Overexpression of the vacuolar sugar carrier AtSWEET16 modifies germination, growth, and stress tolerance in Arabidopsis.

Here, we report that SUGARS WILL EVENTUALLY BE EXPORTED TRANSPORTER (SWEET16) from Arabidopsis (Arabidopsis thaliana) is a vacuole-located carrier, transporting glucose (Glc), fructose (Fru), and sucrose (Suc) after heterologous expression in Xenopus laevis oocytes. The SWEET16 gene, similar to the homologs gene SWEET17, is mainly expressed in vascular parenchyma cells. Application of Glc, Fru, or Suc, as well as cold, osmotic stress, or low nitrogen, provoke the down-regulation of SWEET16 messenger RNA accumulation. SWEET16 overexpressors (35SPro:SWEET16) showed a number of peculiarities related to differences in sugar accumulation, such as less Glc, Fru, and Suc at the end of the night. Under cold stress, 35SPro:SWEET16 plants are unable to accumulate Fru, while under nitrogen starvation, both Glc and Fru, but not Suc, were less abundant. These changes of individual sugars indicate that the consequences of an increased SWEET16 activity are dependent upon the type of external stimulus. Remarkably, 35SPro:SWEET16 lines showed improved germination and increased freezing tolerance. The latter observation, in combination with the modified sugar levels, points to a superior function of Glc and Suc for frost tolerance. 35SPro:SWEET16 plants exhibited increased growth efficiency when cultivated on soil and showed improved nitrogen use efficiency when nitrate was sufficiently available, while under conditions of limiting nitrogen, wild-type biomasses were higher than those of 35SPro:SWEET16 plants. Our results identify SWEET16 as a vacuolar sugar facilitator, demonstrate the substantial impact of SWEET16 overexpression on various critical plant traits, and imply that SWEET16 activity must be tightly regulated to allow optimal Arabidopsis development under nonfavorable conditions.

QTL meta-analysis in Arabidopsis reveals an interaction between leaf senescence and resource allocation to seeds.

Sequential and monocarpic senescence are observed at vegetative and reproductive stages, respectively. Both facilitate nitrogen (N) remobilization and control the duration of carbon (C) fixation. Genetic and environmental factors control N and C resource allocation to seeds. Studies of natural variation in Arabidopsis thaliana revealed differences between accessions for leaf senescence phenotypes, seed N and C contents, and N remobilization efficiency-related traits. Here, a quantitative genetics approach was used to gain a better understanding of seed filling regulation in relation to leaf senescence and resource allocation. For that purpose, three Arabidopsis recombinant inbred line populations (Ct-1×Col-0, Cvi-0×Col-0, Bur-0×Col-0) were used to map QTL (quantitative trait loci) for ten traits related to senescence, resource allocation, and seed filling. The use of common markers across the three different maps allowed direct comparisons of the positions of the detected QTL in a single consensus map. QTL meta-analysis was then used to identify interesting regions (metaQTL) where QTL for several traits co-localized. MetaQTL were compared with positions of candidate genes known to be involved in senescence processes and flowering time. Finally, investigation of the correlation between yield and seed N concentration in the three populations suggests that leaf senescence disrupts the negative correlation generally observed between these two traits.

Resolving the role of plant glutamate dehydrogenase: II. Physiological characterization of plants overexpressing the two enzyme subunits individually or simultaneously.

Glutamate dehydrogenase (GDH; EC 1.4.1.2) is able to carry out the deamination of glutamate in higher plants. In order to obtain a better understanding of the physiological function of GDH in leaves, transgenic tobacco (Nicotiana tabacum L.) plants were constructed that overexpress two genes from Nicotiana plumbaginifolia (GDHA and GDHB under the control of the Cauliflower mosiac virus 35S promoter), which encode the α- and ß-subunits of GDH individually or simultaneously. In the transgenic plants, the GDH protein accumulated in the mitochondria of mesophyll cells and in the mitochondria of the phloem companion cells (CCs), where the native enzyme is normally expressed. Such a shift in the cellular location of the GDH enzyme induced major changes in carbon and nitrogen metabolite accumulation and a reduction in growth. These changes were mainly characterized by a decrease in the amount of sucrose, starch and glutamine in the leaves, which was accompanied by an increase in the amount of nitrate and Chl. In addition, there was an increase in the content of asparagine and a decrease in proline. Such changes may explain the lower plant biomass determined in the GDH-overexpressing lines. Overexpressing the two genes GDHA and GDHB individually or simultaneously induced a differential accumulation of glutamate and glutamine and a modification of the glutamate to glutamine ratio. The impact of the metabolic changes occurring in the different types of GDH-overexpressing plants is discussed in relation to the possible physiological function of each subunit when present in the form of homohexamers or heterohexamers.

Natural variation of Arabidopsis response to nitrogen availability.

Our understanding of plant growth in response to nitrogen (N) supply is mainly based on studies of mutants and transformants. This study explored the natural variability of Arabidopsis thaliana first to find out its global response to N availability and secondly to characterize the plasticity for growth and N metabolism among 23 genetically distant accessions under normal (N+), limited (N-), and starved (N0) N supplies. Plant growth was estimated by eight morphological traits characterizing shoot and root growth and 10 metabolic parameters that represented N and carbon metabolism. Most of the studied traits showed a large variation linked to genotype and nutrition. Furthermore, Arabidopsis growth was coordinated by master traits such as the shoot to root ratio of nitrate content in N+, root fresh matter and root amino acids in N-, and shoot fresh matter together with root thickness in N0. The 23 accessions could be gathered into four different groups, according to their growth in N+, N-, and N0. Phenotypic profiling characterized four different adaptative responses to N- and N0. Class 1 tolerated N limitation with the smallest decrease in shoot and root biomass compared with N+, while class 2 presented the highest resistance to N starvation by preferential increased root growth, huge starch accumulation, and high shoot nitrate content. In contrast, class 3 plants could tolerate neither N limitation nor N starvation. Small plants of class 4 were different, with shoot biomass barely affected in N- and root biomass unaffected in N0.

An SI-traceable reference material for virus-like particles.

A reference material for virus-like particles traceable to the International System of Units (Système International d'Unités - the SI) is reported. The material addresses the need for developing reference standards to benchmark virus-like gene delivery systems and help harmonize measurement approaches for characterization and testing. The material is a major component of synthetic polypeptide virus-like particles produced by the state-of-the-art synthetic and analytical chemistry methods used to generate gene delivery systems. The purity profile of the material is evaluated to the highest metrological order demonstrating traceability to the SI. The material adds to the emerging toolkit of reference standards for quantitative biology.

Transcriptional Plasticity of Autophagy-Related Genes Correlates with the Genetic Response to Nitrate Starvation in Arabidopsis Thaliana.

In eukaryotes, autophagy, a catabolic mechanism for macromolecule and protein recycling, allows the maintenance of amino acid pools and nutrient remobilization. For a better understanding of the relationship between autophagy and nitrogen metabolism, we studied the transcriptional plasticity of autophagy genes (ATG) in nine Arabidopsis accessions grown under normal and nitrate starvation conditions. The status of the N metabolism in accessions was monitored by measuring the relative expression of 11 genes related to N metabolism in rosette leaves. The transcriptional variation of the genes coding for enzymes involved in ammonium assimilation characterize the genetic diversity of the response to nitrate starvation. Starvation enhanced the expression of most of the autophagy genes tested, suggesting a control of autophagy at transcriptomic level by nitrogen. The diversity of the gene responses among natural accessions revealed the genetic variation existing for autophagy independently of the nutritive condition, and the degree of response to nitrate starvation. We showed here that the genetic diversity of the expression of N metabolism genes correlates with that of the ATG genes in the two nutritive conditions, suggesting that the basal autophagy activity is part of the integral response of the N metabolism to nitrate availability.

The Consequences of a Disruption in Cyto-Nuclear Coadaptation on the Molecular Response to a Nitrate Starvation in Arabidopsis.

Mitochondria and chloroplasts are important actors in the plant nutritional efficiency. So, it could be expected that a disruption of the coadaptation between nuclear and organellar genomes impact plant response to nutrient stresses. We addressed this issue using two Arabidopsis accessions, namely Ct1 and Jea, and their reciprocal cytolines possessing the nuclear genome from one parent and the organellar genomes of the other one. We measured gene expression, and quantified proteins and metabolites under N starvation and non-limiting conditions. We observed a typical response to N starvation at the phenotype and molecular levels. The phenotypical response to N starvation was similar in the cytolines compared to the parents. However, we observed an effect of the disruption of genomic coadaptation at the molecular levels, distinct from the previously described responses to organellar stresses. Strikingly, genes differentially expressed in cytolines compared to parents were mainly repressed in the cytolines. These genes encoded more mitochondrial and nuclear proteins than randomly expected, while N starvation responsive ones were enriched in genes for chloroplast and nuclear proteins. In cytolines, the non-coadapted cytonuclear genomic combination tends to modulate the response to N starvation observed in the parental lines on various biological processes.

Resolving the role of plant glutamate dehydrogenase. I. In vivo real time nuclear magnetic resonance spectroscopy experiments.

In higher plants the glutamate dehydrogenase (GDH) enzyme catalyzes the reversible amination of 2-oxoglutarate to form glutamate, using ammonium as a substrate. For a better understanding of the physiological function of GDH either in ammonium assimilation or in the supply of 2-oxoglutarate, we used transgenic tobacco (Nicotiana tabacum L.) plants overexpressing the two genes encoding the enzyme. An in vivo real time (15)N-nuclear magnetic resonance (NMR) spectroscopy approach allowed the demonstration that, when the two GDH genes were overexpressed individually or simultaneously, the transgenic plant leaves did not synthesize glutamate in the presence of ammonium when glutamine synthetase (GS) was inhibited. In contrast we confirmed that the primary function of GDH is to deaminate Glu. When the two GDH unlabeled substrates ammonium and Glu were provided simultaneously with either [(15)N]Glu or (15)NH(4)(+) respectively, we found that the ammonium released from the deamination of Glu was reassimilated by the enzyme GS, suggesting the occurrence of a futile cycle recycling both ammonium and Glu. Taken together, these results strongly suggest that the GDH enzyme, in conjunction with NADH-GOGAT, contributes to the control of leaf Glu homeostasis, an amino acid that plays a central signaling and metabolic role at the interface of the carbon and nitrogen assimilatory pathways. Thus, in vivo NMR spectroscopy appears to be an attractive technique to follow the flux of metabolites in both normal and genetically modified plants.

Leaf fructose content is controlled by the vacuolar transporter SWEET17 in Arabidopsis.

In higher plants, soluble sugars are mainly present as sucrose, glucose, and fructose. Sugar allocation is based on both source-to-sink transport and intracellular transport between the different organelles and depends on actual plant requirements. Under abiotic stress conditions, such as nitrogen limitation, carbohydrates accumulate in plant cells. Despite an increasing number of genetic studies, the genetic architecture determining carbohydrate composition is poorly known. Using a quantitative genetics approach, we determined that the carrier protein SWEET17 is a major factor controlling fructose content in Arabidopsis leaves. We observed that when SWEET17 expression is reduced, either by induced or natural variation, fructose accumulates in leaves, suggesting an enhanced storage capacity. Subcellular localization of SWEET17-GFP to the tonoplast and functional expression in Xenopus oocytes showed that SWEET17 is the first vacuolar fructose transporter to be characterized in plants. Physiological studies in planta provide evidence that SWEET17 acts to export fructose out of the vacuole. Overall, our results suggest that natural variation in leaf fructose levels is controlled by the vacuolar fructose transporter SWEET17. SWEET17 is highly conserved across the plant kingdom; thus, these findings offer future possibilities to modify carbohydrate partitioning in crops.

The Arabidopsis TOR kinase links plant growth, yield, stress resistance and mRNA translation.

Plants, unlike animals, have plastic organ growth that is largely dependent on environmental information. However, so far, little is known about how this information is perceived and transduced into coherent growth and developmental decisions. Here, we report that the growth of Arabidopsis is positively correlated with the level of expression of the TARGET OF RAPAMYCIN (TOR) kinase. Diminished or augmented expression of the AtTOR gene results in a dose-dependent decrease or increase, respectively, in organ and cell size, seed production and resistance to osmotic stress. Strong downregulation of AtTOR expression by inducible RNA interference also leads to a post-germinative halt in growth and development, which phenocopies the action of the plant hormone abscisic acid, to an early senescence and to a reduction in the amount of translated messenger RNA. Thus, we propose that the AtTOR kinase is one of the contributors to the link between environmental cues and growth processes in plants.

Control of the synthesis and subcellular targeting of the two GDH genes products in leaves and stems of Nicotiana plumbaginifolia and Arabidopsis thaliana.

Although the physiological role of the enzyme glutamate dehydrogenase which catalyses in vitro the reversible amination of 2-oxoglutarate to glutamate remains to be elucidated, it is now well established that in higher plants the enzyme preferentially occurs in the mitochondria of phloem companion cells. The Nicotiana plumbaginifolia and Arabidopis thaliana enzyme is encoded by two distinct genes encoding either an alpha- or a beta-subunit. Using antisense plants and mutants impaired in the expression of either of the two genes, we showed that in leaves and stems both the alpha- and beta-subunits are targeted to the mitochondria of the companion cells. In addition, we found in both species that there is a compensatory mechanism up-regulating the expression of the alpha-subunit in the stems when the expression of the beta-subunit is impaired in the leaves, and of the beta-subunit in the leaves when the expression of the alpha-subunit is impaired in the stems. When one of the two genes encoding glutamate dehydrogenase is ectopically expressed, the corresponding protein is targeted to the mitochondria of both leaf and stem parenchyma cells and its production is increased in the companion cells. These results are discussed in relation to the possible signalling and/or physiological function of the enzyme which appears to be coordinated in leaves and stems.

Systematic silencing of a tobacco nitrate reductase transgene in lettuce (Lactuca sativa L.).

A population of 50 independent transgenic lettuces transformed with a nitrate reductase coding sequence under the control of the 35S promoter was studied. None of them showed significantly lower nitrate levels when compared with the untransformed plants, despite the presence of nitrate reductase (NR) activity that derives from the transgene in at least four of the transformants. No repercussion on total NR activity (endogenous+transgenic) was detected in these plants. Nevertheless, 28% of the transformants showed phenotypes characteristic of a general silencing of the NR genes as already described in tobacco and potato, i.e. bleaching of the leaves leading to the death of the plant. By northern blots, it was shown that the transgene was silenced in these chlorotic plants and also in the plants that did not show symptoms of chlorosis. Thus a silencing process specifically directed against the NR mRNA derived from the transgene occurred very early in the development of all the plants studied, whatever homologous endogenous NR mRNA is present in the plant. In some cases this transgene-specific silencing was shown subsequently to extend to the homologous endogenous NR mRNA. These results suggest that, in lettuce, the level of nitrate reductase mRNA is under tight expression control and this is able specifically to target transgenic transcripts by a post-transcriptional gene silencing (PTGS) mechanism during the first stage of development of the plantlet.