Current perspectives on the regulatory mechanisms of sucrose accumulation in sugarcane

Sugars transported from leaves (source) to stems (sink) energize cell growth, elongation, and maintenance. which are regulated by a variety of genes. This review reflects progress and prospects in the regulatory mechanism for maximum sucrose accumulation, including the role of sucrose metabolizing enzymes, sugar transporters and the elucidation of post-transcriptional control of sucrose-induced regulation of translation (SIRT) in the accumulation of sucrose. The current review suggests that SIRT is emerging as a significant mechanism controlling Scbzip44 activities in response to endogenous sugar signals (via the negative feedback mechanism). Sucrose-controlled upstream open reading frame (SC-uORF) exists at the 5' leader region of Scbzip44's main ORF, which inhibits sucrose accumulation through post-transcriptional regulatory mechanisms. Sucrose transporters (SWEET1a/4a/4b/13c, TST, SUT1, SUT4 and SUT5) are crucial for sucrose translocation from source to sink. Particularly, SWEET13c was found to be a major contributor to the efflux in the transportation of stems. Tonoplast sugar transporters (TSTs), which import sucrose into the vacuole, suggest their tissue-specific role from source to sink. Sucrose cleavage has generally been linked with invertase isozymes, whereas sucrose synthase (SuSy)-catalyzed metabolism has been associated with biosynthetic processes such as UDP-Glc, cellulose, hemicellulose and other polymers. However, other two key sucrose-metabolizing enzymes, such as sucrose-6-phosphate phosphohydrolase (S6PP) and sucrose phosphate synthase (SPS) isoforms, have been linked with sucrose biosynthesis. These findings suggest that manipulation of genes, such as overexpression of SPS genes and sucrose transporter genes, silencing of the SC-uORF of Scbzip44 (removing the 5' leader region of the main ORF that is called SIRT-Insensitive) and downregulation of the invertase genes, may lead to maximum sucrose accumulation. This review provides an overview of sugarcane sucrose-regulating systems and baseline information for the development of cultivars with higher sucrose accumulation.

Insights into Sucrose Metabolism and Its Ethylene-Dependent Regulation in Cucumis melo L

The melon (Cucumis melo L.), a fruit crop of significant economic importance, is prized for its sweet and succulent fruits. Among variations of soluble sugars, sucrose, a disaccharide composed of glucose and fructose, is a key carbohydrate present in melon fruits. The sucrose content also determines the quality and value of melon fruits. However, the accumulation of sucrose is a complex process involving the coordinated actions of multiple enzymes and pathways. In melon species, there are two types of fruit ripening modes including climacteric and non-climacteric. Due to this biological characteristic, melon is emerging as a good model for studying the ripening process. Ethylene is a well-known phytohormone regulating the ripening of climacteric fruits. Recently, a few studies have elucidated a primary ethylene-dependent signaling pathway of sucrose accumulation in melon fruits. This review aims to provide a careful overview of the sucrose biosynthesis pathways in melon. It is essential to understand the molecular mechanisms of sucrose metabolism as well as its regulation mode. The information will be useful for developing molecular marker-assisted breeding as well as genetic engineering strategies aiming to improve the sucrose content and quality of melon fruits. In addition, even though limited, the impacts of genetic background and environmental factors on sucrose accumulation in melon fruits are also discussed. These are useful for practical applications in melon cultivation and quality management.

The subgenome Saccharum spontaneum contributes to sugar accumulation in sugarcane as revealed by full-length transcriptomic analysis

INTRODUCTION

Modern sugarcane cultivars (Saccharum spp. hybrids) derived from crosses between S. officinarum and S. spontaneum, with high-sugar traits and excellent stress tolerance inherited respectively. However, the contribution of the S. spontaneum subgenome to sucrose accumulation is still unclear.

OBJECTIVE

To compensate for the absence of a high-quality reference genome, a transcriptome analysis method is needed to analyze the molecular basis of differential sucrose accumulation in sugarcane hybrids and to find clues to the contribution of the S. spontaneum subgenome to sucrose accumulation.

METHODS

PacBio full-length sequencing was used to complement genome annotation, followed by the identification of differential genes between the high and low sugar groups using differential alternative splicing analysis and differential expression analysis. At the subgenomic level, the factors responsible for differential sucrose accumulation were investigated from the perspective of transcriptional and post-transcriptional regulation.

RESULTS

A full-length transcriptome annotated at the subgenomic level was provided, complemented by 263,378 allele-defined transcript isoforms and 139,405 alternative splicing (AS) events. Differential alternative splicing (DA) analysis and differential expression (DE) analysis identified differential genes between high and low sugar groups and explained differential sucrose accumulation factors by the KEGG pathways. In some gene models, different or even opposite expression patterns of alleles from the same gene were observed, reflecting the potential evolution of these alleles toward novel functions in polyploid sugarcane. Among DA and DE genes in the sucrose source-sink complex pathway, we found some alleles encoding sucrose accumulation-related enzymes derived from the S. spontaneum subgenome were differentially expressed or had DA events between the two contrasting sugarcane hybrids.

CONCLUSION

Full-length transcriptomes annotated at the subgenomic level could better characterize sugarcane hybrids, and the S. spontaneum subgenome was found to contribute to sucrose accumulation.

Physiology and whole-plant carbon partitioning during stem sugar accumulation in sweet dwarf sorghum

A greater rate of phloem unloading and storage in the stem, not a higher rate of sugar production by photosynthesis or sugar export from leaves, is the main factor that results in sugar accumulation in sweet dwarf sorghum compared to grain sorghum. At maturity, the stem internodes of sweet sorghum varieties accumulate high concentrations of fermentable sugars and represent an efficient feedstock for bioethanol production. Although stem sugar accumulation is a heritable trait, additional factors that drive sugar accumulation in sorghum have not been identified. To identify the constraints on stem sugar accumulation in sweet sorghum, we used a combination of carbon-11 (¹¹C) radiotracer, physiological and biochemical approaches, and compared a grain sorghum and sweet dwarf sorghum line that have similar growth characteristics including height. Photosynthesis did not increase during development or differ between the sorghum lines. During the developmental transition to the reproductive stage, export of ¹¹C from leaves approximately doubled in both sorghum lines, but ¹¹C export in the sweet dwarf line did not exceed that of the grain sorghum. Defoliation to manipulate relative sink demand did not result in increased photosynthetic rates, indicating that the combined accumulation of C by all sink tissues was limited by the maximum photosynthetic capacity of source leaves. Nearly 3/4 of the ¹¹C exported from leaves was transported to the lower stem in sweet sorghum within 2 h, whereas in grain sorghum nearly 3/4 of the ¹¹C was in the panicle. Accordingly, the transcripts of several sucrose transporter (SUT) genes were more abundant in the stem internodes of the sweet dwarf line compared to the grain sorghum. Overall, these results indicate that sugar accumulation in sweet sorghum stems is influenced by the interplay of different sink tissues for the same sugars, but is likely driven by elevated sugar phloem unloading and uptake capacity in mature stem internodes.

The protein kinase FaSnRK1α regulates sucrose accumulation in strawberry fruits

In strawberry, sucrose is the major form of carbohydrate translocated from the leaves to the fruits and plays an important role in fruit ripening. As a conserved energy sensor, sucrose nonfermenting-1 (SNF1)-related kinase 1 (SnRK1) plays an important role in plant carbon metabolism. However, evidence that SnRK1 regulates sucrose accumulation in fruits is lacking. In this study, we transiently expressed FaSnRK1α in strawberry fruits and found that overexpression (OE) of the FaSnRK1α gene significantly increased the sucrose content, whereas repression of FaSnRK1α by RNA interference (RNAi) decreased the sucrose content. Further analysis revealed that FaSnRK1α increased the expression of FaSUS1 and FaSUS3 as well as the activity of sucrose synthase (SUS; EC 2.4.1.13) and that FaSPS1 expression and sucrose phosphate synthase (SPS; EC 2.4.1.14) activity were strongly downregulated, which decreased the accumulation of sucrose. However, the expression of FaSPS3, which is reported to contribute to sucrose accumulation, was induced by FaSnRK1α, and FaNI expression and invertase (INV; EC 3.2.1.26) activity were upregulated by FaSnRK1α. In addition, FaSnRK1α positively upregulated the expression of the sucrose transporter (SUT) genes FaSUT1 and FaSUT5 and interacted with FaSUS1, FaSPS1 and FaSPS3 proteins but not with FaSUS3, FaNI, FaSUT1 or FaSUT5 proteins. Overall, FaSnRK1α systematically regulates the expression of the genes and activities of key enzymes involved in the sucrose metabolic pathway and promotes the long-distance transport of sucrose, thereby increasing sucrose accumulation and ultimately promoting fruit ripening. However, the mechanisms by which sucrose transport and degradation are regulated by SnRK1 warrant additional research.

Culm transcriptome sequencing of Badila (Saccharum officinarum L.) and analysis of major genes involved in sucrose accumulation

Sugarcane is an important sugar and energy crop worldwide. It utilises highly efficient C4 photosynthesis and accumulates sucrose in its culms. The sucrose content in sugarcane culms is a quantitative trait controlled by multiple genes. The regulatory mechanism underlying the maximum sucrose level in sugarcane culms remains unclear. We used transcriptome sequences to identify the potential regulatory genes involved in sucrose accumulation in Saccarum officinarum L. cv. Badila. The sucrose accumulating internodes at the elongation and mature growth stage and the immature internodes with low sucrose content at the mature stage were used for RNA sequencing. The obtained differentially expressed genes (DEGs) related to sucrose accumulation were analysed. Results showed that the transcripts encoding invertase (beta-fructofuranosidase, EC: 3.2.1.26) which catalyses sucrose hydrolysis and 6-phosphofructokinase (PFK, EC: 2.7.1.11), a key glycolysis regulatory enzyme, were downregulated in the high sucrose accumulation internodes. The transcripts encoding key enzymes for ABA, gibberellin and ethylene synthesis were also downregulated during sucrose accumulation. Furthermore, regulated protein kinase, transcription factor and sugar transporter genes were also obtained. This research can clarify the molecular regulation network of sucrose accumulation in sugarcane.

Ethylene-mediated improvement in sucrose accumulation in ripening sugarcane involves increased sink strength

BACKGROUND

Sugarcane is a major crop producing about 80% of sugar globally. Increasing sugar content is a top priority for sugarcane breeding programs worldwide, however, the progress is extremely slow. Owing to its commercial significance, the physiology of sucrose accumulation has been studied extensively but it did not lead to any significant practical outcomes. Recent molecular studies are beginning to recognize genes and gene networks associated with this phenomenon. To further advance our molecular understanding of sucrose accumulation, we altered sucrose content of sugarcane genotypes with inherently large variation for sucrose accumulation using a sugarcane ripener, ethylene, and studied their transcriptomes to identify genes associated with the phenomenon.

RESULTS

Sucrose content variation in the experimental genotypes was substantial, with the top-performing clone producing almost 60% more sucrose than the poorest performer. Ethylene treatment increased stem sucrose content but that occurred only in low-sugar genotype. Transcriptomic analyses have identified about 160,000 unigenes of which 86,000 annotated genes were classified into functional groups associated with carbohydrate metabolism, signaling, localization, transport, hydrolysis, growth, catalytic activity, membrane and storage, suggesting the structural and functional specification, including sucrose accumulation, occurring in maturing internodes. About 25,000 genes were differentially expressed between all genotypes and treatments combined. Genotype had a dominant effect on differential gene expression than ethylene treatment. Sucrose and starch metabolism genes were more responsive to ethylene treatment in low-sugar genotype. Ethylene caused differential gene expression of many stress-related transcription factors, carbohydrate metabolism, hormone metabolism and epigenetic modification. Ethylene-induced expression of ethylene-responsive transcription factors, cytosolic acid- and cell wall-bound invertases, and ATPase was more pronounced in low- than in high-sugar genotype, suggesting an ethylene-stimulated sink activity and consequent increased sucrose accumulation in low-sugar genotype.

CONCLUSION

Ethylene-induced sucrose accumulation is more pronounced in low-sugar sugarcane genotype, and this is possibly achieved by the preferential activation of genes such as invertases that increase sink strength in the stem. The relatively high enrichment of differentially expressed genes associated with hormone metabolism and signaling and stress suggests a strong hormonal regulation of source-sink activity, growth and sucrose accumulation in sugarcane.

Expression analysis of genes associated with sucrose accumulation and its effect on source-sink relationship in high sucrose accumulating early maturing sugarcane variety

Sucrose synthesis/accumulation in sugarcane depends on the source-sink communication wherein source responds to sink demand for photoassimilate supply. Sucrose in stalk (sink) acts as signal, and sends feedback to restrain further synthesis of sucrose by regulating photosynthetic efficiency of leaves (source). Hence sucrose synthesis/accumulation is controlled by many genes and regulatory sequences including 3 invertases (SAI, CWI, NI), sucrose synthase (SuSy) and sucrose phosphate synthase (SPS). SPS and invertase play key role in enhancing sink strength which ultimately promotes greater sucrose accumulation in the sink tissues. In present study, a significant positive correlation was found between sucrose% of source and sink tissues which was greater in the top (R ² = 0.679) than middle (R ² = 0.580) and bottom (R ² = 0.518) internodes, depicting that sucrose accumulation in the stalk bears a direct relation with sucrose translocation efficiency from source. Results indicated an increased sucrose% with maturity, while reducing sugar content decreased with crop growth. qRT-PCR results exhibited an elevated expression of invertase in immature sink tissues depicting increased sink requirement, which declined with maturity. Similarly, increased PEP carboxylase gene expression as observed supported the fact that higher sink demand results in enhanced photosynthetic rate and thus influences the source activity. SPS was found active at initial stage of cane development indicating its role in sucrose synthesis. Thus by studying expression patterns of the different genes both, in source and sink tissues, a better understanding of the sucrose accumulation pathway in sugarcane is possible, which in turn can help in elucidating ways to enhance sucrose concentration in sink.