Impact of concurrent overexpression of cytosolic glutamine synthetase (GS1) and sucrose phosphate synthase (SPS) on growth and development in transgenic tobacco

MdbZIP44-MdCPRF2-like-Mdα-GP2 regulate starch and sugar metabolism in apple under nitrogen supply

Nitrogen (N) is regarded as an essential macronutrient and is tightly associated with carbon (C) metabolism in plants. The transcriptome data obtained from this study showed that the expression level of the apple basic leucine zipper (bZIP) transcription factor (TF) MdbZIP44 was up-regulated in 'Oregon Spur Delicious' (Malus domestica Borkh.) apple fruits under nitrogen supply. MdbZIP44 bound to the promoter of Mdα-GP2 gene and inhibited its expression, thereby promoting starch accumulation and decreasing glucose content in apple and tomato fruits. Besides, overexpression of MdbZIP44 promoted sucrose accumulation by regulating the activities of sucrose metabolism-related enzymes and the expression of sugar metabolism-related genes in apple callus and tomato fruits. Furthermore, biochemical assays indicated that MdbZIP44 directly interacted with MdCPRF2-like, another bZIP gene in apple. Meanwhile, this study found that MdCPRF2-like, along with the MdbZIP44 and MdCPRF2-like complex, could activate the expression of Mdα-GP2, respectively. In conclusion, this study provides a new reference for potential mechanisms underlying that MdbZIP44-MdCPRF2-like-Mdα-GP2 regulates starch and sugar metabolism under nitrogen supply.

New Insights into MdSPS4-Mediated Sucrose Accumulation under Different Nitrogen Levels Revealed by Physiological and Transcriptomic Analysis

Nitrogen nutrition participates in many physiological processes and understanding the physiological and molecular mechanisms of apple responses to nitrogen is very significant for improving apple quality. This study excavated crucial genes that regulates sugar metabolism in response to nitrogen in apples through physiology and transcriptome analysis, so as to lay a theoretical foundation for improving fruit quality. In this paper, the content of sugar and organic acid in apple fruit at different developmental periods under different nitrogen levels (0, 150, 300, and 600 kg·hm^-2) were determined. Then, the transcriptomic analysis was performed in 120 days after bloom (DAB) and 150 DAB. The results showed that the fructose and glucose content were the highest at 120 DAB under 600 kg·hm^-2 nitrogen level. Meanwhile, different nitrogen treatments decreased malate content in 30 and 60 DAB. RNA-seq analysis revealed a total of 4537 UniGenes were identified as differentially expressed genes (DEGs) under nitrogen treatments. Among these DEGs, 2362 (52.06%) were up-regulated and 2175 (47.94%) were down-regulated. The gene co-expression clusters revealed that most DEGs were significantly annotated in the photosynthesis, glycolysis/gluconeogenesis, pyruvate metabolism, carbon metabolism, carbon fixation in photosynthetic organisms and plant hormone signal transduction pathways. The key transcription factor genes (ERF, NAC, WRKY, and C2H2 genes) were differentially expressed in apple fruit. Sugar and acid metabolism-related genes (e.g., HXK1, SPS4, SS2, PPC16-2, and MDH2 genes) exhibited significantly up-regulated expression at 120 DAB, whereas they were down-regulated at 150 DAB. Furthermore, the MdSPS4 gene overexpression positively promoted sucrose accumulation in apple callus and fruit. In conclusion, the combinational analysis of transcriptome and the functional validation of the MdSPS4 gene provides new insights into apple responses to different nitrogen levels.

Co-overexpression of AtSAT1 and EcPAPR improves seed nutritional value in maize

Maize seeds synthesize insufficient levels of the essential amino acid methionine (Met) to support animal and livestock growth. Serine acetyltransferase1 (SAT1) and 3'-phosphoadenosine-5'-phosphosulfate reductase (PAPR) are key control points for sulfur assimilation into Cys and Met biosynthesis. Two high-MET maize lines pRbcS:AtSAT1 and pRbcS:EcPAPR were obtained through metabolic engineering recently, and their total Met was increased by 1.4- and 1.57-fold, respectively, compared to the wild type. The highest Met maize line, pRbcS:AtSAT1-pRbcS:EcPAPR, was created by stacking the two transgenes, causing total Met to increase 2.24-fold. However, the pRbcS:AtSAT1-pRbcS:EcPAPR plants displayed progressively severe defects in plant growth, including early senescence, stunting, and dwarfing, indicating that excessive sulfur assimilation has an adverse effect on plant development. To explore the mechanism of correlation between Met biosynthesis in maize leaves and storage proteins in developing endosperm, the transcriptomes of the sixth leaf at stage V9 and 18 DAP endosperm of pRbcS:AtSAT1, pRbcS:AtSAT1-pRbcS:EcPAPR, and the null segregants were quantified and analyzed. In pRbcS:AtSAT1-pRbcS:EcPAPR, 3274 genes in leaves (1505 up- and 1769 downregulated) and 679 genes in the endosperm (327 up- and 352 downregulated) were differentially expressed. Gene ontology (GO) and KEGG (Kyoto encyclopedia of genes and genomes) analyses revealed that many genes were associated with Met homeostasis, including transcription factors and genes involved in cysteine and Met metabolism, glutathione metabolism, plant hormone signal transduction, and oxidation-reduction. The data from gene network analysis demonstrated that two genes, serine/threonine-protein kinase (CCR3) and heat shock 70 kDa protein (HSP), were localized in the core of the leaves and endosperm regulation networks, respectively. The results of this study provide insights into the diverse mechanisms that underlie the ideal establishment of enhanced Met levels in maize seeds.

Genetic Engineering and Genome Editing for Improving Nitrogen Use Efficiency in Plants

Low nitrogen availability is one of the main limiting factors for plant growth and development, and high doses of N fertilizers are necessary to achieve high yields in agriculture. However, most N is not used by plants and pollutes the environment. This situation can be improved by enhancing the nitrogen use efficiency (NUE) in plants. NUE is a complex trait driven by multiple interactions between genetic and environmental factors, and its improvement requires a fundamental understanding of the key steps in plant N metabolism—uptake, assimilation, and remobilization. This review summarizes two decades of research into bioengineering modification of N metabolism to increase the biomass accumulation and yield in crops. The expression of structural and regulatory genes was most often altered using overexpression strategies, although RNAi and genome editing techniques were also used. Particular attention was paid to woody plants, which have great economic importance, play a crucial role in the ecosystems and have fundamental differences from herbaceous species. The review also considers the issue of unintended effects of transgenic plants with modified N metabolism, e.g., early flowering—a research topic which is currently receiving little attention. The future prospects of improving NUE in crops, essential for the development of sustainable agriculture, using various approaches and in the context of global climate change, are discussed.

Differential proteomics of tobacco seedling roots at high and low potassium concentrations

The effects of high potassium and normal potassium treatments on protein expression in roots of flue-cured tobacco plant HKDN-5 at the seedling stage were analyzed by an unlabeled protein quantification technique. The results showed that 555 proteins were differentially expressed (245 proteins were down-regulated and 310 proteins were up-regulated) in high potassium treatment compared with normal potassium treatment. Differentially expressed proteins were involved in 96 metabolic pathways (42 metabolic pathways, 21 synthetic pathways as well as catabolic pathways, including fatty acid metabolism, phenylpropane biosynthesis, ketone body synthesis and degradation, and butyric acid metabolism. Root processing of high potassium concentrations leads to increases in the synthesis of peroxidase, superoxide dismutase and acyl-coenzyme-A synthetase. Additional proteomic differences observed in tobacco roots grown in high potassium include proteins involved with genetic information processing as well as environmental sensing. Examples include RNA helicase, ABC transporters and large subunit GTPases. These up-regulated differentially expressed proteins function mainly in protein translation, ribosome structure and protein synthesis. This indicates that under high potassium treatment, root protein synthetic processes are accelerated and substance metabolism pathways are enhanced; thus, providing the material and energetic basis for root growth.

Co-overexpression of AtSHMT1 and AtFDH induces sugar synthesis and enhances the role of original pathways during formaldehyde metabolism in tobacco

Serine hydroxymethyltransferase 1 (SHMT1) is a key enzyme in the photorespiration pathway in higher plants. Our previous study showed that AtSHMT1 controls the assimilation of HCHO to sugars in Arabidopsis. The expression of SHMT1 was induced in Arabidopsis but was inhibited in tobacco under HCHO stress. To investigate whether the function of AtSHMT1 in the HCHO assimilation could be exerted in tobacco, AtSHMT1 was overexpressed alone (S5) or co-overexpressed (SF6) with Arabidopsis formate dehydrogenase (AtFDH) in leaves using a light-inducible promoter in this study. 13C NMR analyses showed that the 13C-metabolic flux from H13CHO was introduced to sugar synthesis in SF6 leaves but not in S5 leaves. The increase in the production of metabolites via the original pathways was particularly greater in SF6 leaves than in S5 leaves, suggesting that co-overexpression of AtSHMT1 and AtFDH is more effective than overexpression of AtSHMT1 alone in the enhancement of HCHO metabolism in tobacco leaves. Consequently, the increase in HCHO uptake and resistance was greater in SF6 leaves than in S5 leaves. The mechanism underlying the role of overexpressed AtSHMT1 and AtFDH was discussed based on changes in photosynthetic parameters, chlorophyll content, antioxidant enzyme activity and the oxidative level in leaves.

Molecular cloning and expression analysis of sucrose phosphate synthase genes in cassava (Manihot esculenta Crantz)

Sucrose phosphate synthase (SPS), a key rate-limiting enzyme in the sucrose biosynthesis pathway in plants, is encoded by a multi-gene family. Until recently, the identification and characterization of the SPS gene family have been performed for dozens of plant species; however, few studies have involved a comprehensive analysis of the SPS family members in tropical crops, such as cassava (Manihot esculenta Crantz). In the current study, five SPS genes (MeSPS1, MeSPS2, MeSPS3, MeSPS4, and MeSPS5) were isolated from cassava, and their sequence characteristics were comprehensively characterized. These MeSPS genes were found distributed on five chromosomes (Chr2, Chr14, Chr15, Chr16, and Chr18). Phylogenetic analysis showed that the MeSPS protein sequences were clustered into three families, together with other SPS sequences from both dicot and monocot species (families A, B, and C). The spatio-temporal expression pattern analysis of MeSPS genes showed a tissue-specific and partially overlapping expression pattern, with the genes mainly expressed in source tissues during cassava growth and development. Correlation analysis revealed that the expression of MeSPS genes correlated positively with root starch content, indicating that the expression of MeSPS genes might accelerate the rate of starch accumulation in the roots of cassava plants.

Transcriptome Analysis of Low- and High-Sucrose Pear Cultivars Identifies Key Regulators of Sucrose Biosynthesis in Fruits

Sucrose accumulation is one of the important factors that determine fruit enlargement and quality. Evaluation of the sugar profile of 105 pear cultivars revealed low-sucrose and high-sucrose (HS) types of pear fruits. To better understand the molecular mechanisms governing the sucrose content of pear fruits, this study performed transcriptome analysis during fruit development using low-sucrose 'Korla' fragrant pear and HS 'Hosui' pear, and a coexpression module uniquely associated with the control of high-sucrose accumulation was identified by weighted gene coexpression network analysis. These results suggested that there are seven candidate genes encoding key enzymes (fructokinase, glucose-6-phosphate isomerase, sucrose phosphate synthase and sucrose synthase) involved in sucrose biosynthesis and several transcription factors (TFs) whose expression patterns correlate with those of genes associated with sucrose biosynthesis. This correlation was confirmed by linear regression analysis between predicted gene expression and sucrose content in different pear cultivars during fruit development. This study provides insight into the molecular mechanism underlying differences in sucrose content across pear cultivars and presents candidate structural genes and TFs that could play important roles in regulating carbohydrate partitioning and sucrose accumulation.

Co-crystal Structure of Thermosynechococcus elongatus Sucrose Phosphate Synthase With UDP and Sucrose-6-Phosphate Provides Insight Into Its Mechanism of Action Involving an Oxocarbenium Ion and the Glycosidic Bond

In green species, sucrose can help antagonize abiotic stress. Sucrose phosphate synthase (SPS) is a well-known rate-limiting enzyme in the synthesis of sucrose. To date, however, there is no known crystal structure of SPS from plant or cyanobacteria. In this study, we report the first co-crystal structure of SPS from Thermosynechococcus elongatus with UDP and sucrose-6-phosphate (S6P). Within the catalytic site, the side chains of His158 and Glu331, along with two phosphate groups from UDP, form hydrogen bonds with the four hydroxyl groups of the glucose moiety in S6P. This association causes these four hydroxyl groups to become partially negatively charged, thus promoting formation of the C1 oxocarbenium ion. Breakage of the hydrogen bond between His158 and one of the hydroxyl groups may trigger covalent bond formation between the C1 oxocarbenium ion and the C2 hydroxyl of fructose-6-phosphate. Consistent with our structural model, we observed that two SPS mutants, H158A and E331A, lost all catalytic activity. Moreover, electron density of residues from two loops (loop1 and loop2) in the SPS A-domain was not observed, suggest their dynamic nature. B-factor analysis and molecular dynamics stimulations of the full-length enzyme and A-domain indicate that both loops are crucial for binding and release of substrate and product. In addition, temperature gradient analysis shows that SPS exhibits its highest activity at 70°C, suggesting that this enzyme has the potential of being used in industrial production of S6P.

An Update on Nitric Oxide Production and Role Under Phosphorus Scarcity in Plants

Phosphate (P) is characterized by its low availability and restricted mobility in soils, and also by a high redistribution capacity inside plants. In order to maintain P homeostasis in nutrient restricted conditions, plants have developed mechanisms which enable P acquisition from the soil solution, and an efficient reutilization of P already present in plant cells. Nitric oxide (NO) is a bioactive molecule with a plethora of functions in plants. Its endogenous synthesis depends on internal and environmental factors, and is closely tied with nitrogen (N) metabolism. Furthermore, there is evidence demonstrating that N supply affects P homeostasis and that P deficiency impacts on N assimilation. This review will provide an overview on how NO levels in planta are affected by P deficiency, the interrelationship with N metabolism, and a summary of the current understanding about the influence of this reactive N species over the processes triggered by P starvation, which could modify P use efficiency.

A 100 bp GAGA motif-containing sequence in AGAMOUS second intron is able to suppress the activity of CaMV35S enhancer in vegetative tissues

Flower-specific promoters enable genetic manipulation of floral organs to improve crop yield and quality without affecting vegetative growth. However, the identification of strong tissue-specific promoters is a challenge. In addition, information on cis elements that is able to repress gene expression in vegetative tissues remains limited. Here, we report that fusing a 35S enhancer to the stamen- and carpel-specific NtAGIP1 promoter derived from the tobacco AGAMOUS second intron (AGI) can significantly increase the promoter activity. Interestingly, although the activity of the new promoter extends to sepals and pedicles, it does not cross the boundary of the reproductive organs. Serial deletion of the AGI and chromatin immunoprecipitation (ChIP) assay reveal a 100-bp fragment that contains a conserved GAGA factor binding motif contributes to the flower specificity by mediating histone H3 lysine 27 trimethylation (H3K27me3) modification of the promoter. Furthermore, this fragment shows significant suppressive effect on the activity of the 35S enhancer in vegetative tissues, consequently, resulting in a significant increase of the activity of 35S enhancer:AGI chimeric promoter without sacrifice of its specificity in inflorescence.

Overexpression of Sucrose Phosphate Synthase Enhanced Sucrose Content and Biomass Production in Transgenic Sugarcane

Sucrose phosphate synthase (SPS) is a key enzyme in sucrose synthesis, which controls sucrose content in plants. This study was designed to examine the efficacy of the overexpression of SoSPS1 gene on sucrose accumulation and carbon partitioning in transgenic sugarcane. The overexpression of SoSPS1 gene increased SPS activity and sucrose content in transgenic sugarcane leaves. More importantly, the overexpression enhanced soluble acid invertase (SAI) activity concomitant with the increase of glucose and fructose levels in the leaves, whereas sucrose synthase activity exhibited almost no change. In the stalk, a similar correlation was observed, but a higher correlation was noted between SPS activity and sugar content. These results suggest that SPS overexpression has both direct and indirect effects on sugar concentration and SAI activity in sugarcane. In addition, SPS overexpression resulted in a significant increase in plant height and stalk number in some transgenic lines compared to those in non-transgenic control. Taken together, these results strongly suggest that enhancing SPS activity is a useful strategy for improving sugarcane yield.

Manipulating Amino Acid Metabolism to Improve Crop Nitrogen Use Efficiency for a Sustainable Agriculture

In a context of a growing worldwide food demand coupled to the need to develop a sustainable agriculture, it is crucial to improve crop nitrogen use efficiency (NUE) while reducing field N inputs. Classical genetic approaches based on natural allelic variations existing within crops have led to the discovery of quantitative trait loci controlling NUE under low nitrogen conditions; however, the identification of candidate genes from mapping studies is still challenging. Amino acid metabolism is the cornerstone of plant N management, which involves N uptake, assimilation, and remobilization efficiencies, and it is finely regulated during acclimation to low N conditions and other abiotic stresses. Over the last two decades, biotechnological engineering of amino acid metabolism has led to promising results for the improvement of crop NUE, and more recently under low N conditions. This review summarizes current work carried out in crops and provides perspectives on the identification of new candidate genes and future strategies for crop improvement.

Biotechnological strategies for improved photosynthesis in a future of elevated atmospheric CO2

The improvement of photosynthesis using biotechnological approaches has been the focus of much research. It is now vital that these strategies be assessed under future atmospheric conditions. The demand for crop products is expanding at an alarming rate due to population growth, enhanced affluence, increased per capita calorie consumption, and an escalating need for plant-based bioproducts. While solving this issue will undoubtedly involve a multifaceted approach, improving crop productivity will almost certainly provide one piece of the puzzle. The improvement of photosynthetic efficiency has been a long-standing goal of plant biotechnologists as possibly one of the last remaining means of achieving higher yielding crops. However, the vast majority of these studies have not taken into consideration possible outcomes when these plants are grown long-term under the elevated CO₂ concentrations (e[CO₂]) that will be evident in the not too distant future. Due to the considerable effect that CO₂ levels have on the photosynthetic process, these assessments should become commonplace as a means of ensuring that research in this field focuses on the most effective approaches for our future climate scenarios. In this review, we discuss the main biotechnological research strategies that are currently underway with the aim of improving photosynthetic efficiency and biomass production/yields in the context of a future of e[CO₂], as well as alternative approaches that may provide further photosynthetic benefits under these conditions.

Distinct nodule and leaf functions of two different sucrose phosphate synthases in alfalfa

In alfalfa, the B form of Sucrose phosphate synthase synthesizes sucrose in the leaves while the A form participates in regulatory cycles of synthesis/breakdown of sucrose/starch in the root nodules. Sucrose (Suc) is the major stable product of photosynthesis that is transported to all heterotrophic organs as a source of energy and carbon. The enzyme sucrose phosphate synthase (SPS) catalyzes the synthesis of Suc. Besides the leaves, SPS is also found in heterotrophic organs. There are two isoforms of SPS in alfalfa (Medicago sativa): SPSA and SPSB. While SPSA is expressed in the vasculature of all the organs and in the N₂-fixing zone in the nodules, SPSB is exclusively expressed in the photosynthetic cells. Two classes of alfalfa transformants were produced, one with a gene construct consisting of the alfalfa SPSA promoter and the other with the SPSB promoter-both driving the maize SPS coding region-referred to as SPSA-ZmSPS and SPSB-ZmSPS, respectively. Both classes of transformants showed increased growth compared to control plants. The SPSB-ZmSPS transformants showed increased SPS protein levels and activity along with a significant increase in the Suc levels in the leaves. The SPSA-ZmSPS transformants showed an increase in the SPS protein level and enzyme activity both in the leaves and the nodules with no increase in Suc content in the leaves but a substantial increase in the nodules. Both SPSA and SPSB have unique roles in the nodules (sink) and leaves (source). SPSB is responsible for the synthesis of Suc in the photosynthetic cells and SPSA participates in a regulatory cycle in which Suc is simultaneously degraded and re-synthesized; both these functions contribute to plant growth in rhizobia nodulated alfalfa plants.

Chile (Capsicum annuum) plants transformed with the RB gene from Solanum bulbocastanum are resistant to Phytophthora capsici

Phytophthora capsici is a soil borne pathogen, and is among the most destructive pathogens for Capsicum annuum (chile). P. capsici is known to cause diseases on all parts of the chile plants. Therefore, it requires independent resistance genes to control disease symptoms that are induced by each of the P. capsici strains. This requirement of multiple resistance genes to confer resistance to P. capsici, in chile makes breeding for resistance a daunting pursuit. Against this backdrop, a genetic engineering approach would be to introduce a broad host resistance gene into chile in order to protect it from different races of P. capsici. Notably, a broad host resistance gene RB from Solanum bulbocastanum has been shown to confer resistance to P. infestans in both S. tuberosum and S. lycopersicum. We agroinfiltrated the RB gene into the leaves of susceptible chile plants, demonstrating that the gene is also capable of lending resistance to P. capsici in chile. We introduced the RB gene into chile by developing an Agrobacterium tumefaciens mediated transformation system. The integration of the RB gene into the genome of the primary transformants and its subsequent transfer to the F1 generation was confirmed by genomic PCR using primers specific for the RB gene. A 3:1 ratio for the presence and absence of the RB gene was observed in the F1 progeny. In addition to showing resistance to P. capsici in a leaf inoculation experiment, about 30% of the F1 progeny also exhibited resistance to root inoculation. Our data, when taken together, suggests that the RB gene from S. bulbocastanum confers resistance against P. capsici in C. annuum, thereby demonstrating that the RB gene has an even broader host range than reported in the literature–both in terms of the host and the pathogen.

Cisgenic overexpression of cytosolic glutamine synthetase improves nitrogen utilization efficiency in barley and prevents grain protein decline under elevated CO2

Cytosolic glutamine synthetase (GS1) plays a central role in nitrogen (N) metabolism. The importance of GS1 in N remobilization during reproductive growth has been reported in cereal species but attempts to improve N utilization efficiency (NUE) by overexpressing GS1 have yielded inconsistent results. Here, we demonstrate that transformation of barley (Hordeum vulgare L.) plants using a cisgenic strategy to express an extra copy of native HvGS1-1 lead to increased HvGS1.1 expression and GS1 enzyme activity. GS1 overexpressing lines exhibited higher grain yields and NUE than wild-type plants when grown under three different N supplies and two levels of atmospheric CO₂ . In contrast with the wild-type, the grain protein concentration in the GS1 overexpressing lines did not decline when plants were exposed to elevated (800-900 μL/L) atmospheric CO₂ . We conclude that an increase in GS1 activity obtained through cisgenic overexpression of HvGS1-1 can improve grain yield and NUE in barley. The extra capacity for N assimilation obtained by GS1 overexpression may also provide a means to prevent declining grain protein levels under elevated atmospheric CO₂ .

Comparison of alfalfa plants overexpressing glutamine synthetase with those overexpressing sucrose phosphate synthase demonstrates a signaling mechanism integrating carbon and nitrogen metabolism between the leaves and nodules

Alfalfa, like other legumes, establishes a symbiotic relationship with the soil bacteria, Sinorhizobium meliloti, which results in the formation of the root nodules. Nodules contain the bacteria enclosed in a membrane-bound vesicle, the symbiosome where it fixes atmospheric N2 and converts it into ammonia using the bacterial enzyme, nitrogenase. The ammonia released into the cytoplasm from the symbiosome is assimilated into glutamine (Gln) using carbon skeletons produced by the metabolism of sucrose (Suc), which is imported into the nodules from the leaves. The key enzyme involved in the synthesis of Suc in the leaves is sucrose phosphate synthase (SPS) and glutamine synthetase (GS) is the enzyme with a role in ammonia assimilation in the root nodules. Alfalfa plants, overexpressing SPS or GS, or both showed increased growth and an increase in nodule function. The endogenous genes for the key enzymes in C/N metabolism showed increased expression in the nodules of both sets of transformants. Furthermore, the endogenous SPS and GS genes were also induced in the leaves and nodules of the transformants, irrespective of the transgene, suggesting that the two classes of plants share a common signaling pathway regulating C/N metabolism in the nodules. This study reaffirms the utility of the nodulated legume plant to study C/N interaction and the cross talk between the source and sink for C and N.

A Larger Root System Is Coupled With Contrasting Expression Patterns of Phosphate and Nitrate Transporters in Foxtail Millet [Setaria italica (L.) Beauv.] Under Phosphate Limitation

Foxtail millet [Setaria italica (L.) Beauv.], a widely cultivated food and fodder crop, develops a smaller root system while enlarges the root diameter facilitating nutrient transport under nitrogen limitation. How foxtail millet responds to phosphate limitation (LP) remains unaddressed. LP seedlings of the sequenced variety Yugu1 had significantly lower P concentrations in both shoots and roots and displayed higher levels of anthocyanin accumulation in leaves, indicating that the seedlings suffered from P limitation under hydroponic culture. One obvious and adaptive phenotype of LP plants was the larger root system mostly as the result of stimulation of lateral root proliferation in terms of the number, density, and length. Preferential biomass accumulation in the root under LP ensured carbon provision for root expansion and resulted in significant increases in the total and specific root length, which substantially extended the absorptive surface of P in the growth medium. Elevation of auxin and gibberellin concentrations might serve as an internal boost underpinning root architectural re-patterning under LP. Not just morphological adaptation, up-regulation of expression of SiPHT1;1 and SiPHT1;4 in roots and that of SiPHT1;2 in roots and shoots preconditioned adaptive enhancement of P uptake and translocation under LP. Interestingly, internal nitrogen surpluses occurred as indicated by dramatic increases in free amino acids in LP shoots and roots and higher concentrations of nitrogen in roots. Such nitrogen surplus 'signals' tended to switch down expression of nitrate transporters SiNRT2.1 and SiNAR2.1 in the root and that of SiNRT1.11 and SiNRT1.12 in the shoot to reduce nitrate mobilization toward or within the shoot. Together, our work provided new insights into adaption of a critical cereal crop to LP and its innate connection with nitrogen nutrition.

The functions of cucumber sucrose phosphate synthases 4 (CsSPS4) in carbon metabolism and transport in sucrose- and stachyose-transporting plants

Sucrose phosphate synthases (SPSs) are rate-limiting sucrose synthesis enzymes present in photosynthetic and non-photosynthetic tissues. The cucumber genome contains three SPSs that can be grouped into families A, B, and C. CsSPS1 and CsSPS2 are highly expressed in flowers and mature leaves, while the expression level of CsSPS4 increased gradually after leaf unfolding in our study and reached its peak after 20 days. In CsSPS4-overexpression tobacco plants, sucrose content and sucrose/starch ratio were increased significantly and resulted in improved leaf yield. By contrast, in CsSPS4-overexpression (CsSPS4-OE) cucumber lines, contents of sucrose and starch were unchanged, and raffinose was increased in transgenic cucumber leaves. The expression of cucumber raffinose family oligosaccharide (RFO)-synthesis-related genes increased obviously in cucumber CsSPS4-OE plants, and the sucrose, raffinose, and stachyose contents increased significantly in the petioles of CsSPS4-OE lines. In CsSPS4-antisense (CsSPS4-A) cucumber lines, decreases occurred in mRNA expression, enzyme activity, sucrose content, sucrose/starch ratio, and stachyose transport, but the RFO-synthesis-related genes were nearly unchanged. Together, these results suggest that overexpression of CsSPS4 can lead to carbon metabolism prioritizing sugar transport in cucumber, and suppression of CsSPS4 likely promotes carbon metabolism to accumulate starch, showing a more complicated carbon distribution model than in transgenic tobacco plants.

Transcriptome analysis reveals the effects of grafting on sugar and α-linolenic acid metabolisms in fruits of cucumber with two different rootstocks

Flavor quality in cucumber is affected by different rootstocks, but the molecular mechanism is largely unclean. To clarify the differences of sugar and aromatic compounds, cucumber (cucumis sativus) fruits from plants of self-grafted (SG) or grafted onto figleaf gourd (Cucurbita ficifolia; G1) or 'Weisheng No.1' rootstock (Cucurbita moschata ⅹCucurbita moschata hybrids; G2) were performed the transcriptome analysis. We obtained 1013 and 920 differentially expressed genes (DEGs) from G1 and G2 compared to SG respectively, in which 453 genes were co-expressed. Functional annotations showed many DEGs were involved in glycolysis/gluconeogenesis metabolism, fructose metabolism and α-Linolenic acid metabolisms, 20 DEGs were selected from the 3 pathways to validate sequencing accuracy by quantitative real-time PCR. The gene relative expression levels were concurrent with RNA-seq results and sugar and aromatic compounds content phenotypes. Moreover, some vital transcript factors and transport proteins were analyzed. These findings indicate that different rootstocks could induce significantly changes in the physiological profiling and transcripts of sugar- and aromatic flavor-related genes. This study provides a novel insight into the molecular mechanisms of fruit quality regulated by candidate genes.

Comprehensive transcriptome profiling of soybean leaves in response to simulated acid rain

As a source of edible oil and protein, soybean is a major globally important economic crop; Improving its production has been an important objective of soybean breeding. Acid rain has been shown to influence soybean growth and productivity, with consequent adverse impacts on its production for use by human populations. In this study, RNA sequencing technology was utilized to examine changes in gene expression when soybean was exposed to simulated acid rain (SAR). We sampled soybean leaves at five time intervals (0, 6, 30, 54, 78, and 102 h), and built the cDNA library. In total, 54,175 expression genes were found, including 2016 genes with differential expression. A total of 416 genes were considered, as they were closely related to the response to SAR. Genes related to the regulation of sulfur and nitrogen metabolism, carbohydrate metabolism, photosynthesis, and reactive oxygen species were among those differentially expressed in response to SAR. In this study, we examined the response mechanisms of soybean under SAR exposure. Our findings will improve our understanding of the molecular mechanisms employed by soybean in responding to abiotic stress, and therefore provides important information in developing soybean breeding to improve tolerance to these stresses.

Nitrogen Supply and Leaf Age Affect the Expression of TaGS1 or TaGS2 Driven by a Constitutive Promoter in Transgenic Tobacco

Glutamine synthetase (GS) plays a key role in nitrogen metabolism. Here, two types of tobacco transformants, overexpressing Triticum aestivum GS1 (TaGS1) or GS2 (TaGS2), were analysed. Four independent transformed lines, GS1-TR1, GS1-TR2, GS2-TR1 and GS2-TR2, were used for the nitrogen treatment. Under nitrogen-sufficient conditions, the leaves of GS2-TR showed high accumulation of the TaGS2 transcript, while those of GS1-TR showed a low TaGS1 transcript levels. However, compared with nitrogen-sufficient conditions, the TaGS1 transcript level increased in the leaves under nitrogen starvation, but the TaGS2 transcript level decreased. In addition, the TaGS1 and TaGS2 transcript levels were highest in the middle leaves under nitrogen-sufficient and starvation conditions. These results show that nitrogen supply and leaf age regulate TaGS expression, even when they are driven by a super-promoter. Additionally, in regard to nitrogen metabolism level, the lower leaves of the GS1-TR exhibited lower NH₄⁺ and higher amino acid contents, while the upper leaves exhibited higher amino acid, soluble protein and chlorophyll contents. The leaves of the GS2-TR exhibited lower NH₄⁺ but higher amino acid, soluble protein and chlorophyll contents. Given the role that GS isoforms play in nitrogen metabolism, these data suggest that TaGS1 overexpression may improve nitrogen transport, and that TaGS2 overexpression may improve nitrogen assimilation under nitrogen stress.

The Effect of Poplar PsnGS1.2 Overexpression on Growth, Secondary Cell Wall, and Fiber Characteristics in Tobacco

The glutamine synthetase (GS1) is a key enzyme that catalyzes the reaction of glutamate and ammonia to produce glutamine in the nitrogen (N) metabolism. Previous studies on GS1s in several plant species suggest that overexpression of GS1s can enhance N utilization, accelerate plant vegetative growth, and change wood formation. In this study, we isolated a GS1 gene, termed PsnGS1.2, from Populus simonii × Populus nigra. This gene was expressed at a higher level in roots, and relatively lower but detectable levels in xylem, leaves and phloem of P. simonii × P. nigra. The protein encoded by PsnGS1.2 is primarily located in the cytoplasm. Overexpression of PsnGS1.2 in tobacco led to the increased GS1 activity and IAA content, the augmented N assimilation, and the enlarged leaves with altered anatomical structures. These changes presumably promoted photosynthetic, growth, and biomass productivity. It was noteworthy that the secondary cell walls and fiber characteristics changed remarkably in PsnGS1.2 transgenic tobacco. These changes aligned well with the altered expression levels of the genes involved in fiber development, secondary cell wall component biosynthesis, IAA biosynthesis, amino acid transport, and starch breakdown. Taken together, the results from our study suggest that catalytic functions of PsnGS1.2 on N assimilation and metabolism in transgenic tobacco had significant effects on vegetative growth, leaf development, and secondary cell wall formation and properties through acceleration of photosynthesis and IAA biosynthesis, and redirection of carbon flux to synthesis of more cellulose and hemicellulose.

Cloning and characterization of the Cerasus humilis sucrose phosphate synthase gene (ChSPS1)

Sucrose is crucial to the growth and development of plants, and sucrose phosphate synthase (SPS) plays a key role in sucrose synthesis. To understand the genetic and molecular mechanisms of sucrose synthesis in Cerasus humilis, ChSPS1, a homologue of SPS, was cloned using RT-PCR. Sequence analysis showed that the open reading frame (ORF) sequence of ChSPS1 is 3174 bp in length, encoding a predicted protein of 1057 amino acids. The predicted protein showed a high degree of sequence identity with SPS homologues from other species. Real-time RT-PCR analysis showed that ChSPS1 mRNA was detected in all tissues and the transcription level was the highest in mature fruit. There is a significant positive correlation between expression of ChSPS1 and sucrose content. Prokaryotic expression of ChSPS1 indicated that ChSPS1 protein was expressed in E. coli and it had the SPS activity. Overexpression of ChSPS1 in tobacco led to upregulation of enzyme activity and increased sucrose contents in transgenic plants. Real-time RT-PCR analysis showed that the expression of ChSPS1 in transgenic tobacco was significantly higher than in wild type plants. These results suggested that ChSPS1 plays an important role in sucrose synthesis in Cerasus humilis.

Expression of the Maize Dof1 Transcription Factor in Wheat and Sorghum

Nitrogen is essential for plant growth and development. Improving the ability of plants to acquire and assimilate nitrogen more efficiently is a key agronomic parameter that will augment sustainability in agriculture. A transcription factor approach was pursued to address improvement of nitrogen use efficiency in two major commodity crops. To this end, the Zea mays Dof1 (ZmDof1) transcription factor was expressed in both wheat (Triticum aestivum) and sorghum (Sorghum bicolor) either constitutively, UBI4 promoter from sugarcane, or in a tissue specific fashion via the maize rbcS1 promoter. The primary transcription activation target of ZmDof1, phosphoenolpyruvate carboxylase (PEPC), is observed in transgenic wheat events. Expression ZmDof1 under control of the rbcs1 promoter translates to increase in biomass and yield components in wheat. However, constitutive expression of ZmDof1 led to the down-regulation of genes involved in photosynthesis and the functional apparatus of chloroplasts, and an outcome that negatively impacts photosynthesis, height, and biomass in wheat. Similar patterns were also observed in sorghum transgenic events harboring the constitutive expression cassette of ZmDof1. These results indicate that transcription factor strategies to boost agronomic phenotypic outcomes in crops need to consider expression patterns of the genetic elements to be introduced.

Sucrose metabolism gene families and their biological functions

Sucrose, as the main product of photosynthesis, plays crucial roles in plant development. Although studies on general metabolism pathway were well documented, less information is available on the genome-wide identification of these genes, their expansion and evolutionary history as well as their biological functions. We focused on four sucrose metabolism related gene families including sucrose synthase, sucrose phosphate synthase, sucrose phosphate phosphatase and UDP-glucose pyrophosphorylase. These gene families exhibited different expansion and evolutionary history as their host genomes experienced differentiated rates of the whole genome duplication, tandem and segmental duplication, or mobile element mediated gene gain and loss. They were evolutionarily conserved under purifying selection among species and expression divergence played important roles for gene survival after expansion. However, we have detected recent positive selection during intra-species divergence. Overexpression of 15 sorghum genes in Arabidopsis revealed their roles in biomass accumulation, flowering time control, seed germination and response to high salinity and sugar stresses. Our studies uncovered the molecular mechanisms of gene expansion and evolution and also provided new insight into the role of positive selection in intra-species divergence. Overexpression data revealed novel biological functions of these genes in flowering time control and seed germination under normal and stress conditions.

Transgenic alfalfa (Medicago sativa) with increased sucrose phosphate synthase activity shows enhanced growth when grown under N2-fixing conditions

Overexpression of SPS in alfalfa is accompanied by early flowering, increased plant growth and an increase in elemental N and protein content when grown under N2-fixing conditions. Sucrose phosphate synthase (SPS; EC 2.3.1.14) is the key enzyme in the synthesis of sucrose in plants. The outcome of overexpression of SPS in different plants using transgenic approaches has been quite varied, but the general consensus is that increased SPS activity is associated with the production of new sinks and increased sink strength. In legumes, the root nodule is a strong C sink and in this study our objective was to see how increasing SPS activity in a legume would affect nodule number and function. Here we have transformed alfalfa (Medicago sativa, cv. Regen SY), with a maize SPS gene driven by the constitutive CaMV35S promoter. Our results showed that overexpression of SPS in alfalfa, is accompanied by an increase in nodule number and mass and an overall increase in nitrogenase activity at the whole plant level. The nodules exhibited an increase in the level of key enzymes contributing to N assimilation including glutamine synthetase and asparagine synthetase. Moreover, the stems of the transformants showed higher level of the transport amino acids, Asx, indicating increased export of N from the nodules. The transformants exhibited a dramatic increase in growth both of the shoots and roots, and earlier flowering time, leading to increased yields. Moreover, the transformants showed an increase in elemental N and protein content. The overall conclusion is that increased SPS activity improves the N status and plant performance, suggesting that the availability of more C in the form of sucrose enhances N acquisition and assimilation in the nodules.