Increased potato tuber size resulting from apoplastic expression of a yeast invertase

Functional characterization of a cell wall invertase inhibitor StInvInh1 revealed its involvement in potato microtuber size in vitro

Cell wall invertase (CWI) is as an essential coordinator in carbohydrate partitioning and sink strength determination, thereby playing key roles in plant development. Emerging evidence revealed that the subtle regulation of CWI activity considerably depends on the post-translational mechanism by their inhibitors (INHs). In our previous research, two putative INHs (StInvInh1 and StInvInh3) were expected as targets of CWI in potato (Solanum tubersum), a model species of tuberous plants. Here, transcript analysis revealed that StInvInh1 showed an overall higher expression than StInhInh3 in all tested organs. Then, StInvInh1 was further selected to study. In accordance with this, the activity of StInvInh1 promoter increased with the development of leaves in plantlets but decreased with the development of microtubers in vitro and mainly appeared in vascular bundle. The recombinant protein StInvInh1 displayed inhibitory activities on the extracted CWI in vitro and StInvInh1 interacted with a CWI StcwINV2 in vivo by bimolecular fluorescence complementation. Furthermore, silencing StInvInh1 in potato dramatically increased the CWI activity without changing activities of vacuolar and cytoplasmic invertase, indicating that StInvInh1 functions as a typical INH of CWI. Releasing CWI activity in StInvInh1 RNA interference transgenic potato led to improvements in potato microtuber size in coordination with higher accumulations of dry matter in vitro. Taken together, these findings demonstrate that StInvInh1 encodes an INH of CWI and regulates the microtuber development process through fine-tuning apoplastic sucrose metabolism, which may provide new insights into tuber development.

Sucrose Metabolism and Transport in Grapevines, with Emphasis on Berries and Leaves, and Insights Gained from a Cross-Species Comparison

In grapevines, as in other plants, sucrose and its constituents glucose and fructose are fundamentally important and carry out a multitude of roles. The aims of this review are three-fold. First, to provide a summary of the metabolism and transport of sucrose in grapevines, together with new insights and interpretations. Second, to stress the importance of considering the compartmentation of metabolism. Third, to outline the key role of acid invertase in osmoregulation associated with sucrose metabolism and transport in plants.

De novo domestication of wild species to create crops with increased resilience and nutritional value

Creating crops with resistance to drought, soil salinity and insect damage, that simultaneously have higher nutritional quality, is challenging to conventional breeding due to the complex and diffuse genetic basis of those traits. Recent advances in gene editing technology, such as base editors and prime-editing, coupled with a deeper understanding of the genetic basis of domestication delivered by the analysis of crop 'pangenomes', open the exciting prospect of creating novel crops via manipulation of domestication-related genes in wild species. A de novo domestication platform may allow rapid and precise conversion of crop wild relatives into crops, while retaining many of the valuable resilience and nutritional traits left behind during domestication and breeding. Using the Solanaceae family as case in point, we discuss how such a knowledge-driven pipeline could be exploited to contribute to food security over the coming decades.

The cytosolic invertase NI6 affects vegetative growth, flowering, fruit set, and yield in tomato

Sucrose metabolism is important for most plants, both as the main source of carbon and via signaling mechanisms that have been proposed for this molecule. A cleaving enzyme, invertase (INV) channels sucrose into sink metabolism. Although acid soluble and insoluble invertases have been largely investigated, studies on the role of neutral invertases (A/N-INV) have lagged behind. Here, we identified a tomato A/N-INV encoding gene (NI6) co-localizing with a previously reported quantitative trait locus (QTL) largely affecting primary carbon metabolism in tomato. Of the eight A/N-INV genes identified in the tomato genome, NI6 mRNA is present in all organs, but its expression was higher in sink tissues (mainly roots and fruits). A NI6-GFP fusion protein localized to the cytosol of mesophyll cells. Tomato NI6-silenced plants showed impaired growth phenotype, delayed flowering and a dramatic reduction in fruit set. Global gene expression and metabolite profile analyses of these plants revealed that NI6 is not only essential for sugar metabolism, but also plays a signaling role in stress adaptation. We also identified major hubs, whose expression patterns were greatly affected by NI6 silencing; these hubs were within the signaling cascade that coordinates carbohydrate metabolism with growth and development in tomato.

Review: The Next Steps in Crop Improvement: Adoption of Emerging Strategies to Identify Bottlenecks in Sugar Flux

Sugar allocation in plants is the fundamental process that transports sugar from source to sink tissues and has a dramatic impact on crop yields. Controlling sugar allocation is required to increase crop yields, as well as biomass for biofuel production. Successful examples have demonstrated that genetic engineering of sugar partitioning offers a promising strategy to achieve this goal. However, improvement has thus far been limited by gaps in understanding of the underlying mechanisms controlling the allocation of sugars. The dynamics of sugar partitioning are minimally predictable under different conditions, between species, or in response to abiotic stresses. Here, we discuss four methodologies that have not been sufficiently exploited for the identification of bottlenecks in sugar flux. Furthermore, we suggest how these strategies can be used and combined to provide the insight needed to maximize crop yields or biomass, especially under conditions of environmental stress.

A Synergistic Genetic Engineering Strategy Induced Triacylglycerol Accumulation in Potato (Solanum tuberosum) Leaf

Potato is the 4th largest staple food in the world currently. As a high biomass crop, potato harbors excellent potential to produce energy-rich compounds such as triacylglycerol as a valuable co-product. We have previously reported that transgenic potato tubers overexpressing WRINKLED1, DIACYLGLYCEROL ACYLTRANSFERASE 1, and OLEOSIN genes produced considerable levels of triacylglycerol. In this study, the same genetic engineering strategy was employed on potato leaves. The overexpression of Arabidopsis thaliana WRINKED1 under the transcriptional control of a senescence-inducible promoter together with Arabidopsis thaliana DIACYLGLYCEROL ACYLTRANSFERASE 1 and Sesamum indicum OLEOSIN driven by the Cauliflower Mosaic Virus 35S promoter and small subunit of Rubisco promoter respectively, resulted in an approximately 30- fold enhancement of triacylglycerols in the senescent transgenic potato leaves compared to the wild type. The increase of triacylglycerol in the transgenic potato leaves was accompanied by perturbations of carbohydrate accumulation, apparent in a reduction in starch content and increased total soluble sugars, as well as changes of polar membrane lipids at different developmental stages. Microscopic and biochemical analysis further indicated that triacylglycerols and lipid droplets could not be produced in chloroplasts, despite the increase and enlargement of plastoglobuli at the senescent stage. Possibly enhanced accumulation of fatty acid phytyl esters in the plastoglobuli were reflected in transgenic potato leaves relative to wild type. It is likely that the plastoglobuli may have hijacked some of the carbon as the result of WRINKED1 expression, which could be a potential factor restricting the effective accumulation of triacylglycerols in potato leaves. Increased lipid production was also observed in potato tubers, which may have affected the tuberization to a certain extent. The expression of transgenes in potato leaf not only altered the carbon partitioning in the photosynthetic source tissue, but also the underground sink organs which highly relies on the leaves in development and energy deposition.

Glucose: Sweet or bitter effects in plants-a review on current and future perspective

Sugars are metabolic substrates playing a part in modulating various processes in plants during different phases of development. Thus, modulating the sugar metabolism can have intense effects on the plant metabolism. Glucose is a soluble sugar, found throughout the plant kingdom. Apart from being a universal carbon source, glucose also operates as a signaling molecule modulating various metabolic processes in plants. From germination to senescence, wide range of processes in plants is regulated by glucose. The effect of glucose is found to be concentration dependent. Photosynthesis and its related attributes, respiration and nitrogen metabolism are influenced by glucose application. Endogenous content of glucose increases upon exposure of plant to various abiotic stresses and also when glucose is supplied exogenously. Glucose accumulation alleviates the damaging effects of stress by enhancing production of antioxidants and compounds similar to that of photosynthetic CO₂ fixation which act as an osmoticum by maintaining osmotic pressure inside the cell, pH homeostasis regulator and reduce membrane permeability during stress. Glucose interaction with various phytohormones has also been discussed in this review.

Sucrose Synthase Enhances Hull Size and Grain Weight by Regulating Cell Division and Starch Accumulation in Transgenic Rice

Grain size and weight are two important determinants of grain yield in rice. Although overexpression of sucrose synthase (SUS) genes has led to several improvements on cellulose and starch-based traits in transgenic crops, little is reported about SUS enhancement of hull size and grain weight in rice. In this study, we selected transgenic rice plants that overexpressed OsSUS1-6 genes driven with the maize Ubi promoter. Compared to the controls (wild type and empty vector line), all independent OsSUS homozygous transgenic lines exhibited considerably increased grain yield and grain weights. Using the representative OsSUS3 overexpressed transgenic plants, four independent homozygous lines showed much raised cell numbers for larger hull sizes, consistent with their enhanced primary cell wall cellulose biosynthesis and postponed secondary wall synthesis. Accordingly, the OsSUS3 transgenic lines contained much larger endosperm volume and higher starch levels than those of the controls in the mature grains, leading to increased brown grain weights by 15–19%. Hence, the results have demonstrated that OsSUS overexpression could significantly improve hull size and grain weight by dynamically regulating cell division and starch accumulation in the transgenic rice.

Primary Metabolism in Citrus Fruit as Affected by Its Unique Structure

Citrus is one of the world's most important fruit crops, contributing essential nutrients, such as vitamin C and minerals, to the human diet. It is characterized by two important traits: first, its major edible part is composed of juice sacs, a unique structure among fruit, and second, relatively high levels of citric acid are accumulated in the vacuole of the juice sac cell. Although the major routes of primary metabolism are generally the same in citrus fruit and other plant systems, the fruit's unique structural features challenge our understanding of carbon flow into the fruit and its movement through all of its parts. In fact, acid metabolism and accumulation have only been summarized in a few reviews. Here we present a comprehensive view of sugar, acid and amino acid metabolism and their connections within the fruit, all in relation to the fruit's unique structure.

Cell wall invertase activity regulates the expression of the transfer cell-specific transcription factor ZmMRP-1

Studies in cell wall bound invertase mutants indicate that the promoter of the transfer cell-specific transcription factor, ZmMRP - 1 , is modulated by the carbohydrate balance. Transfer cells are highly specialized plant cells located at the surfaces that need to support an intensive exchange of nutrients, such as the entrance of fruits, seeds and nodules or the young branching points along the stem. ZmMRP-1 is a one-domain MYB-related transcription factor specifically expressed at the transfer cell layer of the maize endosperm. Previous studies demonstrated that this factor regulates the expression of a large number of transfer cell-specific genes, and suggested that ZmMRP-1 is a key regulator of the differentiation of this tissue. The expression of this gene is largely dominated by positional cues, but within the ZmMRP-1 expressing cells the promoter appears to be modulated by sugars. Here we have investigated in vivo this modulation. Using maize and Arabidopsis mutants for cell wall invertase genes, we found that the absence of cell wall invertase activity is a major inductive signal of the ZmMRP-1 expression.

Genome diversity of tuber-bearing Solanum uncovers complex evolutionary history and targets of domestication in the cultivated potato

Worldwide, potato is the third most important crop grown for direct human consumption, but breeders have struggled to produce new varieties that outperform those released over a century ago, as evidenced by the most widely grown North American cultivar (Russet Burbank) released in 1876. Despite its importance, potato genetic diversity at the whole-genome level remains largely unexplored. Analysis of cultivated potato and its wild relatives using modern genomics approaches can provide insight into the genomic diversity of extant germplasm, reveal historic introgressions and hybridization events, and identify genes targeted during domestication that control variance for agricultural traits, all critical information to address food security in 21st century agriculture.

Cultivated potatoes (Solanum tuberosum L.), domesticated from wild Solanum species native to the Andes of southern Peru, possess a diverse gene pool representing more than 100 tuber-bearing relatives (Solanum section Petota). A diversity panel of wild species, landraces, and cultivars was sequenced to assess genetic variation within tuber-bearing Solanum and the impact of domestication on genome diversity and identify key loci selected for cultivation in North and South America. Sequence diversity of diploid and tetraploid S. tuberosum exceeded any crop resequencing study to date, in part due to expanded wild introgressions following polyploidy that captured alleles outside of their geographic origin. We identified 2,622 genes as under selection, with only 14–16% shared by North American and Andean cultivars, showing that a limited gene set drove early improvement of cultivated potato, while adaptation of upland (S. tuberosum group Andigena) and lowland (S. tuberosum groups Chilotanum and Tuberosum) populations targeted distinct loci. Signatures of selection were uncovered in genes controlling carbohydrate metabolism, glycoalkaloid biosynthesis, the shikimate pathway, the cell cycle, and circadian rhythm. Reduced sexual fertility that accompanied the shift to asexual reproduction in cultivars was reflected by signatures of selection in genes regulating pollen development/gametogenesis. Exploration of haplotype diversity at potato’s maturity locus (StCDF1) revealed introgression of truncated alleles from wild species, particularly S. microdontum in long-day–adapted cultivars. This study uncovers a historic role of wild Solanum species in the diversification of long-day–adapted tetraploid potatoes, showing that extant natural populations represent an essential source of untapped adaptive potential.

Genomics-assisted characterization of a breeding collection of Apios americana, an edible tuberous legume

For species with potential as new crops, rapid improvement may be facilitated by new genomic methods. Apios (Apios americana Medik.), once a staple food source of Native American Indians, produces protein-rich tubers, tolerates a wide range of soils, and symbiotically fixes nitrogen. We report the first high-quality de novo transcriptome assembly, an expression atlas, and a set of 58,154 SNP and 39,609 gene expression markers (GEMs) for characterization of a breeding collection. Both SNPs and GEMs identify six genotypic clusters in the collection. Transcripts mapped to the Phaseolus vulgaris genome-another phaseoloid legume with the same chromosome number-provide provisional genetic locations for 46,852 SNPs. Linkage disequilibrium decays within 10 kb (based on the provisional genetic locations), consistent with outcrossing reproduction. SNPs and GEMs identify more than 21 marker-trait associations for at least 11 traits. This study demonstrates a holistic approach for mining plant collections to accelerate crop improvement.

Intracellular and cell-to-apoplast compartmentation of carbohydrate metabolism

In most plants, carbohydrates represent the major energy store as well as providing the building blocks for essential structural polymers. Although the major pathways for carbohydrate biosynthesis, degradation, and transport are well characterized, several key steps have only recently been discovered. In addition, several novel minor metabolic routes have been uncovered in the past few years. Here we review current studies of plant carbohydrate metabolism detailing the expanding compendium of functionally characterized transport proteins as well as our deeper comprehension of more minor and conditionally activated metabolic pathways. We additionally explore the pertinent questions that will allow us to enhance our understanding of the response of both major and minor carbohydrate fluxes to changing cellular circumstances.

The radish genome and comprehensive gene expression profile of tuberous root formation and development

Understanding the processes that regulate plant sink formation and development at the molecular level will contribute to the areas of crop breeding, food production and plant evolutionary studies. We report the annotation and analysis of the draft genome sequence of the radish Raphanus sativus var. hortensis (long and thick root radish) and transcriptome analysis during root development. Based on the hybrid assembly approach of next-generation sequencing, a total of 383 Mb (N50 scaffold: 138.17 kb) of sequences of the radish genome was constructed containing 54,357 genes. Syntenic and phylogenetic analyses indicated that divergence between Raphanus and Brassica coincide with the time of whole genome triplication (WGT), suggesting that WGT triggered diversification of Brassiceae crop plants. Further transcriptome analysis showed that the gene functions and pathways related to carbohydrate metabolism were prominently activated in thickening roots, particularly in cell proliferating tissues. Notably, the expression levels of sucrose synthase 1 (SUS1) were correlated with root thickening rates. We also identified the genes involved in pungency synthesis and their transcription factors.

Review of recent transgenic studies on abiotic stress tolerance and future molecular breeding in potato

Global warming has become a major issue within the last decade. Traditional breeding programs for potato have focused on increasing productivity and quality and disease resistance, thus, modern cultivars have limited tolerance of abiotic stresses. The introgression of abiotic stress tolerance into modern cultivars is essential work for the future. Recently, many studies have investigated abiotic stress using transgenic techniques. This manuscript focuses on the study of abiotic stress, in particular drought, salinity and low temperature, during this century. Dividing studies into these three stress categories for this review was difficult. Thus, based on the study title and the transgene property, transgenic studies were classified into five categories in this review; oxidative scavengers, transcriptional factors, and above three abiotic categories. The review focuses on studies that investigate confer of stress tolerance and the identification of responsible factors, including wild relatives. From a practical application perspective, further evaluation of transgenic potato with abiotic stress tolerance is required. Although potato plants, including wild species, have a large potential for abiotic stress tolerance, exploration of the factors responsible for conferring this tolerance is still developing. Molecular breeding, including genetic engineering and conventional breeding using DNA markers, is expected to develop in the future.

Crop yield components - photoassimilate supply- or utilisation limited-organ development?

Yield potential is the genome-encoded capacity of a crop species to generate yield in an optimal growth environment. Ninety per cent of plant biomass is derived from the photosynthetic reduction of carbon dioxide to organic carbon (photoassimilates - primarily sucrose). Thus, development of yield components (organ numbers and individual organ masses) can be limited by photoassimilate supply (photosynthesis arranged in series with phloem transport) or by their inherent capacity to utilise imported photoassimilates for growth or storage. To this end, photoassimilate supply/utilisation of crop yield has been quantitatively re-evaluated using published responses of yield components to elevated carbon dioxide concentrations across a selection of key crop species including cereal and pulse grains, fleshy fruits, tubers and sugar storing stems and tap roots. The analysis demonstrates that development of harvested organ numbers is strongly limited by photoassimilate supply. Vegetative branching and, to a lesser extent, flower/pod/fleshy fruit abortion, are the major yield components contributing to sensitivity of organ numbers to photoassimilate supply. In contrast, harvested organ size is partially dependent (eudicots), or completely independent (cereals), of photoassimilate supply. Processes limiting photoassimilate utilisation by harvested organs include membrane transport of soluble sugars and their allocation into polymeric storage products.

Sucrose metabolism: gateway to diverse carbon use and sugar signaling

Sucrose metabolism plays pivotal roles in development, stress response, and yield formation, mainly by generating a range of sugars as metabolites to fuel growth and synthesize essential compounds (including protein, cellulose, and starch) and as signals to regulate expression of microRNAs, transcription factors, and other genes and for crosstalk with hormonal, oxidative, and defense signaling. This review aims to capture the most exciting developments in this area by evaluating (a) the roles of key sucrose metabolic enzymes in development, abiotic stress responses, and plant-microbe interactions; (b) the coupling between sucrose metabolism and sugar signaling from extra- to intracellular spaces; (c) the different mechanisms by which sucrose metabolic enzymes could perform their signaling roles; and (d) progress on engineering sugar metabolism and transport for high yield and disease resistance. Finally, the review outlines future directions for research on sugar metabolism and signaling to better understand and improve plant performance.

Application of metabolic flux analysis to plants

This volume compiles a series of chapters that cover the major aspects of plant metabolic flux analysis, such as but not limited to labeling of plant material, acquisition of labeling data, mathematical modeling of metabolic network at the cell, tissue, and plant level. A short revue, including methodological points and applications of flux analysis to plants, is presented in this introductory chapter.

Constitutive expression of cell wall invertase genes increases grain yield and starch content in maize

Grain size, number and starch content are important determinants of grain yield and quality. One of the most important biological processes that determine these components is the carbon partitioning during the early grain filling, which requires the function of cell wall invertase. Here, we showed the constitutive expression of cell wall invertase-encoding gene from Arabidopsis, rice (Oryza sativa) or maize (Zea mays), driven by the cauliflower mosaic virus (CaMV) 35S promoter, all increased cell wall invertase activities in different tissues and organs, including leaves and developing seeds, and substantially improved grain yield up to 145.3% in transgenic maize plants as compared to the wild-type plants, an effect that was reproduced in our 2-year field trials at different locations. The dramatically increased grain yield is due to the enlarged ears with both enhanced grain size and grain number. Constitutive expression of the invertase-encoding gene also increased total starch content up to 20% in the transgenic kernels. Our results suggest that cell wall invertase gene can be genetically engineered to improve both grain yield and grain quality in crop plants.

Global transcriptome analysis of AtPAP2--overexpressing Arabidopsis thaliana with elevated ATP

BACKGROUND

AtPAP2 is a purple acid phosphatase that is targeted to both chloroplasts and mitochondria. Over-expression (OE) lines of AtPAP2 grew faster, produced more seeds, and contained higher leaf sucrose and glucose contents. The present study aimed to determine how high energy status affects leaf and root transcriptomes.

RESULTS

ATP and ADP levels in the OE lines are 30-50% and 20-50% higher than in the wild-type (WT) plants. Global transcriptome analyses indicated that transcriptional regulation does play a role in sucrose and starch metabolism, nitrogen, potassium and iron uptake, amino acids and secondary metabolites metabolism when there is an ample supply of energy. While the transcript abundance of genes encoding protein components of photosystem I (PS I), photosystem II (PS II) and light harvesting complex I (LHCI) were unaltered, changes in transcript abundance for genes encoding proteins of LHCII are significant. The gene expressions of most enzymes of the Calvin cycle, glycolysis and the tricarboxylic acid (TCA) cycle were unaltered, as these enzymes are known to be regulated by light/redox status or allosteric modulation by the products (e.g. citrate, ATP/ADP ratio), but not at the level of transcription.

CONCLUSIONS

AtPAP2 overexpression resulted in a widespread reprogramming of the transcriptome in the transgenic plants, which is characterized by changes in the carbon, nitrogen, potassium, and iron metabolism. The fast-growing AtPAP2 OE lines provide an interesting tool for studying the regulation of energy system in plant.

Role of metabolite transporters in source-sink carbon allocation

Plants assimilate carbon dioxide during photosynthesis in chloroplasts. Assimilated carbon is subsequently allocated throughout the plant. Generally, two types of organs can be distinguished, mature green source leaves as net photoassimilate exporters, and net importers, the sinks, e.g., roots, flowers, small leaves, and storage organs like tubers. Within these organs, different tissue types developed according to their respective function, and cells of either tissue type are highly compartmentalized. Photoassimilates are allocated to distinct compartments of these tissues in all organs, requiring a set of metabolite transporters mediating this intercompartmental transfer. The general route of photoassimilates can be briefly described as follows. Upon fixation of carbon dioxide in chloroplasts of mesophyll cells, triose phosphates either enter the cytosol for mainly sucrose formation or remain in the stroma to form transiently stored starch which is degraded during the night and enters the cytosol as maltose or glucose to be further metabolized to sucrose. In both cases, sucrose enters the phloem for long distance transport or is transiently stored in the vacuole, or can be degraded to hexoses which also can be stored in the vacuole. In the majority of plant species, sucrose is actively loaded into the phloem via the apoplast. Following long distance transport, it is released into sink organs, where it enters cells as source of carbon and energy. In storage organs, sucrose can be stored, or carbon derived from sucrose can be stored as starch in plastids, or as oil in oil bodies, or - in combination with nitrogen - as protein in protein storage vacuoles and protein bodies. Here, we focus on transport proteins known for either of these steps, and discuss the implications for yield increase in plants upon genetic engineering of respective transporters.

RETRACTED: Molecular strategies in manipulation of the starch synthesis pathway for improving storage starch content in plants (review and prospect for increasing storage starch synthesis)

Starch is the most widespread and abundant storage carbohydrate in plants. We depend upon starch for our nutrition, exploit its unique properties in industry, and use it as a feedstock for bioethanol production. In recent decades, enormous progress has been made in understanding the genetic and biochemical mechanisms of starch synthesis in plants. Yet, despite this remarkable progress and its obvious economic importance, very little has been achieved in terms of adding value to starch or increasing starch content, particularly in cereal crops. In this paper, we first review recent advances in understanding the biochemistry of starch synthesis, particularly in identifying key enzymes involved in starch assembly. We then assess the progress in molecular strategies for improving storage starch content in plants. Finally, we discuss the problems faced in our profession and present ideas to effectively increase storage starch content in the future.

The mode of sucrose degradation in potato tubers determines the fate of assimilate utilization

Cytosolic (U-IN-2) or apoplasmic (U-IN-1) targeting of yeast invertase in potato tubers leads to a reduction in sucrose and an increase in glucose content, but specific phenotypical changes are dependent on the subcellular targeting of the enzyme. Cytosolic expression leads to a more severe phenotype with the most striking aspects being reduced starch content and increased respiration. Despite extensive research, the regulatory mechanisms leading to these changes remain obscure. Recent technological advancements regarding potato transcriptional and genomic research presented us with the opportunity to revisit these lines and perform detailed gene expression analysis, in combination with extensive metabolic profiling, to identify regulatory networks underlying the observed changes. Our results indicate that in both genotypes reduced UDP-glucose production is associated with a reduced expression of cell wall biosynthetic genes. In addition, U-IN-1 tubers are characterized by elevated expression of senescence-associated genes, coupled to reduced expression of genes related to photosynthesis and the cytoskeleton. We provide evidence that increased respiration, observed specifically in U-IN-2 tubers, might be due to sugar signaling via released trehalose-6-phosphate inhibition of the SnRK1 complex. In both genotypes, expression of the plastidic glucose-6-phosphate transporter (GPT) is significantly down-regulated. This leads to a shift in the cytosolic to plastidic glucose-6-phosphate ratio and hence might limit starch synthesis but also the oxidative pentose phosphate pathway. This might explain the observed changes in several additional plastid localized pathways, most notably reduced expression of fatty acid biosynthetic genes and an accumulation of shikimate. Interestingly, a strict negative correlation between invertase and GPT expression could be observed in a wide range of potato tubers. This reciprocal regulation may be part of a more general switch controlling energy versus storage metabolism, suggesting that the fate of assimilate utilization is coordinated at the level of sucrose degradation.

Extracellular invertase is involved in the regulation of clubroot disease in Arabidopsis thaliana

Clubroot disease of Brassicaceae is caused by an obligate biotrophic protist, Plasmodiophora brassicae. During root gall development, a strong sink for assimilates is developed. Among other genes involved in sucrose and starch synthesis and degradation, the increased expression of invertases has been observed in a microarray experiment, and invertase and invertase inhibitor expression was confirmed using promoter::GUS lines of Arabidopsis thaliana. A functional approach demonstrates that invertases are important for gall development. Different transgenic lines expressing an invertase inhibitor under the control of two root-specific promoters, Pyk10 and CrypticT80, which results in the reduction of invertase activity, showed clearly reduced clubroot symptoms in root tissue with highest promoter expression, whereas hypocotyl galls developed normally. These results present the first evidence that invertases are important factors during gall development, most probably in supplying sugars to the pathogen. In addition, root-specific repression of invertase activity could be used as a tool to reduce clubroot symptoms.

Characterization of two cotton (Gossypium hirsutum L) invertase genes

Two cotton vacuolar-invertase genes were identified and sequenced. Both genes had seven exons, including an unusually small second exon typical of acid invertases. These genes encode peptides with many features shared by acid invertases from other species including, leader sequences that probably target the peptide to the vacuole, active site motifs and substrate binding motifs. Expression analyses indicated that one of the genes was expressed in roots during the starch filling stage of development. However, expression of the same gene fluctuated during the starch utilization stage of development. Therefore this gene was unlikely to play a role in determining sink strength of this tissue. Both genes were expressed in elongating fibers where they were likely to play a role in cell expansion. The invertase gene uniquely expressed in fiber had a simple sequence repeat (SSR) in the third intron that was polymorphic among various cotton species. An EST was identified with an expansion of the SSR that included the third intron indicating this SSR is associated with a splice variant. The polymorphic SSR may be useful in investigating the function of this gene in fiber development.

The potential of genetically enhanced plants to address food insecurity

Food insecurity is one of the most important social issues faced today, with 840 million individuals enduring chronic hunger and three billion individuals suffering from nutrient deficiencies. Most of these individuals are poverty stricken and live in developing countries. Strategies to address food insecurity must aim to increase agricultural productivity in the developing world in order to tackle poverty, and must provide long-term improvements in crop yields to keep up with demand as the world's population grows. Genetically enhanced plants provide one route to sustainable higher yields, either by increasing the intrinsic yield capability of crop plants or by protecting them from biotic and abiotic constraints. The present paper discusses a range of transgenic approaches that could increase agricultural productivity if applied on a large scale, including the introduction of genes that confer resistance to pests and diseases, or tolerance of harsh environments, and genes that help to lift the intrinsic yield capacity by increasing metabolic flux towards storage carbohydrates, proteins and oils. The paper also explores how the nutritional value of plants can be improved by genetic engineering. Transgenic plants, as a component of integrated strategies to relieve poverty and deliver sustainable agriculture to subsistence farmers in developing countries, could have a significant impact on food security now and in the future.

Alternative oxidase: a defence against metabolic fluctuations?

An increasing number of oscillating or fluctuating cellular systems have been recently described following the adaptation of fluorescent technology. In diverse organisms, these variously involve signalling factors, heat production, central metabolism and reactive oxygen species (ROS). In response to many plant stresses and primarily via the influence of ROS, changes in mRNA and protein levels or in vivo activity of alternative oxidase are often observed. However, in several investigations, a lack of correlation between the mRNA, protein and in vivo activity has been evident. This discrepancy has made it questionable whether the induction of alternative oxidase has importance in regulating alternative pathway activity in vivo, or being diagnostic for a role of alternative oxidase in stress tolerance and ROS avoidance. Here, we suggest a role of alternative oxidase in counteracting deleterious short-term metabolic fluctuations, especially under stress conditions. This model emphasizes the importance of peak activity for establishing protein levels and allows an amalgamation of the present status of physiological, cellular and molecular knowledge.

RNA interference of LIN5 in tomato confirms its role in controlling Brix content, uncovers the influence of sugars on the levels of fruit hormones, and demonstrates the importance of sucrose cleavage for normal fruit development and fertility

It has been previously demonstrated, utilizing intraspecific introgression lines, that Lycopersicum Invertase5 (LIN5), which encodes a cell wall invertase, controls total soluble solids content in tomato (Solanum lycopersicum). The physiological role of this protein, however, has not yet been directly studied, since evaluation of data obtained from the introgression lines is complicated by the fact that they additionally harbor many other wild species alleles. To allow a more precise comparison, we generated transgenic tomato in which we silenced the expression of LIN5 using the RNA interference approach. The transformants were characterized by an altered flower and fruit morphology, displaying increased numbers of petals and sepals per flower, an increased rate of fruit abortion, and a reduction in fruit size. Evaluation of the mature fruit revealed that the transformants were characterized by a reduction of seed number per plant. Furthermore, detailed physiological analysis revealed that the transformants displayed aberrant pollen morphology and a reduction in the rate of pollen tube elongation. Metabolite profiling of ovaries and green and red fruit revealed that metabolic changes in the transformants were largely confined to sugar metabolism, whereas transcript and hormone profiling revealed broad changes both in the hormones themselves and in transcripts encoding their biosynthetic enzymes and response elements. These results are discussed in the context of current understanding of the role of sugar during the development of tomato fruit, with particular focus given to its impact on hormone levels and organ morphology.

Genomic and functional characterization of StCDPK1

StCDPK1 is a calcium dependent protein kinase expressed in tuberizing potato stolons and in sprouting tubers. StCDPK1 genomic sequence contains eight exons and seven introns, the gene structure is similar to Arabidopsis, rice and wheat CDPKs belonging to subgroup IIa. There is one copy of the gene per genome and it is located in the distal portion of chromosome 12. Western blot and immunolocalization assays (using confocal and transmission electron microscopy) performed with a specific antibody against StCDPK1 indicate that this kinase is mainly located in the plasma membrane of swelling stolons and sprouting tubers. Sucrose (4-8%) increased StCDPK1 protein content in non-induced stolons, however the amount detected in swelling stolons was higher. Transgenic lines with reduced expression of StCDPK1 (beta 7) did not differ from controls when cultured under multiplication conditions, but when grown under tuber inducing conditions some significant differences were observed: the beta 7 line tuberized earlier than controls without the addition of CCC (GA inhibitor), developed more tubers than wild type plants in the presence of hormones that promote tuberization in potato (ABA and BAP) and was more insensitive to GA action (stolons were significantly shorter than those of control plants). StCDPK1 expression was induced by GA, ABA and BAP. Our results suggest that StCDPK1 plays a role in GA-signalling and that this kinase could be a converging point for the inhibitory and promoting signals that influence the onset of potato tuberization.

Rethinking metabolic control

Modulation of metabolic fluxes in plants is usually not a successful business. The main reason is our limited understanding of metabolic plasticity and metabolic control, with the latter still largely influenced by the idea that each pathway has a rate limiting step controlling the flux. Not only is experimental evidence for such steps lacking for most pathways, despite intensive search, but there are also theoretical arguments against the idea that highly regulated enzymes catalyzing reactions far from equilibrium must be considered a priori rate limiting. Conversely, it is argued that reactions close to equilibrium need a lot of enzyme to be maintained close to equilibrium and, contrary to accepted wisdom, begin to limit flux when reduced. Using a few key examples of plant metabolic pathways as case studies, I draw some general conclusions. The approach of augmenting flux by pushing a pathway from above is well exemplified by the attempts at increasing starch content in potato tubers, where several different approaches failed. Also pulling at the other end (close to the end product) has yielded little improvement, while targeting a reaction close to equilibrium (ADP/ATP translocation at the plastid envelope) successfully increased starch content. Rethinking control is equally well applicable to photosynthesis, with prime examples of 'neglected', unregulated enzymes exerting significant control and overprized 'limiting' enzymes having little control in normal conditions like rubisco. In this new paradigm, the role of most control mechanisms is also challenged: feedback inhibition and post-translational modification of enzymes are relevant to metabolite homeostasis rather than flux control, with moiety conservation being a major reason for this constraint. I advocate a more extensive use of control circuitry elements (e.g. sensors like riboswitches), metabolic shortcuts and transcription factors in metabolic engineering.

The effect of sucrose and abscisic acid interaction on sucrose synthase and its relationship to grain filling of rice (Oryza sativa L.)

Rice grain filling is a process of conversion of sucrose into starch catalysed by a series of enzymes. Sucrose synthase (SUS) is considered as a key enzyme regulating this process. This study investigated the possible roles of sucrose and abscisic acid (ABA) in mediating the activity and expression of SUS protein of grains during grain filling in rice (Oryza sativa). Field-grown rice plants and detached cultured panicles were used as experimental materials. Several treatments, including spikelet thinning, leaf cutting, and applications of different concentrations of exogenous sucrose and ABA, were imposed during grain filling. A higher SUS activity was found in superior grains than in inferior grains in the earlier stage of grain filling, which was significantly and closely related to a higher grain filling rate and starch accumulation. An increase in sucrose concentration in grains as a result of different treatments increased both SUS activity and SUS protein expression in grains. An increase in ABA concentration gave similar results. Furthermore, effects of interactions between sucrose and ABA on the activity and expression of SUS protein in grains were also found. It was suggested that sucrose- and ABA-mediated rice grain filling is largely due to an increase in SUS activity and SUS protein expression.

Cell wall-bound invertase limits sucrose export and is involved in symptom development and inhibition of photosynthesis during compatible interaction between tomato and Xanthomonas campestris pv vesicatoria

Cell wall-bound invertase (cw-Inv) plays an important role in carbohydrate partitioning and regulation of sink-source interaction. There is increasing evidence that pathogens interfere with sink-source interaction, and induction of cw-Inv activity has frequently been shown in response to pathogen infection. To investigate the role of cw-Inv, transgenic tomato (Solanum lycopersicum) plants silenced for the major leaf cw-Inv isoforms were generated and analyzed during normal growth and during the compatible interaction with Xanthomonas campestris pv vesicatoria. Under normal growth conditions, activities of sucrolytic enzymes as well as photosynthesis and respiration were unaltered in the transgenic plants compared with wild-type plants. However, starch levels of source leaves were strongly reduced, which was most likely caused by an enhanced sucrose exudation rate. Following X. campestris pv vesicatoria infection, cw-Inv-silenced plants showed an increased sucrose to hexose ratio in the apoplast of leaves. Symptom development, inhibition of photosynthesis, and expression of photosynthetic genes were clearly delayed in transgenic plants compared with wild-type plants. In addition, induction of senescence-associated and pathogenesis-related genes observed in infected wild-type plants was abolished in cw-Inv-silenced tomato lines. These changes were not associated with decreased bacterial growth. In conclusion, cw-Inv restricts carbon export from source leaves and regulates the sucrose to hexose ratio in the apoplast. Furthermore, an increased apoplastic hexose to sucrose ratio can be linked to inhibition of photosynthesis and induction of pathogenesis-related gene expression but does not significantly influence bacterial growth. Indirectly, bacteria may benefit from low invertase activity, since the longevity of host cells is raised and basal defense might be dampened.

Metabolic regulation of pathways of carbohydrate oxidation in potato (Solanum tuberosum) tubers

In the present article we evaluate the consequence of tuber-specific expression of yeast invertase, on the pathways of carbohydrate oxidation, in potato (Solanum tuberosum L. cv. Desiree). We analysed the relative rates of glycolysis and the oxidative pentose phosphate pathway that these lines exhibited as well as the relative contributions of the cytochrome and alternative pathways of mitochondrial respiration. Enzymatic and protein abundance analysis revealed concerted upregulation of the glycolytic pathway and of specific enzymes of the tricarboxylic acid cycle and the alternative oxidase but invariant levels of enzymes of the oxidative pentose phosphate pathway and proteins of the cytochrome pathway. When taken together these experiments suggest that the overexpression of a cytosolic invertase (EC 3.2.1.26) results in a general upregulation of carbohydrate oxidation with increased flux through both the glycolytic and oxidative pentose phosphate pathways as well as the cytochrome and alternative pathways of oxidative phosphorylation. Moreover these data suggest that the upregulation of respiration is a consequence of enhanced efficient mitochondrial metabolism.

The sweet potato sporamin promoter confers high-level phytase expression and improves organic phosphorus acquisition and tuber yield of transgenic potato

The sweet potato sporamin promoter was used to control the expression in transgenic potato of the E. coli appA gene, which encodes a bifunctional enzyme exhibiting both acid phosphatase and phytase activities. The sporamin promoter was highly active in leaves, stems and different size tubers of transgenic potato, with levels of phytase expression ranging from 3.8 to 7.4% of total soluble proteins. Phytase expression levels in transgenic potato tubers were stable over several cycles of propagation. Field tests showed that tuber size, number and yield increased in transgenic potato. Improved phosphorus (P) acquisition when phytate was provided as a sole P source and enhanced microtuber formation in cultured transgenic potato seedlings when phytate was provided as an additional P source were observed, which may account for the increase in leaf chloroplast accumulation (important for photosynthesis) and tuber yield of field-grown transgenic potato supplemented with organic fertilizers. Animal feeding tests indicated that the potato-produced phytase supplement was as effective as a commercially available microbial phytase in increasing the availability of phytate-P to weanling pigs. This study demonstrates that the sporamin promoter can effectively direct high-level recombinant protein expression in potato tubers. Moreover, overexpression of phytase in transgenic potato not only offers an ideal feed additive for improving phytate-P digestibility in monogastric animals but also improves tuber yield, enhances P acquisition from organic fertilizers, and has a potential for phytoremediation.

Analysis of subcellular metabolite levels of potato tubers (Solanum tuberosum) displaying alterations in cellular or extracellular sucrose metabolism

The expression of a heterologous invertase in potato tubers (Solanum tuberosum) in either the cytosol or apoplast leads to a decrease in total sucrose content and to an increase in glucose. Depending on the targeting of the enzyme different changes in phenotype and metabolism of the tubers occur: the cytosolic invertase expressing tubers show an increase in the glycolytic flux, accumulation of amino acids and organic acids, and the appearance of novel disaccharides; however, these changes are not observed when the enzyme is expressed in the apoplast [Roessner et al. (2001). Plant Cell, 13, 11-29]. The analysis of these lines raised several questions concerning the regulation of compartmentation of metabolites in potato tubers. In the current study we addressed these questions by performing comparative subcellular metabolite profiling. We demonstrate that: (i) hexoses accumulate in the vacuole independently of their site of production, but that the cytosolic invertase expression led to a strong increase in the cytosolic glucose concentration and decrease in cytosolic sucrose, whereas these effects were more moderate in the apoplastic expressors; (ii) three out of four of the novel compounds found in the cytosolic overexpressors accumulate in the same compartment; (iii) despite changes in absolute cellular content the subcellular distribution of amino acids was invariant in the invertase overexpressing tubers. These results are discussed in the context of current models of the compartmentation of primary metabolism in heterotrophic plant tissues. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11306-008-0107-5) contains supplementary material, which is available to authorized users.

Prospects for increasing starch and sucrose yields for bioethanol production

In the short term, the production of bioethanol as a liquid transport fuel is almost entirely dependent on starch and sugars from existing food crops. The sustainability of this industry would be enhanced by increases in the yield of starch/sugar per hectare without further inputs into the crops concerned. Efforts to achieve increased yields of starch over the last three decades, in particular via manipulation of the enzyme ADPglucose pyrophosphorylase, have met with limited success. Other approaches have included manipulation of carbon partitioning within storage organs in favour of starch synthesis, and attempts to manipulate source-sink relationships. Some of the most promising results so far have come from manipulations that increase the availability of ATP for starch synthesis. Future options for achieving increased starch contents could include manipulation of starch degradation in organs in which starch turnover is occurring, and introduction of starch synthesis into the cytosol. Sucrose accumulation is much less well understood than starch synthesis, but recent results from research on sugar cane suggest that total sugar content can be greatly increased by conversion of sucrose into a non-metabolizable isomer. A better understanding of carbohydrate storage and turnover in relation to carbon assimilation and plant growth is required, both for improvement of starch and sugar crops and for attempts to increase biomass production in second-generation biofuel crops.

Cell-wall invertases from rice are differentially expressed in Caryopsis during the grain filling stage

Cell-wall invertase plays an important role in sucrose partitioning between source and sink organs in higher plants. To investigate the role of cell-wall invertases for seed development in rice (Oryza sativa L.), cDNAs of three putative cell-wall invertase genes OsCIN1, OsCIN2 and OsCIN3 were isolated. Semi-quantitative reverse transcription-polymerase chain reaction analysis revealed different expression patterns of the three genes in various rice tissues/organs. In developing caryopses, they exhibited similar temporal expression patterns, expressed highly at the early and middle grain filling stages and gradually declined to low levels afterward. However, the spatial expression patterns of them were very different, with OsCIN1 primarily expressed in the caryopsis coat, OsCIN2 in embryo and endosperm, and OsCIN3 in embryo. Further RNA in situ hybridization analysis revealed that a strong signal of OsCIN2 mRNA was detected in the vascular parenchyma surrounding the xylem of the chalazal vein and the aleurone layer, whereas OsCIN3 transcript was strongly detected in the vascular parenchyma surrounding the phloem of the chalazal vein, cross-cells, the aleurone layer and the nucellar tissue. These data indicate that the three cell-wall invertase genes play complementary/synergetic roles in assimilate unloading during the grain filling stage. In addition, the cell type-specific expression patterns of OsCIN3 in source leaf blades and anthers were also investigated, and its corresponding physiological roles were discussed.

Expression of cell wall invertase and several other genes of sugar metabolism in relation to seed development in sorghum (Sorghum bicolor)

We report expression profiles of several genes of carbohydrate metabolism, cell wall invertase (CWI) in particular, to better understand sugar transport and its utilization in developing caryopses of grain sorghum [Sorghum bicolor (L.) Moench]. Gene expression analyses for CWI using RNA gel blot and real-time quantitative PCR approaches on developing caryopses, including the glumes (maternal tissue appended to the seeds), showed expression of SbIncw (ZmIncw2 ortholog) primarily in the basal sugar unloading zone of endosperm. The expression of ZmIncw1 ortholog was significantly less abundant and restricted to the glumes. The protein and enzyme activity data corroborated the temporal transcript expression profile that showed maximal CWI protein (INCW) expression preceding the starch-filling phase of endosperm development, i.e. 6-12d-after-pollination (DAP). Protein gel blot analysis using polyclonal maize INCW1 antibodies showed a single polypeptide of 72kDa. The highest level of enzyme activity was unique to the basal part of the endosperm, in particular the basal endosperm transfer cell (BETC) layer and the maternal pedicel region that were highly enriched for the INCW protein, as seen by immunolocalization. High hexose-to-sucrose ratio in 6-12 DAP seeds, and negligible starch deposition in glumes corroborated the CWI activity data. Additionally, we report transcription profiles of several other genes related to sugar-to-starch metabolism in developing sorghum endosperm. As in maize, the INCW-mediated apoplastic cleavage of sucrose in the BETC and pedicel during the early developmental stages of caryopses is essential for the normal development of filial tissues. The unique cell-specificity of the INCW protein to both proximal and distal ends of placental sac shown here for the first time is likely to greatly increase uptakes of both hexose sugars and water through turgor sensing into developing seed. This trait is unique to sorghum among cereals and may facilitate its survival in drought environment.

Glycolytic enzymes associate dynamically with mitochondria in response to respiratory demand and support substrate channeling

In Arabidopsis thaliana, enzymes of glycolysis are present on the surface of mitochondria and free in the cytosol. The functional significance of this dual localization has now been established by demonstrating that the extent of mitochondrial association is dependent on respiration rate in both Arabidopsis cells and potato (Solanum tuberosum) tubers. Thus, inhibition of respiration with KCN led to a proportional decrease in the degree of association, whereas stimulation of respiration by uncoupling, tissue ageing, or overexpression of invertase led to increased mitochondrial association. In all treatments, the total activity of the glycolytic enzymes in the cell was unaltered, indicating that the existing pools of each enzyme repartitioned between the cytosol and the mitochondria. Isotope dilution experiments on isolated mitochondria, using (13)C nuclear magnetic resonance spectroscopy to monitor the impact of unlabeled glycolytic intermediates on the production of downstream intermediates derived from (13)C-labeled precursors, provided direct evidence for the occurrence of variable levels of substrate channeling. Pull-down experiments suggest that interaction with the outer mitochondrial membrane protein, VDAC, anchors glycolytic enzymes to the mitochondrial surface. It appears that glycolytic enzymes associate dynamically with mitochondria to support respiration and that substrate channeling restricts the use of intermediates by competing metabolic pathways.

Temporal analysis of sucrose-induced phosphorylation changes in plasma membrane proteins of Arabidopsis

Sucrose is the main product of photosynthesis and the most common transport form of carbon in plants. In addition, sucrose is a compound that serves as a signal affecting metabolic flux and development. Here we provide first results of externally induced phosphorylation changes of plasma membrane proteins in Arabidopsis. In an unbiased approach, seedlings were grown in liquid medium with sucrose and then depleted of carbon before sucrose was resupplied. Plasma membranes were purified, and phosphopeptides were enriched and subsequently analyzed quantitatively by mass spectrometry. In total, 67 phosphopeptides were identified, most of which were quantified over five time points of sucrose resupply. Among the identified phosphorylation sites, the well described phosphorylation site at the C terminus of plasma membrane H(+)-ATPases showed a relative increase in phosphorylation level in response to sucrose. This corresponded to a significant increase of proton pumping activity of plasma membrane vesicles from sucrose-supplied seedlings. A new phosphorylation site was identified in the plasma membrane H(+)-ATPase AHA1 and/or AHA2. This phosphorylation site was shown to be crucial for ATPase activity and overrode regulation via the well known C-terminal phosphorylation site. Novel phosphorylation sites were identified for both receptor kinases and cytosolic kinases that showed rapid increases in relative intensities after short times of sucrose treatment. Seven response classes were identified including non-responsive, rapid increase (within 3 min), slow increase, and rapid decrease. Relative quantification of phosphorylation changes by phosphoproteomics provides a means for identification of fast responses to external stimuli in plants as a basis for further functional characterization.

The complex network of non-cellulosic carbohydrate metabolism

Partitioning of carbon dominates intracellular fluxes in both photosynthetic and heterotrophic plant tissues, and has vast influence on both plant growth and development. Recently, much progress has been made in elucidating the structures of the biosynthetic and degradative pathways that link the major and minor pools of soluble carbohydrates to cellular polymers such as starch, heteroglycans and fructans. In most cases, the regulatory properties of these pathways have been elucidated and the enzymes involved have been investigated using reverse genetics approaches. Although many of the results from these approaches were merely confirmatory, several of them were highly unexpected. The challenge ahead is to achieve better understanding of metabolic regulation at the network level in order to develop more rational strategies for metabolic engineering.

Regulation of arbuscular mycorrhization by carbon. The symbiotic interaction cannot be improved by increased carbon availability accomplished by root-specifically enhanced invertase activity

The mutualistic interaction in arbuscular mycorrhiza (AM) is characterized by an exchange of mineral nutrients and carbon. The major benefit of AM, which is the supply of phosphate to the plant, and the stimulation of mycorrhization by low phosphate fertilization has been well studied. However, less is known about the regulatory function of carbon availability on AM formation. Here the effect of enhanced levels of hexoses in the root, the main form of carbohydrate used by the fungus, on AM formation was analyzed. Modulation of the root carbohydrate status was performed by expressing genes encoding a yeast (Saccharomyces cerevisiae)-derived invertase, which was directed to different subcellular locations. Using tobacco (Nicotiana tabacum) alcc::wINV plants, the yeast invertase was induced in the whole root system or in root parts. Despite increased hexose levels in these roots, we did not detect any effect on the colonization with Glomus intraradices analyzed by assessment of fungal structures and the level of fungus-specific palmitvaccenic acid, indicative for the fungal carbon supply, or the plant phosphate content. Roots of Medicago truncatula, transformed to express genes encoding an apoplast-, cytosol-, or vacuolar-located yeast-derived invertase, had increased hexose-to-sucrose ratios compared to beta-glucuronidase-transformed roots. However, transformations with the invertase genes did not affect mycorrhization. These data suggest the carbohydrate supply in AM cannot be improved by root-specifically increased hexose levels, implying that under normal conditions sufficient carbon is available in mycorrhizal roots. In contrast, tobacco rolC::ppa plants with defective phloem loading and tobacco pyk10::InvInh plants with decreased acid invertase activity in roots exhibited a diminished mycorrhization.

Assessing the potential for unintended effects in genetically modified potatoes perturbed in metabolic and developmental processes. Targeted analysis of key nutrients and anti-nutrients

Targeted compositional analysis was carried out on transgenic potato tubers of either cultivar (cv.) Record or cv. Desirée to assess the potential for unintended effects caused by the genetic modification process. The range of transgenic lines analysed included those modified in primary carbohydrate metabolism, polyamine biosynthesis and glycoprotein processing. Controls included wildtype tubers, tubers produced from plants regenerated through tissue culture (including a callus phase) and tubers derived from transformation with the 'empty vector' i.e. no specific target gene included (with the exception of the kanamycin resistance gene as a selectable marker). Metabolite analysis included soluble carbohydrates, glycoalkaloids, vitamin C, total nitrogen and fatty acids. Trypsin inhibitor activity was also assayed. These cover the major compounds recommended by the OECD in their Consensus Document on Compositional Considerations for New Varieties of Potatoes: Key Food and Feed Nutrients, Anti-Nutrients and Toxicants (2002). Data was statistically analysed using analysis of variance (ANOVA) for individual compounds and, where applicable, principal component analysis (PCA). In general, targeted compositional analysis revealed no consistent differences between GM lines and respective controls. No construct specifically induced unintended effects. Statistically significant differences between wildtype controls and specific GM lines did occur but appeared to be random and not associated with any specific construct. Indeed such significant differences were also found between wildtypes and both tissue culture derived tubers and tubers derived from transformation with the empty vector. This raises the possibility that somaclonal variation (known to occur significantly in potato, depending on genotype) may be responsible for an unknown proportion of any differences observed between specific GM lines and the wildtype. The most obvious differences seen in GC-MS profiles were between the two potato varieties used in the study.

Varied growth, biomass and cellulose content in tobacco expressing yeast-derived invertases

The effects of the expression of yeast-derived apoplastic (AI) and cytosolic (CI) invertases (EC 3.2.1.26) on biomass and structural carbohydrate accumulation in tobacco (Nicotiana tabacum L. cv. Xanthi) were evaluated. Transgenic tobacco plants expressing AI or CI under the control of either a tandem repeat of the Cauliflower Mosaic Virus 35S promoter (2X35S), or a promoter that drives xylem-localized expression (Petroselinum crispum 4-coumarate:CoA ligase promoter; 4CL) were generated. Yeast-derived invertase transcript levels, invertase protein, enzyme activity, growth parameters as well as both structural and soluble carbohydrates of stem tissue of all transformed lines were quantified. Transgenic tobacco lines expressing invertase under the control of 4CL displayed severe growth retardation with both yeast-derived isogenes. Similarly, several transformed lines expressing either AI or CI regulated by the 2X35S promoter were also shorter than wild-type (WT) plants. Despite the decreases in height, some transformed lines had significant increases in biomass. One line (2X35S::AI-1) had a biomass/height increase of 88% and an increase in stem diameter of over 40%, while a second line (2X35S::CI-5) had a biomass/height increase of 21%. A separate line (2X35S::AI-2) had a 36% increase in cellulose content, while two others (4CL::AI-2 and 4CL::AI-3) displayed significant decreases in cellulose content. The observed phenotypes can be in part explained by the levels of foreign invertase present, subcellular localization and the carbohydrate status of the tissues.