Growth and metabolism in sugarcane are altered by the creation of a new hexose-phosphate sink

Physiological Changes of Bamboo (Fargesia yunnanensis) Shoots During Storage and the Related Cold Storage Mechanisms

The mechanisms for physiological senescence in bamboo shoots after harvest remain unclear. This study investigated physiological changes in Fargesia yunnanensis shoots during storage at different temperatures. The relationship between morphological and physiological changes in bamboo shoots during storage was also analyzed. The results show that cold storage can reduce weight loss, browning, respiration rates, and sugar degradation in bamboo shoots; decrease related enzymatic activities; and inhibit the increase in lignin and cellulose content. The quality of bamboo shoots declines more during the first 3d after harvesting than it does during subsequent periods. The increase in the degree of lignification and fibrosis is the main reason for senescence and for the decline in quality of bamboo shoots after harvest. The bamboo shoots under RT conditions began browning during the third 3d of storage, with a browning rate of 688gkg^-1 even in the upper parts; the increase in shoot browning degrees significantly decreased the quality. Low temperatures had better inhibitory effects on browning than they did on lignification and fibrosis. Nonstructural carbohydrates in bamboo shoots are degraded and flow into sheath and shoot respiration, phenols, and shoot fibrosis and lignification at room temperature, but only flow into sheath respiration, shoot fibrosis, and lignification at cold temperature. Soluble protein and free amino acids are primarily distributed into shoot and sheath respiration and into phenols at room temperature, but that process is well inhibited at cold temperature. Bamboo shoots, once removed from cold storage, should be consumed rapidly because enzyme activity recovers quickly. This research provides new theoretical information on the preservation of bamboo shoots.

Current and possible approaches for improving photosynthetic efficiency

One of the most important tasks laying ahead today's biotechnology is to improve crop productivity with the aim of meeting increased food and energy demands of humankind. Plant productivity depends on many genetic factors, including life cycle, harvest index, stress tolerance and photosynthetic activity. Many approaches were already tested or suggested to improve either. Limitations of photosynthesis have also been uncovered and efforts been taken to increase its efficiency. Examples include decreasing photosynthetic antennae size, increasing the photosynthetically available light spectrum, countering oxygenase activity of Rubisco by implementing C4 photosynthesis to C3 plants and altering source to sink transport of metabolites. A natural and effective photosynthetic adaptation, the sugar alcohol metabolism got however remarkably little attention in the last years, despite being comparably efficient as C4, and can be considered easier to introduce to new species. We also propose root to shoot carbon-dioxide transport as a means to improve photosynthetic performance and drought tolerance at the same time. Different suggestions and successful examples are covered here for improving plant photosynthesis as well as novel perspectives are presented for future research.

Isolation and Characterization of ScGluD2, a New Sugarcane beta-1,3-Glucanase D Family Gene Induced by Sporisorium scitamineum, ABA, H2O2, NaCl, and CdCl2 Stresses

Beta-1,3-glucanases (EC 3.2.1.39), commonly known as pathogenesis-related (PR) proteins, play an important role not only in plant defense against fungal pathogens but also in plant physiological and developmental processes. However, only a limited number of sugarcane beta-1,3-glucanase genes have been isolated. In the present study, we identified and characterized a new beta-1,3-glucanase gene ScGluD2 (GenBank Acc No. KF664181) from sugarcane. An X8 domain was present at the C terminal region of ScGluD2, suggesting beta-1,3-glucan-binding function. Phylogenetic analysis showed that the predicted ScGluD2 protein was classified into subfamily D beta-1,3-glucanase. Localization of the ScGluD2 protein in the plasma membrane was determined by tagging it with green fluorescent protein. The expression of ScGluD2 was more up-regulated in sugarcane smut-resistant cultivars in the early stage (1 or 3 days) than in the susceptible ones after being challenged by the smut pathogen, revealing that ScGluD2 may be involved in defense against the invasion of Sporisorium scitamineum. Transient overexpression of ScGluD2 in Nicotiana benthamiana leaves induced a defense response and exhibited antimicrobial action on the tobacco pathogens Pseudomonas solanacearum and Botrytis cinerea, further demonstrating that ScGluD2 was related to the resistance to plant pathogens. However, the transcripts of ScGluD2 partially increased (12 h) under NaCl stress, and were steadily up-regulated from 6 to 24 h upon ABA, H2O2, and CdCl2 treatments, suggesting that ABA may be a signal molecule regulating oxidative stress and play a role in the salt and heavy metal stress-induced stimulation of ScGluD2 transcripts. Taken together, ScGluD2, a novel member of subfamily D beta-1,3-glucanase, was a stress-related gene of sugarcane involved in plant defense against smut pathogen attack and salt and heavy metal stresses.

Plant aldo-keto reductases (AKRs) as multi-tasking soldiers involved in diverse plant metabolic processes and stress defense: A structure-function update

The aldo-keto reductase (AKR) superfamily comprises of a large number of primarily monomeric protein members, which reduce a broad spectrum of substrates ranging from simple sugars to potentially toxic aldehydes. Plant AKRs can be broadly categorized into four important functional groups, which highlight their roles in diverse plant metabolic reactions including reactive aldehyde detoxification, biosynthesis of osmolytes, secondary metabolism and membrane transport. Further, multiple overlapping functional aspects of plant AKRs including biotic and abiotic stress defense, production of commercially important secondary metabolites, iron acquisition from soil, plant-microbe interactions etc. are discussed as subcategories within respective major groups. Owing to the broad substrate specificity and multiple stress tolerance of the well-characterized AKR4C9 from Arabidopsis thaliana, protein sequences of all the homologues of AKR4C9 (A9-like proteins) from forty different plant species (Phytozome database) were analyzed. The analysis revealed that all A9-like proteins possess strictly conserved key catalytic residues (D-47, Y-52 and K-81) and belong to the pfam00248 and cl00470 AKR superfamilies. Based on structural homology of the three flexible loops of AKR4C9 (Loop A, B and C) responsible for broad substrate specificity, A9-like proteins found in Brassica rapa, Phaseolus vulgaris, Cucumis sativus, Populus trichocarpa and Solanum lycopersicum were predicted to have a similar range of substrate specificity. Thus, plant AKRs can be considered as potential breeding targets for developing stress tolerant varieties in the future. The present review provides a consolidated update on the current research status of plant AKRs with an emphasis on important functional aspects as well as their potential future prospects and an insight into the overall structure-function relationships of A9-like proteins.

Phosphomannose isomerase affects the key enzymes of glycolysis and sucrose metabolism in transgenic sugarcane overexpressing the manA gene

Sugarcane is one of the most important crops cultivated for the production of sugar and ethanol. In our previous studies, an innovative positive selection system for obtaining transgenic sugarcane, which utilized the E. coli-derived manA gene as the selectable marker and mannose as the selective agent, was developed and patented in China. In this paper, the influence of phosphomannose isomerase (PMI) overexpression on the key enzymes of both glycolysis and sucrose metabolism was investigated in transgenic sugarcane through the manA gene. Overexpressed PMI increased hexokinase activity by approximately 24 % compared with non-transgenic control plants, but pyruvate kinase (PK) activity was reduced by approximately 14 %. In comparison with the non-transgenic control plants, the activities of sucrose synthase, sucrose-phosphate synthase, and acid invertase were also modestly affected in the PMI-overexpressing transgenic plants, but no significant differences were observed at the stalk elongation and maturity stages. However, agronomic and technical traits were not affected by manA gene overexpression in the transgenic sugarcane. In conclusion, PMI overexpression significantly affected the hexokinase and PK activities by catalyzing the reversible interconversion between mannose-6-phosphate and fructose-6-phosphate, which is an intermediate of glycolysis. However, it had no significant effects on sucrose accumulation in sugarcane.

Factors affecting polyhydroxybutyrate accumulation in mesophyll cells of sugarcane and switchgrass

Background

Polyhydroxyalkanoates are linear biodegradable polyesters produced by bacteria as a carbon store and used to produce a range of bioplastics. Widespread polyhydroxyalkanoate production in C₄ crops would decrease petroleum dependency by producing a renewable supply of biodegradable plastics along with residual biomass that could be converted into biofuels or energy. Increasing yields to commercial levels in biomass crops however remains a challenge. Previously, lower accumulation levels of the short side chain polyhydroxyalkanoate, polyhydroxybutyrate (PHB), were observed in the chloroplasts of mesophyll (M) cells compared to bundle sheath (BS) cells in transgenic maize (Zea mays), sugarcane (Saccharum sp.), and switchgrass (Panicum virgatum L.) leading to a significant decrease in the theoretical yield potential. Here we explore various factors which might affect polymer accumulation in mesophyll cells, including targeting of the PHB pathway enzymes to the mesophyll plastid and their access to substrate.

Results

The small subunit of Rubisco from pea effectively targeted the PHB biosynthesis enzymes to both M and BS chloroplasts of sugarcane and switchgrass. PHB enzyme activity was retained following targeting to M plastids and was equivalent to that found in the BS plastids. Leaf total fatty acid content was not affected by PHB production. However, when fatty acid synthesis was chemically inhibited, polymer accumulated in M cells.

Conclusions

In this study, we provide evidence that access to substrate and neither poor targeting nor insufficient activity of the PHB biosynthetic enzymes may be the limiting factor for polymer production in mesophyll chloroplasts of C₄ plants.

Field performance of transgenic sugarcane produced using Agrobacterium and biolistics methods

Future genetic improvement of sugarcane depends, in part, on the ability to produce high-yielding transgenic cultivars with improved traits such as herbicide and insect resistance. Here, transgenic sugarcane plants generated by different transformation methods were assessed for field performance over 3 years. Agrobacterium-mediated (Agro) transgenic events (35) were produced using four different Agrobacterium tumefaciens strains, while biolistic (Biol) transgenic events (48) were produced using either minimal linearized DNA (LDNA) transgene cassettes with 5', 3' or blunt ends or whole circular plasmid (PDNA) vectors containing the same transgenes. A combined analysis showed a reduction in growth and cane yield in Biol, Agro as well as untransformed tissue culture (TC) events, compared with the parent clone (PC) Q117 (no transformation or tissue culture) in the plant, first ratoon and second ratoon crops. However, when individual events were analysed separately, yields of some transgenic events from both Agro and Biol were comparable to PC, suggesting that either transformation method can produce commercially suitable clones. Interestingly, a greater percentage of Biol transformants were similar to PC for growth and yield than Agro clones. Crop ratoonability and sugar yield components (Brix%, Pol%, and commercial cane sugar (CCS)) were unaffected by transformation or tissue culture. Transgene expression remained stable over different crop cycles and increased with plant maturity. Transgene copy number did not influence transgene expression, and both transformation methods produced low transgene copy number events. No consistent pattern of genetic changes was detected in the test population using three DNA fingerprinting techniques.

Non-invasive monitoring of sucrose mobilization from culm storage parenchyma by magnetic resonance spectroscopy

Because sucrose stored in mature stalks (in excess of 40% of stalk dry weight) can be wholly mobilized to supply carbon for the growth of heterotrophic tissues, we propose that sucrose mobilization requires a net sink-to-source transition that acts in toto within sett internode storage parenchyma. Based on our data we propose that mobilization of sucrose from culm storage parenchyma requires minimal investment of metabolic resources, and that the mechanism of sucrose mobilization is metabolically neutral. By magnetic resonance spectroscopy and phloem-specific tracer dyes, strong evidence was found that sucrose is mobilized from sett storage parenchyma via phloem to the growing shoot tissue. An analysis of the enzyme activities involved in sucrose metabolism and glycolysis suggested that sucrose synthase activity is downregulated due to the effects of sucrose mobilization. Overall, metabolism in storage parenchyma shifts from futile cycling to a more quiescent state during sucrose mobilization.

Control limits for accumulation of plant metabolites: brute force is no substitute for understanding

Which factors limit metabolite accumulation in plant cells? Are theories on flux control effective at explaining the results? Many biotechnologists cling to the idea that every pathway has a rate limiting enzyme and target such enzymes first in order to modulate fluxes. This often translates into large effects on metabolite concentration, but disappointing small increases in flux. Rate limiting enzymes do exist, but are rare and quite opposite to what predicted by biochemistry. In many cases however, flux control is shared among many enzymes. Flux control and concentration control can (and must) be distinguished and quantified for effective manipulation. Flux control for several 'building blocks' of metabolism is placed on the demand side, and therefore increasing demand can be very successful. Tampering with supply, particularly desensitizing supply enzymes, is usually not very effective, if not dangerous, because supply regulatory mechanisms function to control metabolite homeostasis. Some important, but usually unnoticed, metabolic constraints shape the responses of metabolic systems to manipulation: mass conservation, cellular resource allocation and, most prominently, energy supply, particularly in heterotrophic tissues. The theoretical basis for this view shall be explored with recent examples gathered from the manipulation of several metabolites (vitamins, carotenoids, amino acids, sugars, fatty acids, polyhydroxyalkanoates, fructans and sugar alcohols). Some guiding principles are suggested for an even more successful engineering of plant metabolism.

Metabolic engineering of sugars and simple sugar derivatives in plants

Carbon captured through photosynthesis is transported, and sometimes stored in plants, as sugar. All organic compounds in plants trace to carbon from sugars, so sugar metabolism is highly regulated and integrated with development. Sugars stored by plants are important to humans as foods and as renewable feedstocks for industrial conversion to biofuels and biomaterials. For some purposes, sugars have advantages over polymers including starches, cellulose or storage lipids. This review considers progress and prospects in plant metabolic engineering for increased yield of endogenous sugars and for direct production of higher-value sugars and simple sugar derivatives. Opportunities are examined for enhancing export of sugars from leaves. Focus then turns to manipulation of sugar metabolism in sugar-storing sink organs such as fruits, sugarcane culms and sugarbeet tubers. Results from manipulation of suspected 'limiting' enzymes indicate a need for clearer understanding of flux control mechanisms, to achieve enhanced levels of endogenous sugars in crops that are highly selected for this trait. Outcomes from in planta conversion to novel sugars and derivatives range from severe interference with plant development to field demonstration of crops accumulating higher-value sugars at high yields. The differences depend on underlying biological factors including the effects of the novel products on endogenous metabolism, and on biotechnological fine-tuning including developmental expression and compartmentation patterns. Ultimately, osmotic activity may limit the accumulation of sugars to yields below those achievable using polymers; but results indicate the potential for increases above current commercial sugar yields, through metabolic engineering underpinned by improved understanding of plant sugar metabolism.

RNAi-mediated abrogation of trehalase expression does not affect trehalase activity in sugarcane

To engineer trehalose metabolism in sugarcane (Saccharum spp. hybrids) two transgenes were introduced to the genome: trehalose-6-phosphate synthase- phosphatase (TPSP), to increase trehalose biosynthesis and an RNAi transgene specific for trehalase, to abrogate trehalose catabolism. In RNAi-expressing lines trehalase expression was abrogated in many plants however no decrease in trehalase activity was observed. In TPSP lines trehalase activity was significantly higher. No events of co-integration of TPSP and RNAi transgenes were observed. We suggest trehalase activity is essential to mitigate embryonic lethal effects of trehalose metabolism and discuss the implications for engineering trehalose metabolism.

Enhanced polyhydroxybutyrate production in transgenic sugarcane

Polyhydroxybutyrate (PHB) is a bacterial polyester that has properties similar to some petrochemically produced plastics. Plant-based production has the potential to make this biorenewable plastic highly competitive with petrochemical-based plastics. We previously reported that transgenic sugarcane produced PHB at levels as high as 1.8% leaf dry weight without penalty to biomass accumulation, suggesting scope for improving PHB production in this species. In this study, we used different plant and viral promoters, in combination with multigene or single-gene constructs to increase PHB levels. Promoters tested included the maize and rice polyubiquitin promoters, the maize chlorophyll A/B-binding protein promoter and a Cavendish banana streak badnavirus promoter. At the seedling stage, the highest levels of polymer were produced in sugarcane plants when the Cavendish banana streak badnavirus promoter was used. However, in all cases, this promoter underwent silencing as the plants matured. The rice Ubi promoter enabled the production of PHB at levels similar to the maize Ubi promoter. The maize chlorophyll A/B-binding protein promoter enabled the production of PHB to levels as high as 4.8% of the leaf dry weight, which is approximately 2.5 times higher than previously reported levels in sugarcane. This is the first time that this promoter has been tested in sugarcane. The highest PHB-producing lines showed phenotypic differences to the wild-type parent, including reduced biomass and slight chlorosis.

Peroxisomal polyhydroxyalkanoate biosynthesis is a promising strategy for bioplastic production in high biomass crops

Polyhydroxyalkanoates (PHAs) are bacterial carbon storage polymers with diverse plastic-like properties. PHA biosynthesis in transgenic plants is being developed as a way to reduce the cost and increase the sustainability of industrial PHA production. The homopolymer polyhydroxybutyrate (PHB) is the simplest form of these biodegradable polyesters. Plant peroxisomes contain the substrate molecules and necessary reducing power for PHB biosynthesis, but peroxisomal PHB production has not been explored in whole soil-grown transgenic plants to date. We generated transgenic sugarcane (Saccharum sp.) with the three-enzyme Ralstonia eutropha PHA biosynthetic pathway targeted to peroxisomes. We also introduced the pathway into Arabidopsis thaliana, as a model system for studying and manipulating peroxisomal PHB production. PHB, at levels up to 1.6%-1.8% dry weight, accumulated in sugarcane leaves and A. thaliana seedlings, respectively. In sugarcane, PHB accumulated throughout most leaf cell types in both peroxisomes and vacuoles. A small percentage of total polymer was also identified as the copolymer poly (3-hydroxybutyrate-co-3-hydroxyvalerate) in both plant species. No obvious deleterious effect was observed on plant growth because of peroxisomal PHA biosynthesis at these levels. This study highlights how using peroxisomal metabolism for PHA biosynthesis could significantly contribute to reaching commercial production levels of PHAs in crop plants.

Transgenic sugarcane plants expressing high levels of modified cry1Ac provide effective control against stem borers in field trials

To improve transgene expression level, we synthesized a truncated insecticidal gene m-cry1Ac by increasing its GC content from 37.4 to 54.8%, based on the codon usage pattern of sugarcane genes, and transferred it into two sugarcane cultivars (ROC16 and YT79-177) by microprojectile bombardment. The integration sites and expression pattern of the transgene were determined, respectively, by Southern, northern and western blot analyses. The transgenic sugarcane lines produced up to 50 ng Cry1Ac protein per mg soluble proteins, which was about fivefold higher than that produced by the partially modified s-cry1Ac (GC% = 47.5%). In greenhouse plant assay, about 62% of the transgenic lines exhibited excellent resistance to heavy infestation by stem borers. In field trials, the m-cry1Ac transgenic sugarcane lines expressing high levels of Cry1Ac were immune from insect attack. In contrast, expression of s-cry1Ac in transgenic sugarcane plants resulted in moderately decreased damages in internodes (0.4-1.7%) and stalks (13.3-26.7%) in comparison with the untransformed sugarcane controls, which showed about 4 and 26-40% damaged internodes and stalks, respectively. Significantly, these transgenic sugarcane lines with high levels of insect resistance showed similar agronomic and industrial traits as untransformed control plants. Taken together, the findings from this study indicate a promising potential of engineering an insect-resistant gene to tailor its protein expression levels in transgenic sugarcane to combat insect infestations.

Co-ordinated synthesis of gentiobiitol and sorbitol, evidence of sorbitol glycosylation in transgenic sugarcane

Sugarcane (a Saccharum spp. interspecific hybrid) was previously engineered to synthesize sorbitol (designated as sorbitolcane). Motivated by the atypical development of the leaves in some sorbitolcane, the polar metabolite profiles in the leaves of those plants were compared against a group of control sugarcane plants. Eighty-six polar metabolites were detected in leaf extracts by GC-MS. Principal component analysis of the metabolites indicated that three compounds were strongly associated with sorbitolcane. Two were identified as sorbitol and gentiobiose and the third was unknown. Gentiobiose and the unknown compound were positively correlated with sorbitol accumulation. The unknown compound was only abundant in sorbitolcane. This compound was structurally characterized and found to be a sorbitol-glucose conjugate. (13)C NMR analysis indicated that the glucopyranose and glucitol moieties were 1,6-linked. Ligand exchange chromatography confirmed that the compound was a beta-anomer, thus identifying the compound as 6-O-beta-d-glucopyranosyl-D-glucitol, or gentiobiitol.

Production of renewable polymers from crop plants

Plants produce a range of biopolymers for purposes such as maintenance of structural integrity, carbon storage, and defense against pathogens and desiccation. Several of these natural polymers are used by humans as food and materials, and increasingly as an energy carrier. In this review, we focus on plant biopolymers that are used as materials in bulk applications, such as plastics and elastomers, in the context of depleting resources and climate change, and consider technical and scientific bottlenecks in the production of novel or improved materials in transgenic or alternative crop plants. The biopolymers discussed are natural rubber and several polymers that are not naturally produced in plants, such as polyhydroxyalkanoates, fibrous proteins and poly-amino acids. In addition, monomers or precursors for the chemical synthesis of biopolymers, such as 4-hydroxybenzoate, itaconic acid, fructose and sorbitol, are discussed briefly.