Autoregulation of the gene for cystathionine gamma-synthase in Arabidopsis: post-transcriptional regulation induced by S-adenosylmethionine

CYSTATHIONINE GAMMA-SYNTHASE activity in rice is developmentally regulated and strongly correlated with sulfate

An important goal of rice cultivar development is improvement of protein quality, especially with respect to essential amino acids such as methionine. With the goal of increasing seed methionine content, we generated Oryza sativa ssp. japonica cv. Taipei 309 transgenic lines expressing a feedback-desensitized CYSTATHIONINE GAMMA-SYNTHASE from Arabidopsis thaliana (AtD-CGS) under the control of the maize ubiquitin promoter. Despite persistently elevated cystathionine gamma-synthase (CGS) activity in the AtD-CGS transgenic lines relative to untransformed Taipei, sulfate was the only sulfur-containing compound found to be elevated throughout vegetative development. Accumulation of methionine and other sulfur-containing metabolites was limited to the leaves of young plants. Sulfate concentration was found to strongly and positively correlate with CGS activity across vegetative development, irrespective of whether the activity was provided by the endogenous rice CGS or by a combination of endogenous and AtD-CGS. Conversely, the concentrations of glutathione, valine, and leucine were clearly negatively correlated with CGS activity in the same tissues. We also observed a strong decrease in CGS activity in both untransformed Taipei and the AtD-CGS transgenic lines as the plants approached heading stage. The mechanism for this downregulation is currently unknown and of potential importance for efforts to increase methionine content in rice.

Protein synthesis is the most sensitive process when potassium is substituted by sodium in the nutrition of sugar beet (Beta vulgaris)

Potassium ions (K(+)) and sodium ions (Na(+)) share many physical and chemical similarities. However, their interchangeability in plant nutrition is restricted. Substitution studies showed that K(+) can be replaced by Na(+) to a large extent in the nutrition of Beta vulgaris L. However, the extent of substitution without negative impacts is not unlimited. The aim of the present study was to identify the process which is most sensitive during the substitution of K(+) by Na(+) in nutrition of young sugar beet plants. We focused on transpiration, growth, and net protein synthesis. Plants were grown under controlled environmental conditions. With transfer of seedlings into nutrient solution, plants were cultivated in different substitution treatments. For all treatments the sum of K(+) and Na(+) (applied as chloride) was fixed to 4 mM. The extent of substitution of K(+) by Na(+) in the nutrient solution was varied from low (0.25% substitution: 3.99 mM K(+), 0.01 mM Na(+)) to almost complete substitution (99.75% substitution: 0.01 mM K(+), 3.99 mM Na(+)). The supply of 3.99 mM K(+) in 0.25% substitution treatment guaranteed the absence of K(+) deficiency. Transpiration was not affected by the substitution. Growth was inhibited at a substitution level of 99.75%. Net protein synthesis was already affected at a substitution level of 97.50% (0.10 mM K(+), 3.90 mM Na(+)). Hence, net protein synthesis was most sensitive to the substitution and limited the extent of substitution of K(+) by Na(+) in the nutrition of young sugar beet plants.

The Regulation of Essential Amino Acid Synthesis and Accumulation in Plants

Although amino acids are critical for all forms of life, only proteogenic amino acids that humans and animals cannot synthesize de novo and therefore must acquire in their diets are classified as essential. Nine amino acids-lysine, methionine, threonine, phenylalanine, tryptophan, valine, isoleucine, leucine, and histidine-fit this definition. Despite their nutritional importance, several of these amino acids are present in limiting quantities in many of the world's major crops. In recent years, a combination of reverse genetic and biochemical approaches has been used to define the genes encoding the enzymes responsible for synthesizing, degrading, and regulating these amino acids. In this review, we describe recent advances in our understanding of the metabolism of the essential amino acids, discuss approaches for enhancing their levels in plants, and appraise efforts toward their biofortification in crop plants.

Multi-scale modeling of Arabidopsis thaliana response to different CO2 conditions: From gene expression to metabolic flux

Multi-scale investigation from gene transcript level to metabolic activity is important to uncover plant response to environment perturbation. Here we integrated a genome-scale constraint-based metabolic model with transcriptome data to explore Arabidopsis thaliana response to both elevated and low CO2 conditions. The four condition-specific models from low to high CO2 concentrations show differences in active reaction sets, enriched pathways for increased/decreased fluxes, and putative post-transcriptional regulation, which indicates that condition-specific models are necessary to reflect physiological metabolic states. The simulated CO2 fixation flux at different CO2 concentrations is consistent with the measured Assimilation-CO2intercellular curve. Interestingly, we found that reactions in primary metabolism are affected most significantly by CO2 perturbation, whereas secondary metabolic reactions are not influenced a lot. The changes predicted in key pathways are consistent with existing knowledge. Another interesting point is that Arabidopsis is required to make stronger adjustment on metabolism to adapt to the more severe low CO2 stress than elevated CO2 . The challenges of identifying post-transcriptional regulation could also be addressed by the integrative model. In conclusion, this innovative application of multi-scale modeling in plants demonstrates potential to uncover the mechanisms of metabolic response to different conditions.

Inhibition of 5'-methylthioadenosine metabolism in the Yang cycle alters polyamine levels, and impairs seedling growth and reproduction in Arabidopsis

The methionine or Yang cycle recycles Met from 5'-methylthioadenosine (MTA) which is produced from S-adenosyl-L-methionine (SAM) as a by-product of ethylene, polyamines, and nicotianamine (NA) synthesis. MTA nucleosidase is encoded by two genes in Arabidopsis thaliana, MTN1 and MTN2. Analysis of T-DNA insertion mutants and of wt revealed that MTN1 provides approximately 80% of the total MTN activity. Severe knock down of MTN enzyme activity in the mtn1-1 and mtn1-2 allelic lines resulted in accumulation of SAM/dSAM (decarboxylated SAM) and of MTA in seedlings grown on MTA as sulfur source. While ethylene and NA synthesis were not altered in mtn1-1 and mtn1-2 seedlings grown on MTA, putrescine and spermine were elevated. By contrast, mtn2-1 and mtn2-2 seedlings with near wt enzyme activity had wt levels of SAM/dSAM, MTA, and polyamines. In addition to the metabolic phenotypes, mtn1-1 and mtn1-2 seedlings were growth retarded, while seedlings of wt, mtn2-1, and mtn2-2 showed normal growth on 500 microm MTA. The double knock down mutant mtn1-1/mtn2-1 was sterile. In conclusion, the data presented identify MTA as a crucial metabolite that acts as a regulatory link between the Yang cycle and polyamine biosynthesis and identifies MTA nucleosidase as a crucial enzyme of the Yang cycle.

Recent progress in deciphering the biosynthesis of aspartate-derived amino acids in plants

Plants are either directly or indirectly the source of most of the essential amino acids in animal diets. Four of these essential amino acids-methionine, threonine, isoleucine, and lysine-are all produced from aspartate via a well studied biosynthesis pathway. Given the nutritional interest in essential amino acids, the aspartate-derived amino acid pathway has been the subject of extensive research. Additionally, several pathway enzymes serve as targets for economically important herbicides, and some of the downstream products are biosynthetic precursors for other essential plant metabolites such as ethylene and S-adenosylmethionine. Recent and ongoing research on the aspartate-derived family of amino acids has identified new enzyme activities, regulatory mechanisms, and in vivo metabolic functions. Together, these discoveries will open up new possibilities for plant metabolic engineering.

Nascent peptide-dependent translation arrest leads to Not4p-mediated protein degradation by the proteasome

The potentially deleterious effects of aberrant mRNA lacking a termination codon (nonstop mRNA) are ameliorated by translation arrest, proteasome-mediated protein destabilization, and rapid mRNA degradation. Because polylysine synthesis via translation of the poly(A) mRNA tail leads to translation arrest and protein degradation by the proteasome, we examined the effects of other amino acid sequences. Insertion of 12 consecutive basic amino acids between GFP and HIS3 reporter genes, but not a stem-loop structure, resulted in degradation of the truncated green fluorescent protein (GFP) products by the proteasome. Translation arrest products derived from GFP-R12-FLAG-HIS3 or GFP-K12-FLAG-HIS3 mRNA were detected in a not4Delta mutant, and MG132 treatment did not affect the levels of the truncated arrest products. Deletion of other components of the Ccr4-Not complex did not increase the levels of the translation arrest products or reporter mRNAs. A L35A substitution in the Not4p RING finger domain, which disrupted its interaction with the Ubc4/Ubc5 E2 enzyme and its activity as an ubiquitin-protein ligase, also abrogated the degradation of arrest products. These results suggest that Not4p, a component of the Ccr4-Not complex, may act as an E3 ubiquitin-protein ligase for translation arrest products. The results let us propose that the interaction between basic amino acid residues and the negatively charged exit tunnel of the ribosome leads to translation arrest followed by Not4p-mediated ubiquitination and protein degradation by the proteasome.

Aspartate-Derived Amino Acid Biosynthesis in Arabidopsis thaliana

The aspartate-derived amino acid pathway in plants leads to the biosynthesis of lysine, methionine, threonine, and isoleucine. These four amino acids are essential in the diets of humans and other animals, but are present in growth-limiting quantities in some of the world's major food crops. Genetic and biochemical approaches have been used for the functional analysis of almost all Arabidopsis thaliana enzymes involved in aspartate-derived amino acid biosynthesis. The branch-point enzymes aspartate kinase, dihydrodipicolinate synthase, homoserine dehydrogenase, cystathionine gamma synthase, threonine synthase, and threonine deaminase contain well-studied sites for allosteric regulation by pathway products and other plant metabolites. In contrast, relatively little is known about the transcriptional regulation of amino acid biosynthesis and the mechanisms that are used to balance aspartate-derived amino acid biosynthesis with other plant metabolic needs. The aspartate-derived amino acid pathway provides excellent examples of basic research conducted with A. thaliana that has been used to improve the nutritional quality of crop plants, in particular to increase the accumulation of lysine in maize and methionine in potatoes.

Overexpression of mutated forms of aspartate kinase and cystathionine gamma-synthase in tobacco leaves resulted in the high accumulation of methionine and threonine

Methionine and threonine are two essential amino acids, the levels of which limit the nutritional quality of plants. Both amino acids diverge from the same branch of the aspartate family biosynthesis pathway; therefore, their biosynthesis pathways compete for the same carbon/amino substrate. To further elucidate the regulation of methionine biosynthesis and seek ways of increasing the levels of these two amino acids, we crossed transgenic tobacco plants overexpressing the bacterial feedback-insensitive aspartate kinase (bAK), containing a significantly higher threonine level, with plants overexpressing Arabidopsis cystathionine gamma-synthase (AtCGS), the first unique enzyme of methionine biosynthesis. Plants co-expressing bAK and the full-length AtCGS (F-AtCGS) have significantly higher methionine and threonine levels compared with the levels found in wild-type plants, but the methionine level does not increase beyond that found in plants expressing F-AtCGS alone. This finding can be explained through the feedback inhibition regulation mediated by the methionine metabolite on the transcript level of AtCGS. To test this assumption, plants expressing bAK were crossed with plants expressing two mutated forms of AtCGS in which the domains responsible for the feedback regulation have been deleted. Indeed, significantly higher methionine contents and its metabolites levels accumulated in the newly produced plants, and the levels of threonine were also significantly higher than in the wild-type plants. The transcript level of the two mutated forms of AtCGS significantly increased when there was a high content of threonine in the plants, suggesting that threonine modulates, probably indirectly, the transcript level of AtCGS.

Regulatory role of cystathionine-gamma-synthase and de novo synthesis of methionine in ethylene production during tomato fruit ripening

The essential amino acid methionine is a substrate for the synthesis of S-adenosyl-methionine (SAM), that donates its methyl group to numerous methylation reactions, and from which polyamines and ethylene are generated. To study the regulatory role of methionine synthesis in tomato fruit ripening, which requires a sharp increase in ethylene production, we cloned a cDNA encoding cystathionine gamma-synthase (CGS) from tomato and analysed its mRNA and protein levels during tomato fruit ripening. CGS mRNA and protein levels peaked at the "turning" stage and declined as the fruit ripened. Notably, the tomato CGS mRNA level in both leaves and fruit was negatively affected by methionine feeding, a regulation that Arabidopsis, but not potato CGS mRNA is subject to. A positive correlation was found between elevated ethylene production and increased CGS mRNA levels during the ethylene burst of the climacteric ripening of tomato fruit. In addition, wounding of pericarp from tomato fruit at the mature green stage stimulated both ethylene production and CGS mRNA level. Application of exogenous methionine to pericarp of mature green fruit increased ethylene evolution, suggesting that soluble methionine may be a rate limiting metabolite for ethylene synthesis. Moreover, treatment of mature green tomato fruit with the ethylene-releasing reagent Ethephon caused an induction of CGS mRNA level, indicating that CGS gene expression is regulated by ethylene. Taken together, these results imply that in addition to recycling of the methionine moieties via the Yang pathway, operating during synthesis of ethylene, de novo synthesis of methionine may be required when high rates of ethylene production are induced.

An in vivo internal deletion in the N-terminus region of Arabidopsis cystathionine gamma-synthase results in CGS expression that is insensitive to methionine

Cystathionine gamma-synthase (CGS), the first enzyme of methionine biosynthesis in higher plants, plays an important role in the biosynthesis pathway and in regulating methionine metabolism in plant cells. In response to methionine, the expression of this enzyme is regulated via amino acid sequences located in its N-terminal. Here, using reverse transcription PCR and ribonuclease protection analysis, we demonstrate that, in addition to the full-length CGS transcript, a deleted form exists in Arabidopsis. The deleted transcript of CGS that lacks 90 or 87 nt located internally in the regulatory N-terminal region of CGS maintains the reading frame of the protein. Its association with polyribosomes indicates that this deleted form of CGS is translated. In order to study the function of this deleted form of CGS, we overexpressed it in transgenic tobacco plants. We found that the transgenic plants engineered to express only the deleted form of CGS accumulated methionine to a much higher level than those that expressed the full-length CGS. Furthermore, in vitro feeding experiments revealed that the deleted form of CGS is not subject to feedback regulation by methionine, as reported for the full-length transcript. Therefore, although most likely produced from the full-length CGS, the transcript of the deleted form is insensitive to methionine application and its expression may be important for maintaining methionine metabolism even in the presence of a high level of methionine.

Synthesis of the sulfur amino acids: cysteine and methionine

This review will assess new features reported for the molecular and biochemical aspects of cysteine and methionine biosynthesis in Arabidopsis thaliana with regards to early published data from other taxa including crop plants and bacteria (Escherichia coli as a model). By contrast to bacteria and fungi, plant cells present a complex organization, in which the sulfur network takes place in multiple sites. Particularly, the impact of sulfur amino-acid biosynthesis compartmentalization will be addressed in respect to localization of sulfur reduction. To this end, the review will focus on regulation of sulfate reduction by synthesis of cysteine through the cysteine synthase complex and the synthesis of methionine and its derivatives. Finally, regulatory aspects of sulfur amino-acid biosynthesis will be explored with regards to interlacing processes such as photosynthesis, carbon and nitrogen assimilation.

Improving the levels of essential amino acids and sulfur metabolites in plants

Plants represent the major source of food for humans, either directly or indirectly through their use as livestock feeds. Plant foods are not nutritionally balanced because they contain low proportions of a number of essential metabolites, such as vitamins and amino acids, which humans and a significant proportion of their livestock cannot produce on their own. Among the essential amino acids needed in human diets, Lys, Met, Thr and Trp are considered as the most important because they are present in only low levels in plant foods. In the present review, we discuss approaches to improve the levels of the essential amino acids Lys and Met, as well as of sulfur metabolites, in plants using metabolic engineering approaches. We also focus on specific examples for which a deeper understanding of the regulation of metabolic networks in plants is needed for tailor-made improvements of amino acid metabolism with minimal interference in plant growth and productivity.