2S Arginine-rich proteins from Pinus pinaster seeds

Identification of Metabolic Pathways Differentially Regulated in Somatic and Zygotic Embryos of Maritime Pine

Embryogenesis is a complex phase of conifer development involving hundreds of genes, and a proper understanding of this process is critical not only to produce embryos with different applied purposes but also for comparative studies with angiosperms. A global view of transcriptome dynamics during pine somatic and zygotic embryogenesis is currently missing. Here, we present a genome-wide transcriptome analysis of somatic and zygotic embryos at three developmental stages to identify conserved biological processes and gene functions during late embryogenesis. Most of the differences became more significant as the developmental process progressed from early to cotyledonary stages, and a higher number of genes were differentially expressed in somatic than in zygotic embryos. Metabolic pathways substantially affected included those involved in amino acid biosynthesis and utilization, and this difference was already observable at early developmental stages. Overall, this effect was found to be independent of the line (genotype) used to produce the somatic embryos. Additionally, transcription factors differentially expressed in somatic versus zygotic embryos were analyzed. Some potential hub regulatory genes were identified that can provide clues as to what transcription factors are controlling the process and to how the observed differences between somatic and zygotic embryogenesis in conifers could be regulated.

What we know about plant arginases?

Nitrogen is one of the essential element required for plant growth and development. In plants, most of the nitrogen is stored in arginine. Hence, metabolism of arginine to urea by arginase and its further hydrolysis to ammonia by urease is involved in nitrogen recycling to meet the metabolic demands of growing plants. In this respect, plant arginases differ from that of animals. Animals excrete urea while plants recycle the urea. However, the studies on the biochemical and biophysical characteristics of plant arginase are limited when compared to animal arginase(s). In this review, the structural and biochemical characteristics of various plant arginases are discussed. Moreover, the significance of arginase in nitrogen recycling is explained and recent literature on function and activation of plant arginases in response to various environmental (biotic and abiotic) insults is also presented.

Enzymes Involved in the Biosynthesis of Arginine from Ornithine in Maritime Pine (Pinus pinaster Ait.)

The amino acids arginine and ornithine are the precursors of a wide range of nitrogenous compounds in all living organisms. The metabolic conversion of ornithine into arginine is catalyzed by the sequential activities of the enzymes ornithine transcarbamylase (OTC), argininosuccinate synthetase (ASSY) and argininosuccinate lyase (ASL). Because of their roles in the urea cycle, these enzymes have been purified and extensively studied in a variety of animal models. However, the available information about their molecular characteristics, kinetic and regulatory properties is relatively limited in plants. In conifers, arginine plays a crucial role as a main constituent of N-rich storage proteins in seeds and serves as the main source of nitrogen for the germinating embryo. In this work, recombinant PpOTC, PpASSY and PpASL enzymes from maritime pine (Pinus pinaster Ait.) were produced in Escherichia coli to enable study of their molecular and kinetics properties. The results reported here provide a molecular basis for the regulation of arginine and ornithine metabolism at the enzymatic level, suggesting that the reaction catalyzed by OTC is a regulatory target in the homeostasis of ornithine pools that can be either used for the biosynthesis of arginine in plastids or other nitrogenous compounds in the cytosol.

The role of arginine metabolic pathway during embryogenesis and germination in maritime pine (Pinus pinaster Ait.)

Vegetative propagation through somatic embryogenesis is critical in conifer biotechnology towards multivarietal forestry that uses elite varieties to cope with environmental and socio-economic issues. An important and still sub-optimal process during in vitro maturation of somatic embryos (SE) is the biosynthesis and deposition of storage proteins, which are rich in amino acids with high nitrogen (N) content, such as arginine. Mobilization of these N-rich proteins is essential for the germination and production of vigorous somatic seedlings. Somatic embryos accumulate lower levels of N reserves than zygotic embryos (ZE) at a similar stage of development. To understand the molecular basis for this difference, the arginine metabolic pathway has been characterized in maritime pine (Pinus pinaster Ait.). The genes involved in arginine metabolism have been identified and GFP-fusion constructs were used to locate the enzymes in different cellular compartments and clarify their metabolic roles during embryogenesis and germination. Analysis of gene expression during somatic embryo maturation revealed high levels of transcripts for genes involved in the biosynthesis and metabolic utilization of arginine. By contrast, enhanced expression levels were only observed during the last stages of maturation and germination of ZE, consistent with the adequate accumulation and mobilization of protein reserves. These results suggest that arginine metabolism is unbalanced in SE (simultaneous biosynthesis and degradation of arginine) and could explain the lower accumulation of storage proteins observed during the late stages of somatic embryogenesis.

Pine as fast food: foraging ecology of an endangered cockatoo in a forestry landscape

Pine plantations near Perth, Western Australia have provided an important food source for endangered Carnaby’s Cockatoos (Calyptorhynchus latirostris) since the 1940s. Plans to harvest these plantations without re-planting will remove this food source by 2031 or earlier. To assess the impact of pine removal, we studied the ecological association between Carnaby’s Cockatoos and pine using behavioural, nutritional, and phenological data. Pine plantations provided high densities of seed (158 025 seeds ha⁻¹) over a large area (c. 15 000 ha). Carnaby’s Cockatoos fed throughout these plantations and removed almost the entire annual crop of pine cones. Peak cockatoo abundance coincided with pine seed maturation. Pine seed had energy and protein contents equivalent to native food sources and, critically, is available in summer when breeding pairs have young offspring to feed. This strong and enduring ecological association clearly suggests that removing pine will have a significant impact on this endangered species unless restoration strategies, to establish alternative food sources, are implemented.

Evidence for an operative glutamine translocator in chloroplasts from maritime pine (Pinus pinaster Ait.) cotyledons

In higher plants, ammonium is assimilated into amino acids through the glutamine synthetase (GS)/glutamate synthase (GOGAT) cycle. This metabolic cycle is distributed in different cellular compartments in conifer seedlings: glutamine synthesis occurs in the cytosol and glutamate synthesis within the chloroplast. A method for preparing intact chloroplasts of pine cotyledons is presented with the aim of identifying a glutamine-glutamate translocator. Glutamine-glutamate exchange has been studied using the double silicone layer system, suggesting the existence of a translocator that imports glutamine into the chloroplast and exports glutamate to the cytoplasm. The translocator identified is specific for glutamine and glutamate, and the kinetic constants for both substrates indicate that it is unsaturated at intracellular concentrations. Thus, the experimental evidence obtained supports the model of the GS/GOGAT cycle in developing pine seedlings that accounts for the stoichiometric balance of metabolites. As a result, the efficient assimilation of free ammonia produced by photorespiration, nitrate reduction, storage protein mobilisation, phenylpropanoid pathway or S-adenosylmethionine synthesis is guaranteed.

Molecular and functional analyses support a role of Ornithine-{delta}-aminotransferase in the provision of glutamate for glutamine biosynthesis during pine germination

We report the molecular characterization and functional analysis of a gene (PsdeltaOAT) from Scots pine (Pinus sylvestris) encoding Orn-delta-aminotransferase (delta-OAT; EC 2.6.1.13), an enzyme of arginine metabolism. The deduced amino acid sequence contains a putative N-terminal signal peptide for mitochondrial targeting. The polypeptide is similar to other delta-OATs from plants, yeast, and mammals and encoded by a single-copy gene in pine. PsdeltaOAT encodes a functional delta-OAT as determined by expression of the recombinant protein in Escherichia coli and analysis of the active enzyme. The expression of PsdeltaOAT was undetectable in the embryo, but highly induced at early stages of germination and seedling development in all different organs. Transcript levels decreased in later developmental stages, although an increase was observed in lignified stems of 90-d-old plants. An increase of delta-OAT activity was observed in germinating embryos and seedlings and appears to mirror the observed alterations in PsdeltaOAT transcript levels. Similar expression patterns were also observed for genes encoding arginase and isocitrate dehydrogenase. Transcripts of PsdeltaOAT and the arginase gene were found widely distributed in different cell types of pine organs. Consistent with these results a metabolic pathway is proposed for the nitrogen flow from the megagametophyte to the developing seedling, which is also supported by the relative abundance of free amino acids in embryos and seedlings. Taken together, our data support that delta-OAT plays an important role in this process providing glutamate for glutamine biosynthesis during early pine growth.

Antisense repression of the Medicago truncatula nodule-enhanced sucrose synthase leads to a handicapped nitrogen fixation mirrored by specific alterations in the symbiotic transcriptome and metabolome

We analyzed the role of the sucrose (Suc) synthase MtSucS1 during nodulation of the model legume Medicago truncatula, integrating data for the developmental, transcriptional, and metabolic processes affected downstream of an impaired Suc cleavage in root nodules. To reduce carbohydrate supply to nodule tissues, transgenic plants expressing a p35S-driven MtSucS1-antisense fusion were constructed. These plants displayed an up to 90% reduction of MtSucS1 proteins in roots and nodules. Phenotypic studies of two independent MtSucS1-reduced lines demonstrated that only under conditions depending on nodulation, these plants appeared to be impaired in above-ground growth. Specifically plant height, shoot weight, leaf development, flowering, as well as seed maturation were reduced, and the efficiency of photosynthesis was affected. Concomitantly, a significantly enhanced root to shoot ratio with a marked increase in root tip numbers was observed. Root nodule formation was found retarded and the impaired nodulation was accompanied by a less efficient nitrogen (N) acquisition. The decreased total N content of MtSucS1-antisense lines and an enhanced carbon to N ratio in roots, nodules, and shoots correlated with the extent of MtSucS1 knockdown. On the level of transcription, effects of an MtSucS1 reduction were evident for genes representing important nodes of the nodule carbon and N metabolism, while metabolite profiling revealed significantly lower levels of amino acids and their derivatives particularly in strongly MtSucS1-reduced nodules. Our results support the model that nodule-enhanced Suc synthase 1 of the model legume M. truncatula is required for the establishment and maintenance of an efficient N-fixing symbiosis.

Les protéines de réserve du pin pignon (Pinus pinea L.)

The Mediterranean stone pine Pinus pinea L. (gymnosperm, Pinaceae) is much appreciated for its seed production, widely used in food preparation in the Mediterranean Basin. Seeds contain 25% proteins on a dry-weight basis. Pinus pinea accumulate globulins as major storage proteins in seeds (75% of total storage proteins), composed of several subunits of 10 to 150 kDa, revealed by SDS-PAGE. The albumin fraction (15%) represents three subunits of 14, 24 and 46 kDa. Glutelins, the least soluble fraction, represents a small proportion (10%). Their constitutive units have frequent PM of 43 kDa. Prolamins also represent a very small percentage (1 to 2%).

Expression patterns of two glutamine synthetase genes in zygotic and somatic pine embryos support specific roles in nitrogen metabolism during embryogenesis

Here, embryo-specific patterns of glutamine synthetase (GS) genes were studied for the first time using pine somatic and zygotic embryogenesis as model systems. GS1a expression was absent in zygotic embryos whereas it was detected in the cotyledons of somatic embryos at late developmental stages along with transcripts for photosynthesis genes and arginase. These findings suggest that germination was initiated in maturing somatic embryos. GS1b transcripts were found mainly in procambial cells in both zygotic and somatic embryos. Expression of the GS1b in procambial cells before the differentiation of mature vascular elements indicated that this gene could be useful as a molecular marker for early stages of vascular differentiation in pine. Accordingly, a correlation was found between the quality of somatic embryos generated from three different cell lines and the pattern and level of GS1b expression. Our data suggest that GS1a and GS1b genes play distinct functional roles in the biosynthesis and mobilization of seed nitrogen reserves. Furthermore, the results presented may have potential application for improving conifer somatic embryogenesis.

Molecular aspects of nitrogen mobilization and recycling in trees

Plants have developed a variety of molecular strategies to use limiting nutrients with a maximum efficiency. N assimilated into biomolecules can be released in the form of ammonium by plant metabolic activities in various physiological processes such as photorespiration, the biosynthesis of phenylpropanoids or the mobilization of stored reserves. Thus, efficient reassimilation mechanisms are required to reincorporate liberated ammonium into metabolism and maintain N plant economy. Although the biochemistry and molecular biology of ammonium recycling in annual herbaceous plants has been previously reported, the recent advances in woody plants need to be reviewed. Moreover, it is important to point out that N recycling is quantitatively massive during some of these metabolic processes in trees, including seed germination, the onset of dormancy and resumption of active growth or the biosynthesis of lignin that takes place during wood formation. Therefore, woody plants constitute an excellent system as a model to study N mobilization and recycling. The aim of this paper is to provide an overview of different physiological processes in woody perennials that challenge the overall plant N economy by releasing important amounts of inorganic N in the form of ammonium.

Electrophoretic analysis of seed storage proteins from gymnosperms

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), pore gradient gel electrophoresis (PGGE) followed by SDS-PAGE and Western blot analysis were used to characterize the seed storage proteins from seven gymnosperm species from the families Pinaceae (Abies alba, Cedrus atlantica and Picea abies), Cupressaceae (Biota orientalis, Chamaecyparis lawsoniana and Cupressus arizonica), and Taxaceae (Taxus baccata). SDS-PAGE and PGGE X SDS-PAGE indicate the presence of proteins with characteristics similar to the 7S globulins in all the species studied. Antibodies to a 7S globulin subunit from Pinus pinaster cross-reacted with homologous polypeptides from Pinaceae species, but not with corresponding subunits from species belonging to other families. Also detected in each of the studied species, with the exception of A. alba and T. baccata, were those of C. atlantica may be extracted by saline buffer, while the remainder require a dissociating agent. Antibodies raised against the small subunit from P. pinaster 11S protein recognized only the corresponding polypeptides from Pinaceae species. Overall, these results help clarify our knowledge of gymnosperm seed storage proteins.