The breakdown of starch in leaves

NMR Fingerprinting of Conventional and Genetically Modified Soybean Plants with AtAREB1 Transcription Factors

Drought stress impacts soybean yields and physiological processes. However, the insertion of the activated form of the AtAREB1 gene in the soybean cultivar BR16, which is sensitive to water deficit, improved the drought response of the genetically modified plants. Thus, in this study, we used 1H NMR in solution and solid-state NMR to investigate the response of genetically modified soybean overexpressing AtAREB1 under water deficiency conditions. We achieved that drought-tolerant soybean yields high content of amino acids isoleucine, leucine, threonine, valine, proline, glutamate, aspartate, asparagine, tyrosine, and phenylalanine after 12 days of drought stress conditions, as compared to drought-sensitive soybean under the same conditions. Specific target compounds, including sugars, organic acids, and phenolic compounds, were identified as involved in controlling sensitive soybean during the vegetative stage. Solid-state NMR was used to study the impact of drought stress on starch and cellulose contents in different soybean genotypes. The findings provide insights into the metabolic adjustments of soybean overexpressing AREB transcription factors in adapting to dry climates. This study presents NMR techniques for investigating the metabolome of transgenic soybean plants in response to the water deficit. The approach allowed for the identification of physiological and morphological changes in drought-resistant and drought-tolerant soybean tissues. The findings indicate that drought stress significantly alters micro- and macromolecular metabolism in soybean plants. Differential responses were observed among roots and leaves as well as drought-tolerant and drought-sensitive cultivars, highlighting the complex interplay between overexpressed transcription factors and drought stress in soybean plants.

Co-overexpression of SWEET sucrose transporters modulates sucrose synthesis and defence responses to enhance immunity against bacterial blight in rice

Enhancing carbohydrate export from source to sink tissues is considered to be a realistic approach for improving photosynthetic efficiency and crop yield. The rice sucrose transporters OsSUT1, OsSWEET11a and OsSWEET14 contribute to sucrose phloem loading and seed filling. Crucially, Xanthomonas oryzae pv. oryzae (Xoo) infection in rice enhances the expression of OsSWEET11a and OsSWEET14 genes, and causes leaf blight. Here we show that co-overexpression of OsSUT1, OsSWEET11a and OsSWEET14 in rice reduced sucrose synthesis and transport leading to lower growth and yield but reduced susceptibility to Xoo relative to controls. The immunity-related hypersensitive response (HR) was enhanced in the transformed lines as indicated by the increased expression of defence genes, higher salicylic acid content and presence of HR lesions on the leaves. The results suggest that the increased expression of OsSWEET11a and OsSWEET14 in rice is perceived as a pathogen (Xoo) attack that triggers HR and results in constitutive activation of plant defences that are related to the signalling pathways of pathogen starvation. These findings provide a mechanistic basis for the trade-off between plant growth and immunity because decreased susceptibility against Xoo compromised plant growth and yield.

Integrated bacterial transcriptome and host metabolome analysis reveals insights into "Candidatus Liberibacter asiaticus" population dynamics in the fruit pith of three citrus cultivars with different tolerance

"Candidatus Liberibacter asiaticus" (CLas), the causal agent of citrus Huanglongbing (HLB), is able to multiply to a high abundance in citrus fruit pith. However, little is known about the biological processes and phytochemical substances that are vital for CLas colonization and growth in fruit pith. In this study, CLas-infected fruit pith of three citrus cultivars ("Shatangju" mandarin, "Guanxi" pomelo, and "Shatian" pomelo) exhibiting different tolerance to CLas were collected and used for dual RNA-Seq and untargeted metabolome analysis. Comparative transcriptome analysis found that the activation of the CLas noncyclic TCA pathway and pathogenic-related effectors could contribute to the colonization and growth of CLas in fruit pith. The pre-established Type 2 prophage in the CLas genome and the induction of its CRISPR/cas system could enhance the phage resistance of CLas and, in turn, facilitate CLas population growth in fruit pith. CLas infection caused the accumulation of amino acids that were correlated with tolerance to CLas. The accumulation of most sugars and organic acids in CLas-infected fruit pith, which could be due to the phloem blockage caused by CLas infection, was thought to be beneficial for CLas growth in localized phloem tissue. The higher levels of flavonoids and terpenoids in the fruit pith of CLas-tolerant cultivars, particularly those known for their antimicrobial properties, could hinder the growth of CLas. This study advances our understanding of CLas multiplication in fruit pith and offers novel insight into metabolites that could be responsible for tolerance to CLas or essential to CLas population growth.IMPORTANCECitrus Huanglongbing (HLB, also called citrus greening disease) is a highly destructive disease currently threatening citrus production worldwide. HLB is caused by an unculturable bacterial pathogen, "Candidatus Liberibacter asiaticus" (CLas). However, the mechanism of CLas colonization and growth in citrus hosts is poorly understood. In this study, we utilized the fruit pith tissue, which was able to maintain the CLas at a high abundance, as the materials for dual RNA-Seq and untargeted metabolome analysis, aiming to reveal the biological processes and phytochemical substances that are vital for CLas colonization and growth. We provided a genome-wide CLas transcriptome landscape in the fruit pith of three citrus cultivars with different tolerance and identified the important genes/pathways that contribute to CLas colonization and growth in the fruit pith. Metabolome profiling identified the key metabolites, which were mainly affected by CLas infection and influenced the population dynamic of CLas in fruit pith.

Daylength variation affects growth, photosynthesis, leaf metabolism, partitioning, and metabolic fluxes

Daylength, a seasonal and latitudinal variable, exerts a substantial impact on plant growth. However, the relationship between daylength and growth is nonproportional, suggesting the existence of adaptive mechanisms. Thus, our study aimed to comprehensively investigate the adaptive strategies employed by plants in response to daylength variation. We grew false flax (Camelina sativa) plants, a model oilseed crop, under long-day (LD) and short-day (SD) conditions and used growth measurements, gas exchange measurements, and isotopic labeling techniques, including 13C, 14C, and 2H2O, to determine responses to different daylengths. Our findings revealed that daylength influences various growth parameters, photosynthetic physiology, carbon partitioning, metabolic fluxes, and metabolite levels. SD plants employed diverse mechanisms to compensate for reduced CO2 fixation in the shorter photoperiod. These mechanisms included enhanced photosynthetic rates and reduced respiration in the light (RL), leading to increased shoot investment. Additionally, SD plants exhibited reduced rates of the glucose 6-phosphate (G6P) shunt and greater partitioning of sugars into starch, thereby sustaining carbon availability during the longer night. Isotopic labeling results further demonstrated substantial alterations in the partitioning of amino acids and TCA cycle intermediates between rapidly and slowly turning over pools. Overall, the results point to multiple developmental, physiological, and metabolic ways in which plants adapt to different daylengths to maintain growth.

Transcription factors ABF4 and ABR1 synergistically regulate amylase-mediated starch catabolism in drought tolerance

β-Amylase (BAM)-mediated starch degradation is a main source of soluble sugars that help plants adapt to environmental stresses. Here, we demonstrate that dehydration-induced expression of PtrBAM3 in trifoliate orange (Poncirus trifoliata (L.) Raf.) functions positively in drought tolerance via modulation of starch catabolism. Two transcription factors, PtrABF4 (P. trifoliata abscisic acid-responsive element-binding factor 4) and PtrABR1 (P. trifoliata ABA repressor 1), were identified as upstream transcriptional activators of PtrBAM3 through yeast one-hybrid library screening and protein-DNA interaction assays. Both PtrABF4 and PtrABR1 played a positive role in plant drought tolerance by modulating soluble sugar accumulation derived from BAM3-mediated starch decomposition. In addition, PtrABF4 could directly regulate PtrABR1 expression by binding to its promoter, leading to a regulatory cascade to reinforce the activation of PtrBAM3. Moreover, PtrABF4 physically interacted with PtrABR1 to form a protein complex that further promoted the transcriptional regulation of PtrBAM3. Taken together, our finding reveals that a transcriptional cascade composed of ABF4 and ABR1 works synergistically to upregulate BAM3 expression and starch catabolism in response to drought condition. The results shed light on the understanding of the regulatory molecular mechanisms underlying BAM-mediated soluble sugar accumulation for rendering drought tolerance in plants.

Metabolomic Analysis Revealed Distinct Physiological Responses of Leaves and Roots to Huanglongbing in a Citrus Rootstock

Huanglongbing (HLB) is an obstinate disease in the citrus industry. No resistant citrus resources were currently available, but various degrees of Huanglongbing tolerance exist in different germplasm. Citrus junos is emerging as one of the popular rootstocks widely used in the citrus production. However, its responses to the HLB causal agent, Candidatus Liberibacter asiaticus (CLas), were still elusive. In the current study, we investigated the physiological, anatomical, and metabolomic responses of a C. junos rootstock ‘Pujiang Xiangcheng’ by a controlled CLas grafting inoculation. The summer flushes and roots were impaired at 15 weeks after inoculation, although typical leaf symptomatic phenotypes were not obvious. The chlorophyll pigments and the photosynthetic rate were compromised. The phloem sieve tubes were still working, despite the fact that the callose was deposited and the starch granules were accumulated in the phloem cells. A wide, targeted metabolomic analysis was carried out to explore the systematic alterations of the metabolites at this early stage of infection in the leaves and root system. The differentially accumulated metabolites in the CLas-affected leaves and roots compared with the mock-inoculation control tissues revealed that distinct responses were obvious. Besides the commonly observed alteration of sugar and amino acids, the active break down of starch in the roots was discovered. The different types of fatty acids were altered in the two tissues, with a more pronounced content decline in the roots. Our results not only provided fundamental knowledge about the response of the C. junos rootstock to the HLB disease, but also presented new insights into the host–pathogen interaction in the early stages.

Production of Large-Ring Cyclodextrins by Amylomaltases

Amylomaltase is a well-known glucan transferase that can produce large ring cyclodextrins (LR-CDs) or so-called cycloamyloses via cyclization reaction. Amylomaltases have been found in several microorganisms and their optimum temperatures are generally around 60–70 °C for thermostable amylomaltases and 30–45 °C for the enzymes from mesophilic bacteria and plants. The optimum pHs for mesophilic amylomaltases are around pH 6.0–7.0, while the thermostable amylomaltases are generally active at more acidic conditions. Size of LR-CDs depends on the source of amylomaltases and the reaction conditions including pH, temperature, incubation time, and substrate. For example, in the case of amylomaltase from Corynebacterium glutamicum, LR-CD productions at alkaline pH or at a long incubation time favored products with a low degree of polymerization. In this review, we explore the synthesis of LR-CDs by amylomaltases, structural information of amylomaltases, as well as current applications of LR-CDs and amylomaltases.

Disruption of Brachypodium lichenase alters metabolism of mixed-linkage glucan and starch

Mixed-linkage glucan, which is widely distributed in grasses, is a polysaccharide highly abundant in cell walls of grass endosperm and young vegetative tissues. Lichenases are enzymes that hydrolyze mixed-linkage glucan first identified in mixed-linkage glucan-rich lichens. In this study, we identify a gene encoding a lichenase we name Brachypodium distachyon LICHENASE 1 (BdLCH1), which is highly expressed in the endosperm of germinating seeds and coleoptiles and at lower amounts in mature shoots. RNA in situ hybridization showed that BdLCH1 is primarily expressed in chlorenchyma cells of mature leaves and internodes. Disruption of BdLCH1 resulted in an eight-fold increase in mixed-linkage glucan content in senesced leaves. Consistent with the in situ hybridization data, immunolocalization results showed that mixed-linkage glucan was not removed in chlorenchyma cells of lch1 mutants as it was in wild type and implicate the BdLCH1 enzyme in removing mixed-linkage glucan in chlorenchyma cells in mature vegetative tissues. We also show that mixed-linkage glucan accumulation in lch1 mutants was resistant to dark-induced degradation, and 8-week-old lch1 plants showed a faster rate of starch breakdown than wild type in darkness. Our results suggest a role for BdLCH1 in modifying the cell wall to support highly metabolically active cells.

Synergistic effects of multiple enzymes from industrial Aspergillus niger strain O1 on starch saccharification

BACKGROUND

Starch is one of the most important renewable polysaccharides in nature for production of bio-ethanol. The starch saccharification step facilitates the depolymerization of starch to yield glucose for biofuels production. The filamentous fungus Aspergillus niger (A. niger) is the most used microbial cell factory for production of the commercial glucoamylase. However, the role of each component in glucoamylases cocktail of A. niger O1 for starch saccharification remains unclear except glucoamylase.

RESULTS

In this study, we identified the key enzymes contributing to the starch saccharification process are glucoamylase, α-amylase and acid α-amylase out of 29 glycoside hydrolases from the 6-day fermentation products of A. niger O1. Through the synergistic study of the multienzymes for the starch saccharification in vitro, we found that increasing the amount of α-amylase by 5-10 times enhanced the efficiency of starch saccharification by 14.2-23.2%. Overexpression of acid α-amylase in strain O1 in vivo increased the total glucoamylase activity of O1 cultures by 15.0%.

CONCLUSIONS

Our study clarifies the synergistic effects among the components of glucoamylases cocktail, and provides an effective approach to optimize the profile of saccharifying enzymes of strain O1 for improving the total glucoamylase activity.

A Holistic Approach to Study Photosynthetic Acclimation Responses of Plants to Fluctuating Light

Plants in natural environments receive light through sunflecks, the duration and distribution of these being highly variable across the day. Consequently, plants need to adjust their photosynthetic processes to avoid photoinhibition and maximize yield. Changes in the composition of the photosynthetic apparatus in response to sustained changes in the environment are referred to as photosynthetic acclimation, a process that involves changes in protein content and composition. Considering this definition, acclimation differs from regulation, which involves processes that alter the activity of individual proteins over short-time periods, without changing the abundance of those proteins. The interconnection and overlapping of the short- and long-term photosynthetic responses, which can occur simultaneously or/and sequentially over time, make the study of long-term acclimation to fluctuating light in plants challenging. In this review we identify short-term responses of plants to fluctuating light that could act as sensors and signals for acclimation responses, with the aim of understanding how plants integrate environmental fluctuations over time and tailor their responses accordingly. Mathematical modeling has the potential to integrate physiological processes over different timescales and to help disentangle short-term regulatory responses from long-term acclimation responses. We review existing mathematical modeling techniques for studying photosynthetic responses to fluctuating light and propose new methods for addressing the topic from a holistic point of view.

Effects of Deficit Irrigation and Huanglongbing on Sweet Orange Trees

This study addresses the interactive effects of deficit irrigation and huanglongbing (HLB) infection on the physiological, biochemical, and oxidative stress responses of sweet orange trees. We sought to answer: (i) What are the causes for the reduction in water uptake in HLB infected plants? (ii) Is the water status of plants negatively affected by HLB infection? (iii) What are the key physiological traits impaired in HLB-infected plants? and (iv) What conditions can mitigate both disease severity and physiological/biochemical impairments in HLB-infected plants? Two water management treatments were applied for 11 weeks to 1-year-old-trees that were either healthy (HLB-) or infected with HLB (+) and grown in 12-L pots. Half of the trees were fully irrigated (FI) to saturation, whereas half were deficit-irrigated (DI) using 40% of the water required to saturate the substrate. Our results demonstrated that: reduced water uptake capacity in HLB+ plants was associated with reduced root growth, leaf area, stomatal conductance, and transpiration. Leaf water potential was not negatively affected by HLB infection. HLB increased leaf respiration rates (ca. 41%) and starch synthesis, downregulated starch breakdown, blocked electron transport, improved oxidative stress, and reduced leaf photosynthesis (ca. 57%) and photorespiration (ca.57%). Deficit irrigation reduced both leaf respiration (ca. 45%) and accumulation of starch (ca.53%) by increasing maltose (ca. 20%), sucrose, glucose, and fructose contents in the leaves, decreasing bacterial population (ca. 9%) and triggering a series of protective measures against further impairments in the physiology and biochemistry of HLB-infected plants. Such results provide a more complete physiological and biochemical overview of HLB-infected plants and can guide future studies to screen genetic tolerance to HLB and improve management strategies under field orchard conditions.

Exogenous supplementation of melatonin alters representative organic acids and enzymes of respiratory cycle as well as sugar metabolism during arsenic stress in two contrasting indica rice cultivars

The objective of the present investigation was to understand the impact of exogenously applied melatonin on mitochondrial respiration and sugar metabolism in two contrasting rice cultivars, viz., Khitish (arsenic-susceptible) and Muktashri (arsenic-tolerant) under arsenic-stress. Melatonin effectively restored the level of organic acids like pyruvic acid, malic acid and more particularly citric acid by 33 % in Khitish which were lowered during arsenic-stress, whereas their levels were further elevated in Muktashri to provide energy for defence against arsenic-induced injury. Arsenic-exposure led to a significant inhibition in enzyme activities as well as corresponding transcript level of key respiratory enzymes, viz., pyruvate dehydrogenase, citrate synthase, isocitrate dehydrogenase, succinate dehydrogenase and malate dehydrogenase, intriguingly more prominently in case of Khitish. Conversely, melatonin supplementation, irrespective of cultivars, considerably improved the activity of the above enzymes and corresponding gene expressions during stress, indicating acceleration in the rate of Krebs cycle. Melatonin supplementation also stimulated the accumulation of total soluble sugars by 62 % and 25 %, reducing sugars by 50 % and 44 % and non-reducing sugars by 75 % and 14 % in Khitish and Muktashri respectively, concomitant with higher activities of acid invertase, sucrose synthase and sucrose phosphate synthase enzymes, along with the expression of corresponding genes. Enhanced starch accumulation via regulation of alpha amylase and starch phosphorylase activities and gene expression, by melatonin also contributed towards better stress tolerance. Overall, this work illustrated the efficacy of melatonin in the regulation of representative organic acids and enzymes of respiratory cycle along with starch and sugar metabolism in rice cultivars under arsenic toxicity.

Structural analysis and reaction mechanism of the disproportionating enzyme (D-enzyme) from potato

Starch produced by plants is a stored form of energy and is an important dietary source of calories for humans and domestic animals. Disproportionating enzyme (D-enzyme) catalyzes intramolecular and intermolecular transglycosylation reactions of α-1, 4-glucan. D-enzyme is essential in starch metabolism in the potato. We present the crystal structures of potato D-enzyme, including two different types of complex structures: a primary Michaelis complex (substrate binding mode) for 26-meric cycloamylose (CA26) and a covalent intermediate for acarbose. Our study revealed that the acarbose and CA26 reactions catalyzed by potato D-enzyme involve the formation of a covalent intermediate with the donor substrate. HPAEC of reaction substrates and products revealed the activity of the potato D-enzyme on acarbose and CA26 as donor substrates. The structural and chromatography analyses provide insight into the mechanism of the coupling reaction of CA and glucose catalyzed by the potato D-enzyme. The enzymatic reaction mechanism does not involve residual hydrolysis. This could be particularly useful in preventing unnecessary starch degradation leading to reduced crop productivity. Optimization of this mechanism would be important for improvements of starch storage and productivity in crops.

Spectral differentiation of oak wilt from foliar fungal disease and drought is correlated with physiological changes

Hyperspectral reflectance tools have been used to detect multiple pathogens in agricultural settings and single sources of infection or broad declines in forest stands. However, differentiation of any one disease from other sources of tree stress is integral for stand and landscape-level applications in mixed species systems. We tested the ability of spectral models to differentiate oak wilt, a fatal disease in oaks caused by Bretziella fagacearum ``Bretz'', from among other mechanisms of decline. We subjected greenhouse-grown oak seedlings (Quercus ellipsoidalis ``E.J. Hill'' and Quercus macrocarpa ``Michx.'') to chronic drought or inoculation with the oak wilt fungus or bur oak blight fungus (Tubakia iowensis ``T.C. Harr. & D. McNew''). We measured leaf and canopy spectroscopic reflectance (400-2400 nm) and instantaneous photosynthetic and stomatal conductance rates, then used partial least-squares discriminant analysis to predict treatment from hyperspectral data. We detected oak wilt before symptom appearance, and classified the disease with high accuracy in symptomatic leaves. Classification accuracy from spectra increased with declines in photosynthetic function in oak wilt-inoculated plants. Wavelengths diagnostic of oak wilt were only found in non-visible spectral regions and are associated with water status, non-structural carbohydrates and photosynthetic mechanisms. We show that hyperspectral models can differentiate oak wilt from other causes of tree decline and that detection is correlated with biological mechanisms of oak wilt infection and disease progression. We also show that within the canopy, symptom heterogeneity can reduce detection, but that symptomatic leaves and tree canopies are suitable for highly accurate diagnosis. Remote application of hyperspectral tools can be used for specific detection of disease across a multi-species forest stand exhibiting multiple stress symptoms.

Carbon partitioning mechanisms in POTATO under drought stress

Potato (Solanum tuberosum) is an important food crop consumed all over the world, but it is generally sensitive to drought conditions. One of the major physiological processes affected by drought stress is carbon partitioning: the plant's choice of where to allocate its photoassimilates. Our aim was to investigate the molecular factors and possible bottlenecks affecting carbon partitioning during drought. We studied potato cultivars with contrasting drought responses in the greenhouse in the years 2013-2015, and further investigated the expression of genes involved in carbon partitioning and metabolite levels. Our results indicate that one of the most severe effects of drought stress on potato is the arrest of stolon differentiation and formation of tubers. We also identified some physiological traits like stomatal conductance and chlorophyll content as affecting carbon assimilation, partitioning and eventual tuber yield. The gene expressions and biochemical analyses highlight the various tissues prioritized by the plant for assimilate transport during drought stress, and give indications of what distinguishes drought tolerance and sensitivity of cultivated potato. Some of the key genes studied (like Sucrose synthase and Sucrose transporters) may be inclusive breeding targets for drought tolerance in potato.

A rapid and universal method for isolating starch granules in plant tissues

Starch is the major form of carbohydrate storage in plants and exists as discrete starch granules (SGs). Isolation of high-quality SGs in different plant tissues is a prerequisite for studying the roles of SGs during plant growth, development, and responses to abiotic stress. However, it is difficult to isolate transitory SGs from leaves and storage SGs from pollen grains due to their small sizes and low quantities. Herein, we develop a novel method for isolating SGs by using the aqueous two-phase system (ATS) of ethanol/NaH₂ PO₄ . The ATS method efficiently separated SGs from contaminants based on their differences in density, solubility, and polarity. Using this method, we first isolated and purified three kinds of SGs from maize seeds, pollen, and leaves. The biochemical, microscopic, and proteomic analyses demonstrated the high purity of the isolated SGs. Proteomic analysis revealed distinct differences in SG-bound proteins between seed SGs and pollen SGs. As a simple, rapid, and low-cost method, the ATS-based method exhibits highly universal and reproducible results for starch-containing tissues in various plant species.

Elevated CO2-induced changes in mesophyll conductance and anatomical traits in wild type and carbohydrate-metabolism mutants of Arabidopsis

Decreases in photosynthetic rate, stomatal conductance (gs), and mesophyll conductance (gm) are often observed under elevated CO2 conditions. However, which anatomical and/or physiological factors contribute to the decrease in gm is not fully understood. Arabidopsis thaliana wild-type and carbon-metabolism mutants (gwd1, pgm1, and cfbp1) with different accumulation patterns of non-structural carbohydrates were grown at ambient (400 ppm) and elevated (800 ppm) CO2. Anatomical and physiological traits of leaves were measured to investigate factors causing the changes in gm and in the mesophyll resistance (expressed as the reciprocal of mesophyll conductance per unit chloroplast surface area facing to intercellular space, Sc/gm). When grown at elevated CO2, all the lines showed increases in cell wall mass, cell wall thickness, and starch content, but not in leaf thickness. gm measured at 800 ppm CO2 was significantly lower than at 400 ppm CO2 in all the lines. Changes in Sc/gm were associated with thicker cell walls rather than with excess starch content. The results indicate that the changes in gm and Sc/gm that occur in response to elevated CO2 are independent of non-structural carbohydrates, and the cell wall represents a greater limitation factor for gm than starch.

De novo transcriptome provides insights into the growth behaviour and resveratrol and trans-stilbenes biosynthesis in Dactylorhiza hatagirea - An endangered alpine terrestrial orchid of western Himalaya

This is the first report on de novo transcriptome of Dactylorhiza hatagirea, a critically-endangered, terrestrial orchid of alpine Himalayas. The plant is acclaimed for medicinal properties but little is known about its secondary-metabolites profile or cues regulating their biosynthesis. De novo transcriptome analysis was therefore, undertaken to gain basic understanding on these aspects, while circumventing the acute limitation of plant material availability. 65,384 transcripts and finally, 37,371 unigenes were assembled de novo from a total of 236 million reads obtained from shoot, tuber and leaves of the plant. Dominance of differentially-expressing-genes (DEGs) related to cold-stress-response and plant-hormone-signal-transduction; and those involved in photosynthesis, sugar-metabolism and secondary-metabolite-synthesis provided insights into carbohydrate-partitioning in the plant during its preparation for freezing winter at natural habitat. DEGs of glucomannan, ascorbic acid, carotenoids, phylloquinone/naphthoquinones, indole alkaloids, resveratrol and stilbene biosynthesis revealed the secondary-metabolite profile of D. hatagirea. UHPLC results confirmed appreciable amounts of resveratrol and trans-stilbene in D. hatagirea tubers, for the first time. Expression analysis of 15 selected genes including those of phenylpropanoid pathway confirmed the validity of RNA-seq data. Opportunistic growth, temperature- and tissue-specific-differential-expression of secondary metabolite biosynthesis and stress tolerant genes were confirmed using clonal plants growing at 8, 15 and 25 °C.

Response of plasmodesmata formation in leaves of C4 grasses to growth irradiance

Rapid metabolite diffusion across the mesophyll (M) and bundle sheath (BS) cell interface in C₄ leaves is a key requirement for C₄ photosynthesis and occurs via plasmodesmata (PD). Here, we investigated how growth irradiance affects PD density between M and BS cells and between M cells in two C₄ species using our PD quantification method, which combines three-dimensional laser confocal fluorescence microscopy and scanning electron microscopy. The response of leaf anatomy and physiology of NADP-ME species, Setaria viridis and Zea mays to growth under different irradiances, low light (100 μmol m^-2  s^-1 ), and high light (1,000 μmol m^-2  s^-1 ), was observed both at seedling and established growth stages. We found that the effect of growth irradiance on C₄ leaf PD density depended on plant age and species. The high light treatment resulted in two to four-fold greater PD density per unit leaf area than at low light, due to greater area of PD clusters and greater PD size in high light plants. These results along with our finding that the effect of light on M-BS PD density was not tightly linked to photosynthetic capacity suggest a complex mechanism underlying the dynamic response of C₄ leaf PD formation to growth irradiance.

Metabolomic foundation for differential responses of lipid metabolism to nitrogen and phosphorus deprivation in an arachidonic acid-producing green microalga

The green oleaginous microalga Lobosphaera incisa accumulates storage lipids triacylglycerols (TAG) enriched in the long-chain polyunsaturated fatty acid arachidonic acid under nitrogen (N) deprivation. In contrast, under phosphorous (P) deprivation, the production of the monounsaturated oleic acid prevails. We compared physiological responses, ultrastructural, and metabolic consequences of L. incisa acclimation to N and P deficiency to provide novel insights into the key determinants of ARA accumulation. Differential responses to nutrient deprivation on growth performance, carbon-to-nitrogen stoichiometry, membrane lipid composition and TAG accumulation were demonstrated. Ultrastructural analyses suggested a dynamic role for vacuoles in sustaining cell homeostasis under conditions of different nutrient availability and their involvement in autophagy in L. incisa. Paralleling ARA-rich TAG accumulation in lipid droplets, N deprivation triggered intensive chloroplast dismantling and promoted catabolic processes. Metabolome analysis revealed depletion of amino acids and pyrimidines, and repression of numerous biosynthetic hubs to favour TAG biosynthesis under N deprivation. Under P deprivation, despite the relatively low growth penalties, the presence of the endogenous P reserves and the characteristic lipid remodelling, metabolic signatures of energy deficiency were revealed. Metabolome adjustments to P deprivation included depletion in ATP and phosphorylated nucleotides, increased levels of TCA-cycle intermediates and osmoprotectants. We conclude that characteristic cellular and metabolome adjustments tailor the adaptive responses of L. incisa to N and P deprivation modulating its LC-PUFA production.

Effects of Light Quality and Intensity on Diurnal Patterns and Rates of Photo-Assimilate Translocation and Transpiration in Tomato Leaves

Translocation of assimilates is a fundamental process involving carbon and water balance affecting source/sink relationships. Diurnal patterns of CO2 exchange, translocation (carbon export), and transpiration of an intact tomato source leaf were determined during 14CO2 steady-state labeling under different wavelengths at three pre-set photosynthetic rates. Daily patterns showed that photosynthesis and export were supported by all wavelengths of light tested including orange and green. Export in the light, under all wavelengths was always higher than that at night. Export in the light varied from 65-83% of the total daily carbon fixed, depending on light intensity. Photosynthesis and export were highly correlated under all wavelengths (r = 0.90-0.96). Export as a percentage of photosynthesis (relative export) decreased as photosynthesis increased by increasing light intensity under all wavelengths. These data indicate an upper limit for export under all spectral conditions. Interestingly, only at the medium photosynthetic rate, relative export under the blue and the orange light-emitting diodes (LEDs) were higher than under white and red-white LEDs. Stomatal conductance, transpiration rates, and water-use-efficiency showed similar daily patterns under all wavelengths. Illuminating tomato leaves with different spectral quality resulted in similar carbon export rates, but stomatal conductance and transpiration rates varied due to wavelength specific control of stomatal function. Thus, we caution that the link between transpiration and C-export may be more complex than previously thought. In summary, these data indicate that orange and green LEDs, not simply the traditionally used red and blue LEDs, should be considered and tested when designing lighting systems for optimizing source leaf strength during plant production in controlled environment systems. In addition, knowledge related to the interplay between water and C-movement within a plant and how they are affected by environmental stimuli, is needed to develop a better understanding of source/sink relationships.

A new mechanistic understanding of light-limitation in the seagrass Zostera muelleri

In this study we investigated the effect of light-limitation (∼20 μmol photons m^-2 s^-1) on the southern hemisphere seagrass, Zostera muelleri. RNA sequencing, chlorophyll fluorometry and HPLC techniques were used to investigate how the leaf-specific transcriptome drives changes in photosynthesis and photo-pigments in Z. muelleri over 6 days. 1593 (7.51%) genes were differentially expressed on day 2 and 1481 (6.98%) genes were differentially expressed on day 6 of the experiment. Differential gene expression correlated with significant decreases in rETR_Max, I_k, an increase in Yi (initial photosynthetic quantum yield of photosystem II), and significant changes in pigment composition. Regulation of carbohydrate metabolism was observed along with evidence that abscisic acid may serve a role in the low-light response of this seagrass. This study provides a novel understanding of how Z. muelleri responds to light-limitation in the marine water column and provides potential molecular markers for future conservation monitoring efforts.

A comprehensive investigation of starch degradation process and identification of a transcriptional activator MabHLH6 during banana fruit ripening

Although starch degradation has been well studied in model systems such as Arabidopsis leaves and cereal seeds, this process in starchy fruits during ripening, especially in bananas, is largely unknown. In this study, 38 genes encoding starch degradation-related proteins were identified and characterized from banana fruit. Expression analysis revealed that 27 candidate genes were significantly induced during banana fruit ripening, with concomitant conversion of starch-to-sugars. Furthermore, iTRAQ-based proteomics experiments identified 18 starch degradation-associated enzymes bound to the surface of starch granules, of which 10 were markedly up-regulated during ripening. More importantly, a novel bHLH transcription factor, MabHLH6, was identified based on a yeast one-hybrid screening using MaGWD1 promoter as a bait. Transcript and protein levels of MabHLH6 were also increased during fruit ripening. Electrophoretic mobility shift assays, chromatin immunoprecipitation and transient expression experiments confirmed that MabHLH6 activates the promoters of 11 starch degradation-related genes, including MaGWD1, MaLSF2, MaBAM1, MaBAM2, MaBAM8, MaBAM10, MaAMY3, MaAMY3C, MaISA2, MaISA3 and MapGlcT2-2 by recognizing their E-box (CANNTG) motifs present in the promoters. Collectively, these findings suggest that starch degradation during banana fruit ripening may be attributed to the complex actions of numerous enzymes related to starch breakdown at transcriptional and translational levels, and that MabHLH6 may act as a positive regulator of this process via direct activation of a series of starch degradation-related genes.

Deep Assessment of Genomic Diversity in Cassava for Herbicide Tolerance and Starch Biosynthesis

Cassava is one of the most important food security crops in tropical countries, and a competitive resource for the starch, food, feed and ethanol industries. However, genomics research in this crop is much less developed compared to other economically important crops such as rice or maize. The International Center for Tropical Agriculture (CIAT) maintains the largest cassava germplasm collection in the world. Unfortunately, the genetic potential of this diversity for breeding programs remains underexploited due to the difficulties in phenotypic screening and lack of deep genomic information about the different accessions. A chromosome-level assembly of the cassava reference genome was released this year and only a handful of studies have been made, mainly to find quantitative trait loci (QTL) on breeding populations with limited variability. This work presents the results of pooled targeted resequencing of more than 1500 cassava accessions from the CIAT germplasm collection to obtain a dataset of more than 2000 variants within genes related to starch functional properties and herbicide tolerance. Results of twelve bioinformatic pipelines for variant detection in pooled samples were compared to ensure the quality of the variant calling process. Predictions of functional impact were performed using two separate methods to prioritize interesting variation for genotyping and cultivar selection. Targeted resequencing, either by pooled samples or by similar approaches such as Ecotilling or capture, emerges as a cost effective alternative to whole genome sequencing to identify interesting alleles of genes related to relevant traits within large germplasm collections.

Production and Catalytic Properties of Amylases from Lichtheimia ramosa and Thermoascus aurantiacus by Solid-State Fermentation

The present study compared the production and the catalytic properties of amylolytic enzymes obtained from the fungi Lichtheimia ramosa (mesophilic) and Thermoascus aurantiacus (thermophilic). The highest amylase production in both fungi was observed in wheat bran supplemented with nutrient solution (pH 4.0) after 96 hours of cultivation, reaching 417.2 U/g of dry substrate (or 41.72 U/mL) and 144.5 U/g of dry substrate (or 14.45 U/mL) for L. ramosa and T. aurantiacus, respectively. The enzymes showed higher catalytic activity at pH 6.0 at 60°C. The amylases produced by L. ramosa and T. aurantiacus were stable between pH 3.5-10.5 and pH 4.5-9.5, respectively. The amylase of L. ramosa was stable at 55°C after 1 hour of incubation, whereas that of T. aurantiacus maintained 60% of its original activity under the same conditions. Both enzymes were active in the presence of ethanol. The enzymes hydrolyzed starch from different sources, with the best results obtained with corn starch. The enzymatic complex produced by L. ramosa showed dextrinizing and saccharifying potential. The enzymatic extract produced by the fungus T. aurantiacus presented only saccharifying potential, releasing glucose monomers as the main hydrolysis product.

Characterization of ubiquitin ligase SlATL31 and proteomic analysis of 14-3-3 targets in tomato fruit tissue (Solanum lycopersicum L.)

The 14-3-3 proteins participate in many aspects of plant physiology by interacting with phosphorylated proteins and thereby regulating target protein functions. In Arabidopsis plant, the ubiquitin ligase ATL31 controls 14-3-3 stability via both direct interaction and ubiquitination, and this consequently regulates post-germinative growth in response to carbon and nitrogen nutrient availability. Since 14-3-3 proteins regulate the activities of many key enzymes related to nutrient metabolism, one would anticipate that they should play an essential role not only in vegetative but also in reproductive tissue. Because fruit yield largely depends on carbon and nitrogen availability and their utilization, the function of 14-3-3 proteins was analyzed in tomato fruit tissue. Here, we isolated and characterized an ubiquitin ligase SlATL31 (Solyc03g112340) from tomato and demonstrated that SlATL31 has ubiquitin ligase activity as well as interaction with tomato 14-3-3 proteins, suggesting the possibility that the SlATL31 functions as an ubiquitin ligase for 14-3-3 similarly to its Arabidopsis ortholog. Furthermore, we performed proteomic analysis of 14-3-3 interacting proteins and identified 106 proteins as putative 14-3-3 targets including key enzymes for carbon metabolism and photosynthesis. This 14-3-3 interactome result and available transcriptome profile suggest a considerable yet complex role of 14-3-3 proteins in tomato fruit tissue.

BIOLOGICAL SIGNIFICANCE

Considerable cumulative evidence exists which implies that 14-3-3 proteins are involved in the regulation of plant primary metabolism. Here we provide the first report of 14-3-3 interactome analysis and identify putative 14-3-3 targets in tomato fruit tissue, which may be highly important given the documented metabolic shifts, which occur during fruit development and ripening. These data open future research avenues by which to understand the regulation of the role of post-translational regulation in tomato fruit development.

Diurnal changes in Sorghum leaf starch molecular structure

Control of the fine structure of transitory starch synthesized during the day in leaves is required for its normal degradation during the subsequent night. In this study, the molecular structure of transitory starch from Sorghum leaves over the diurnal cycle was characterized using size-exclusion chromatography. This is the first study of diurnal changes in the chain-length distribution (CLD) of amylopectin and amylose over the entire range of chain lengths, and in the size distribution of whole starch molecules. It was found that the outer layers of leaf starch granules, which were synthesized during the daytime and degraded during the night, contained more large molecules, including amylopectin with more short chains and more branching, than those in the inner layers. The outer layers also had lower amylose content. Starch molecular sizes in leaves are much smaller than in grain starch. The starch structures observed are likely to give optimal energy control during plant growth. Lack of this control may contribute to poor plant growth.

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.

Transcriptional response to petiole heat girdling in cassava

To examine the interactions of starch and sugar metabolism on photosynthesis in cassava, a heat-girdling treatment was applied to petioles of cassava leaves at the end of the light cycle to inhibit starch remobilization during the night. The inhibition of starch remobilization caused significant starch accumulation at the beginning of the light cycle, inhibited photosynthesis, and affected intracellular sugar levels. RNA-seq analysis of heat-treated and control plants revealed significantly decreased expression of genes related to photosynthesis, as well as N-metabolism and chlorophyll biosynthesis. However, expression of genes encoding TCA cycle enzymes and mitochondria electron transport components, and flavonoid biosynthetic pathway enzymes were induced. These studies reveal a dynamic transcriptional response to perturbation of sink demand in a single leaf, and provide useful information for understanding the regulations of cassava under sink or source limitation.

Growing duckweed for biofuel production: a review

Duckweed can be utilised to produce ethanol, butanol and biogas, which are promising alternative energy sources to minimise dependence on limited crude oil and natural gas. The advantages of this aquatic plant include high rate of nutrient (nitrogen and phosphorus) uptake, high biomass yield and great potential as an alternative feedstock for the production of fuel ethanol, butanol and biogas. The objective of this article is to review the published research on growing duckweed for the production of the biofuels, especially starch enrichment in duckweed plants. There are mainly two processes affecting the accumulation of starch in duckweed biomass: photosynthesis for starch generation and metabolism-related starch consumption. The cost of stimulating photosynthesis is relatively high based on current technologies. Considerable research efforts have been made to inhibit starch degradation. Future research need in this area includes duckweed selection, optimisation of duckweed biomass production, enhancement of starch accumulation in duckweeds and use of duckweeds for production of various biofuels.

Photoperiodic variations induce shifts in the leaf metabolic profile of Chrysanthemum morifolium

Plants have a high ability to adjust their metabolism, growth and development to changes in the light environment and to photoperiodic variation, but the current knowledge on how changes in metabolite contents are associated with growth and development is limited. We investigated the effect of three different photoperiodic treatments with similar daily light integral (DLI) on the growth responses and diurnal patterns in detected leaf metabolites in the short day plant Chrysanthemum×morifolium Ramat. Treatments were long day (LD, 18h light/6h dark), short day (SD, 12h light/12h dark) and short day with irregular night interruptions (NI-SD,12h light/12h dark, applied in a weekly pattern, shifting from day-to-day). Photoperiodic variation resulted in changes in the phenotypic development of the plants. The plants grown in the SD treatment started to initiate reproductive development of the meristems and a decrease in leaf expansion resulted in lower leaf area of expanding leaves. In contrast, plants in the NI-SD and LD treatments did not show reproductive development at any stage and final leaf area of the expanding leaves was intermediate for the NI-SD plants and largest for the LD plants. Photoperiodic variation also resulted in changes in the leaf metabolic profile for most of the analysed metabolites, but only carbohydrates, citrate and some amino acids displayed a shift in their diurnal pattern. Further, our results illustrated that short days (SD) increased the diurnal turnover of 1-kestose after 2 weeks, and decreased the overall contents of leaf hexoses after 3 weeks. In the two other treatments a diurnal turnover of 1-kestose was not stimulated before after 3 weeks, and hexoses together with the hexose:sucrose ratio steadily increased during the experiment. Our results enlighten the plasticity of leaf growth and metabolism to environmental changes, and demonstrate that diurnally regulated metabolites not always respond to photoperiodic variation.

SNPs in genes functional in starch-sugar interconversion associate with natural variation of tuber starch and sugar content of potato (Solanum tuberosum L.)

Starch accumulation and breakdown are vital processes in plant storage organs such as seeds, roots, and tubers. In tubers of potato (Solanum tuberosum L.) a small fraction of starch is converted into the reducing sugars glucose and fructose. Reducing sugars accumulate in response to cold temperatures. Even small quantities of reducing sugars affect negatively the quality of processed products such as chips and French fries. Tuber starch and sugar content are inversely correlated complex traits that are controlled by multiple genetic and environmental factors. Based on in silico annotation of the potato genome sequence, 123 loci are involved in starch-sugar interconversion, approximately half of which have been previously cloned and characterized. By means of candidate gene association mapping, we identified single-nucleotide polymorphisms (SNPs) in eight genes known to have key functions in starch-sugar interconversion, which were diagnostic for increased tuber starch and/or decreased sugar content and vice versa. Most positive or negative effects of SNPs on tuber-reducing sugar content were reproducible in two different collections of potato cultivars. The diagnostic SNP markers are useful for breeding applications. An allele of the plastidic starch phosphorylase PHO1a associated with increased tuber starch content was cloned as full-length cDNA and characterized. The PHO1a-HA allele has several amino acid changes, one of which is unique among all known starch/glycogen phosphorylases. This mutation might cause reduced enzyme activity due to impaired formation of the active dimers, thereby limiting starch breakdown.

A Diel Flux Balance Model Captures Interactions between Light and Dark Metabolism during Day-Night Cycles in C3 and Crassulacean Acid Metabolism Leaves

Although leaves have to accommodate markedly different metabolic flux patterns in the light and the dark, models of leaf metabolism based on flux-balance analysis (FBA) have so far been confined to consideration of the network under continuous light. An FBA framework is presented that solves the two phases of the diel cycle as a single optimization problem and, thus, provides a more representative model of leaf metabolism. The requirement to support continued export of sugar and amino acids from the leaf during the night and to meet overnight cellular maintenance costs forces the model to set aside stores of both carbon and nitrogen during the day. With only minimal constraints, the model successfully captures many of the known features of C3 leaf metabolism, including the recently discovered role of citrate synthesis and accumulation in the night as a precursor for the provision of carbon skeletons for amino acid synthesis during the day. The diel FBA model can be applied to other temporal separations, such as that which occurs in Crassulacean acid metabolism (CAM) photosynthesis, allowing a system-level analysis of the energetics of CAM. The diel model predicts that there is no overall energetic advantage to CAM, despite the potential for suppression of photorespiration through CO2 concentration. Moreover, any savings in enzyme machinery costs through suppression of photorespiration are likely to be offset by the higher flux demand of the CAM cycle. It is concluded that energetic or nitrogen use considerations are unlikely to be evolutionary drivers for CAM photosynthesis.

The plastid-localized NAD-dependent malate dehydrogenase is crucial for energy homeostasis in developing Arabidopsis thaliana seeds

In the absence of photosynthesis, ATP is imported into chloroplasts and non-green plastids by ATP/ADP transporters or formed during glycolysis, the latter requiring continuous regeneration of NAD(+), supplied by the plastidial isoform of NAD-MDH. During screening for T-DNA insertion mutants in the plNAD-MDH gene of Arabidopsis, only heterozygous plants could be isolated and homozygous knockout mutants grew only after complementation. These heterozygous plants show higher transcript levels of an alternative NAD(+)-regenerating enzyme, NADH-GOGAT, and, remarkably, improved growth when ammonium is the sole N-source. In situ hybridization and GUS-histochemical staining revealed that plNAD-MDH was particularly abundant in male and female gametophytes. Knockout plNAD-MDH pollen exhibit impaired tube growth in vitro, which can be overcome by adding the substrates of NADH-GOGAT. In vivo, knockout pollen is able to fertilize the egg cell. Young siliques of selfed heterozygous plants contain both green and white seeds corresponding to wild-type/heterozygous (green) and homozygous knockout mutants (white) in a (1:2):1 ratio. Embryos of the homozygous knockout seeds only reached the globular stage, did not green, and developed to tiny wrinkled seeds. Complementation with the gene under the native promoter rescued this defect, and all seeds developed as wild-type. This suggests that a blocked major physiological process in plNAD-MDH mutants stops both embryo and endosperm development, thus avoiding assimilate investment in compromised offspring.

The glucan phosphorylation mediated by α-glucan, water dikinase (GWD) is also essential in the light phase for a functional transitory starch turn-over

Starch phosphorylation mediated by the α-glucan, water dikinase (GWD) is crucial for transitory starch metabolism. The impact of the GWD action on transitory starch metabolism was analyzed in Arabidopsis mutants either lacking or revealing different reduced levels of GWD activity. In these mutants, glucose 6-phosphate (G6P) levels of the transitory leaf starch, the average leaf starch content, as well as alterations in the growth phenotype were determined under different light length conditions, including continuous light. Based on biochemical and growth phenotypical data, we found that the length of the light phase affects the phosphorylation state of the transitory starch and, by this, the average leaf starch content and the resulting growth of the plants. Additionally, we discuss data referring to an involvement of the GWD mediated glucan phosphorylation in starch synthesis, as, e.g., starch phosphorylation occurred even when a dark phase was omitted.

Translucent tissue defect in potato (Solanum tuberosum L.) tubers is associated with oxidative stress accompanying an accelerated aging phenotype

Translucent tissue defect (TTD) is an undesirable postharvest disorder of potato tubers characterized by the development of random pockets of semi-transparent tissue containing high concentrations of reducing sugars. Translucent areas turn dark during frying due to the Maillard reaction. The newly released cultivar, Premier Russet, is highly resistant to low temperature sweetening, but susceptible to TTD. Symptoms appeared as early as 170 days after harvest and worsened with time in storage (4-9 °C, 95 % RH). In addition to higher concentrations of glucose, fructose and sucrose, TTD resulted in lower dry matter, higher specific activities of starch phosphorylase and glc-6-phosphate dehydrogenase, higher protease activity, loss of protein, and increased concentrations of free amino acids (esp. asparagine and glutamine). The mechanism of TTD is unknown; however, the disorder has similarities with the irreversible senescent sweetening that occurs in tubers during long-term storage, where much of the decline in quality is a consequence of progressive increases in oxidative stress with advancing age. The respiration rate of non-TTD 'Premier Russet' tubers was inherently higher (ca. 40 %) than that of 'Russet Burbank' tubers (a non-TTD cultivar). Moreover, translucent tissue from 'Premier Russet' tubers had a 1.9-fold higher respiration rate than the average of non-translucent tissue and tissue from non-TTD tubers. Peroxidation of membrane lipids during TTD development resulted in increased levels of malondialdehyde and likely contributed to a measurable increase in membrane permeability. Superoxide dismutase and catalase activities and the ratio of oxidized to total glutathione were substantially higher in translucent tissue. TTD tubers also contained twofold less ascorbate than non-TTD tubers. TTD appears to be a consequence of oxidative stress associated with accelerated aging of 'Premier Russet' tubers.

Excess nickel modulates activities of carbohydrate metabolizing enzymes and induces accumulation of sugars by upregulating acid invertase and sucrose synthase in rice seedlings

The effects of increasing concentrations of nickel sulfate, NiSO(4) (200 and 400 μM) in the growth medium on the content of starch and sugars and activity levels of enzymes involved in starch and sugar metabolism were examined in seedlings of the two Indica rice cvs. Malviya-36 and Pant-12. During a 5-20 day growth period of seedlings in sand cultures, with Ni treatment, no definite pattern of alteration in starch level could be observed in the seedlings. In both roots and shoots of the seedlings Ni treatment led to a significant decrease in activities of starch degrading enzymes α-amylase, β-amylase, whereas starch phosphorylase activity increased. The contents of reducing, non-reducing, and total sugars increased in Ni-treated rice seedlings with a concomitant increase in the activities of sucrose degrading enzymes acid invertase and sucrose synthase. However, the activity of sucrose synthesizing enzyme sucrose phosphate synthase declined. These results suggest that Ni toxicity in rice seedlings causes marked perturbation in metabolism of carbohydrates leading to increased accumulation of soluble sugars. Such perturbation could serve as a limiting factor for growth of rice seedlings in Ni polluted environments and accumulating soluble sugars could serve as compatible solutes in the cells under Ni toxicity conditions.

High levels of sugars and fructan in mature seed of sweet wheat lacking GBSSI and SSIIa enzymes

Sweet wheat (SW), which lacks functional granule-bound starch synthase I (GBSSI) and starch synthase IIa (SSIIa), accumulates high levels of free sugars in immature seeds. Here, we examined the effects of the lack of these two enzymes on mature kernel composition. Whole grain flour of SW had higher levels of sugars, particularly maltose, slightly higher ash and protein content, approximately two to three times higher lipid levels, and about twice as much total dietary fiber as parental or wild-type lines. Considerably higher levels of low-molecular-weight soluble dietary fiber (LMW-SDF), largely consisting of fructan, were also detected in SW. Although there were no differences in total amino acid levels, the free amino acid content of SW was approximately 4-fold higher than that of wild type, and the levels of certain free amino acids such as proline were particularly high. Thus, we were able to clearly demonstrate that the lack of GBSSI and SSIIa caused dramatic changes in mature seed composition in SW. These compositional changes suggest that SW flour may provide health benefits when used as a food ingredient.

Thioredoxin-regulated beta-amylase (BAM1) triggers diurnal starch degradation in guard cells, and in mesophyll cells under osmotic stress

BAM1 is a plastid-targeted β-amylase of Arabidopsis thaliana specifically activated by reducing conditions. Among eight different chloroplast thioredoxin isoforms, thioredoxin f1 was the most efficient redox mediator, followed by thioredoxins m1, m2, y1, y2, and m4. Plastid-localized NADPH-thioredoxin reductase (NTRC) was also able partially to restore the activity of oxidized BAM1. Promoter activity of BAM1 was studied by reporter gene expression (GUS and YFP) in Arabidopsis transgenic plants. In young (non-flowering) plants, BAM1 was expressed both in leaves and roots, but expression in leaves was mainly restricted to guard cells. Compared with wild-type plants, bam1 knockout mutants were characterized by having more starch in illuminated guard cells and reduced stomata opening, suggesting that thioredoxin-regulated BAM1 plays a role in diurnal starch degradation which sustains stomata opening. Besides guard cells, BAM1 appears in mesophyll cells of young plants as a result of a strongly induced gene expression under osmotic stress, which is paralleled by an increase in total β-amylase activity together with its redox-sensitive fraction. Osmotic stress impairs the rate of diurnal starch accumulation in leaves of wild-type plants, but has no effect on starch accumulation in bam1 mutants. It is proposed that thioredoxin-regulated BAM1 activates a starch degradation pathway in illuminated mesophyll cells upon osmotic stress, similar to the diurnal pathway of starch degradation in guard cells that is also dependent on thioredoxin-regulated BAM1.

Chemical genetics and cereal starch metabolism: structural basis of the non-covalent and covalent inhibition of barley β-amylase

There are major issues regarding the proposed pathway for starch degradation in germinating cereal grain. Given the commercial importance but genetic intractability of the problem, we have embarked on a program of chemical genetics studies to identify and dissect the pathway and regulation of starch degradation in germinating barley grains. As a precursor to in vivo studies, here we report systematic analysis of the reversible and irreversible inhibition of the major β-amylase of the grain endosperm (BMY1). The molecular basis of inhibitor action was defined through high resolution X-ray crystallography studies of unliganded barley β-amylase, as well as its complexes with glycone site binder disaccharide iminosugar G1M, irreversible inhibitors α-epoxypropyl and α-epoxybutyl glucosides, which target the enzyme's catalytic residues, and the aglycone site binders acarbose and α-cyclodextrin.

Internal management of non-structural carbohydrate resources in apple leaves and branch wood under a broad range of sink and source manipulations

Apple (Malus x domestica Borkh.) grown in a Mediterranean climate depends on regular irrigation throughout the growing season. The objective of the current study was to elucidate the changes in carbohydrate storage and utilization by mature, field-grown apple trees in response to water availability to the trees and to the level of cropping. Fourteen-year-old apple trees cv. 'Golden Delicious' were grown under various combinations of irrigation rate (11, 33 or 77 l day(-)(1) per tree) and crop level ( approximately 100, approximately 300 or >1000 fruits per tree) beginning 47 days after full bloom (DAFB). Non-structural carbohydrate concentrations were measured at 78 (leaves and branch wood), 102 (leaves), 183 (branch wood) and 214 (branch wood) DAFB. Midday stem water potential (SWP) was measured at 2-week intervals between June and October. Trunk cross-sectional area was measured 47 and 265 DAFB. At harvest, 139 DAFB, the fruits of each tree were counted and weighed. SWP at 102 DAFB ranged between -0.6 and -2.7 MPa. Fruit fresh weight at harvest was positively related to SWP measured 37 days before harvest with distinct slopes for light/intermediate and heavy crop levels. Leaf and branch wood starch concentrations 78 and 102 DAFB were positively related to irrigation rate and negatively related to crop level. Mean fruit weight at harvest was positively related to branch wood starch concentration and neared maximum at a concentration of 40 mg g(-)(1) dry weight. Branch wood starch concentration recovered after harvest, especially in water-stressed trees. Sorbitol concentration was negatively related to irrigation rate. The sorbitol-to-starch concentration ratio in leaves at 102 DAFB was closely proportional to SWP. It is suggested that branch wood starch concentration represents the overall balance between carbon sources and sinks and may therefore serve as a reliable indicator of photo-assimilate availability. In water-stressed trees, sorbitol is prioritized over starch, probably to support osmotic adjustment, thereby suppressing fruit growth even further.

Starch phosphorylase: role in starch metabolism and biotechnological applications

The alpha-glucan phosphorylases of the glycosyltransferase family are important enzymes of carbohydrate metabolism in prokaryotes and eukaryotes. The plant alpha-glucan phosphorylase, commonly called starch phosphorylase (EC 2.4.1.1), is largely known for the phosphorolytic degradation of starch. Starch phosphorylase catalyzes the reversible transfer of glucosyl units from glucose-1-phosphate to the nonreducing end of alpha-1,4-D-glucan chains with the release of phosphate. Two distinct forms of starch phosphorylase, plastidic phosphorylase and cytosolic phosphorylase, have been consistently observed in higher plants. Starch phosphorylase is industrially useful and a preferred enzyme among all glucan phosphorylases for phosphorolytic reactions for the production of glucose-1-phosphate and for the development of engineered varieties of glucans and starch. Despite several investigations, the precise functional mechanisms of its characteristic multiple forms and the structural details are still eluding us. Recent discoveries have shed some light on their physiological substrates, precise biological functions, and regulatory aspects. In this review, we have highlighted important developments in understanding the role of starch phosphorylases and their emerging applications in industry.

Starch: its metabolism, evolution, and biotechnological modification in plants

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. Here, we review recent advances in research in three key areas. First, we assess progress in identifying the enzymatic machinery required for the synthesis of amylopectin, the glucose polymer responsible for the insoluble nature of starch. Second, we discuss the pathways of starch degradation, focusing on the emerging role of transient glucan phosphorylation in plastids as a mechanism for solubilizing the surface of the starch granule. We contrast this pathway in leaves with the degradation of starch in the endosperm of germinated cereal seeds. Third, we consider the evolution of starch biosynthesis in plants from the ancestral ability to make glycogen. Finally, we discuss how this basic knowledge has been utilized to improve and diversify starch crops.

Exploiting the potential of plants with crassulacean acid metabolism for bioenergy production on marginal lands

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation that facilitates the uptake of CO(2) at night and thereby optimizes the water-use efficiency of carbon assimilation in plants growing in arid habitats. A number of CAM species have been exploited agronomically in marginal habitats, displaying annual above-ground productivities comparable with those of the most water-use efficient C(3) or C(4) crops but with only 20% of the water required for cultivation. Such attributes highlight the potential of CAM plants for carbon sequestration and as feed stocks for bioenergy production on marginal and degraded lands. This review highlights the metabolic and morphological features of CAM that contribute towards high biomass production in water-limited environments. The temporal separation of carboxylation processes that underpins CAM provides flexibility for modulating carbon gain over the day and night, and poses fundamental questions in terms of circadian control of metabolism, growth, and productivity. The advantages conferred by a high water-storage capacitance, which translate into an ability to buffer fluctuations in environmental water availability, must be traded against diffusive (stomatal plus internal) constraints imposed by succulent CAM tissues on CO(2) supply to the cellular sites of carbon assimilation. The practicalities for maximizing CAM biomass and carbon sequestration need to be informed by underlying molecular, physiological, and ecological processes. Recent progress in developing genetic models for CAM are outlined and discussed in light of the need to achieve a systems-level understanding that spans the molecular controls over the pathway through to the agronomic performance of CAM and provision of ecosystem services on marginal lands.

STARCH-EXCESS4 is a laforin-like Phosphoglucan phosphatase required for starch degradation in Arabidopsis thaliana

Starch is the major storage carbohydrate in plants. It is comprised of glucans that form semicrystalline granules. Glucan phosphorylation is a prerequisite for normal starch breakdown, but phosphoglucan metabolism is not understood. A putative protein phosphatase encoded at the Starch Excess 4 (SEX4) locus of Arabidopsis thaliana was recently shown to be required for normal starch breakdown. Here, we show that SEX4 is a phosphoglucan phosphatase in vivo and define its role within the starch degradation pathway. SEX4 dephosphorylates both the starch granule surface and soluble phosphoglucans in vitro, and sex4 null mutants accumulate phosphorylated intermediates of starch breakdown. These compounds are linear alpha-1,4-glucans esterified with one or two phosphate groups. They are released from starch granules by the glucan hydrolases alpha-amylase and isoamylase. In vitro experiments show that the rate of starch granule degradation is increased upon simultaneous phosphorylation and dephosphorylation of starch. We propose that glucan phosphorylating enzymes and phosphoglucan phosphatases work in synergy with glucan hydrolases to mediate efficient starch catabolism.