-3280 were analyzed. Table 1 list the name, genomic cultivar origin, number of genes and transposable elements (TEs) identified in each genomic fragment after manual inspection and annotation. All genes predicted during BAC annotation were curated against NCBI databases and the TEs were analyzed in GIRI RePBASE [34] databases, considering 80-80-80 rule [35]. A Maximum Likelihood phylogenetic tree was carried out with MEGA7 software [36] using T92 + G as the best model, with 1000 bootstraps to determine the phylogeny and possible grouping and segregations between the fragments. The alignment included the tandem duplication genomic region from the start codon of the first gene sequence to the stop codon of the second tandem duplicated gene including exon, intron and intergenic regions. For comparative purposes Neighbor Joining and Parsimony phylogenetic tree were produced and no differences in grouping was observed. Aligned sequence size is indicated in the

S6PP domain phylogeny in Poaceae
The phylogenetic analysis was carried out by Maximum Likelihood method, JTT model, with 1000 bootstrap pseudo-replicates using the S6PP catalytic domain from Poaceae genomes obtained from Phytozome13. S6PP gene notation variants were numbered 1 and 2 according to their position and orientation in the tandem region. S6PP.1 and S6PP.2 denote first gene in the genomic fragment and its tandem duplication in the genome. Evolutionary analysis was inferred in MEGAX software [40].

RNA extraction and cDNA synthesis
Total RNA was isolate employing TRIzol® reagent (Invi-trogen®), following manufacturer instructions, and digested with DNAse TURBO DNA-free (Ambion®, Life Technologies™) to remove any contaminating genomic DNA. Total RNA was visualized by agarose gel, to determine integrity. Total RNA was used as cDNA template by using reverse transcriptase SuperScript™ III (Invitro-gen®). cDNA samples were kept at − 20°C until used.
Expression data was obtained from two independent biological replicates each with three technical replicates in a QuantStudio 7 Flex Real-Time PCR (Applied Biosystems) comprised 2 min denaturation at 50°C, 30 s at 98°C and then followed by 40 cycles (98°C for 15 s and 60°C for 60 s). To check the specificity of the amplicon, the qRT-PCR products of each gene were used for analysis of melting curves in each reaction (data not shown).

Data analysis
The dCT values were obtained across difference of CT values between the target genes and housekeeping (CT target -CT housekeeping) for each sample. A heat map was made for these data, the values were plotted logarithmic scale (Log dCT) [24] and clusterized by correlation method. A three-way ANOVA [25] was conducted to determine the effects of cultivar, age and tissues factors on expression genes S6PP.1 and S6PP_2D. Residual analysis was performed to test for the assumptions of the three-way ANOVA. Normality was assessed using Shapiro-Wilk's normality test.