Transgenic potato plants reveal the indispensable role of cystathionine beta-lyase in plant growth and development

Transcriptional Profile of Soybean Seeds with Contrasting Seed Coat Color

Soybean is the primary source of vegetable protein and is used for various purposes, mainly to feed animals. This crop can have diverse seed coat colors, varying from yellow, black, brown, and green to bicolor. Black seed coat cultivars have already been assigned as favorable for both seed and grain production. Thus, this work aimed to identify genes associated with soybean seed quality by comparing the transcriptomes of soybean seeds with contrasting seed coat colors. The results from RNA-seq analyses were validated with real-time PCR using the cultivar BRS 715A (black seed coat) and the cultivars BRS 413 RR and DM 6563 IPRO (yellow seed coat). We found 318 genes differentially expressed in all cultivars (freshly harvested seeds and seeds stored in cold chamber). From the in silico analysis of the transcriptomes, the following genes were selected and validated with RT-qPCR: ACS1, ACSF3, CYP90A1, CYP710A1, HCT, CBL, and SAHH. These genes are genes induced in the black seed coat cultivar and are part of pathways responsible for ethylene, lipid, brassinosteroid, lignin, and sulfur amino acid biosynthesis. The BRSMG 715A gene has almost 4times more lignin than the yellow seed coat cultivars. These attributes are related to the BRSMG 715A cultivar's higher seed quality, which translates to more longevity and resistance to moisture and mechanical damage. Future silencing studies may evaluate the knockout of these genes to better understand the biology of soybean seeds with black seed coat.

Identification and functional characterization of a cystathionine β-lyase (CBL) enzyme for H2S production in Arabidopsis thaliana

Sulfide or sulfur metabolism plays an important role in the growth and development of plants. Cystathionine β-lyase (CBL) is an important enzyme in methionine synthesis, but a comprehensive understanding of CBL functions is limited. As the third gasotransmitter, hydrogen sulfide (H₂S) plays important physiological roles in plants. In this study, we found that the endogenous H₂S content in Arabidopsis thaliana cbl mutants was lower than that in the wild type. Under PEG-based osmotic stress conditions, the H₂S contents of CBL-overexpression (OE-CBL) plants increased significantly compared with the wild type. Additionally, the OE-CBL plants increased their tolerance to osmotic stress by increasing the transcription levels of drought-related genes and their relative water-loss rates. Compared with cbl and wild type, OE-CBL plants resisted drought stress by significantly closing their stomata, resulting in improved survival rates. Root tip-bending experiments showed that CBL overexpression relieved osmotic, heavy metal and cold stresses in Arabidopsis. The recombinant CBL activity in vitro revealed that CBL produced H₂S using L-cysteine as a substrate. Thus, CBL had a very strong cysteine desulfhydrase activity that could produce endogenous H₂S using L-cysteine as a substrate, and it played an important role in plant abiotic stress resistance.

Global Survey, Expressions and Association Analysis of CBLL Genes in Peanut

Cystathionine γ-synthase (CGS), methionine γ-lyase (MGL), cystathionine β-lyase (CBL) and cystathionine γ-lyase (CGL) share the Cys_Met_Meta_PP domain and play important roles in plant stress response and development. In this study, we defined the genes containing the Cys_Met_Meta_PP domain (PF01053.20) as CBL-like genes (CBLL). Twenty-nine CBLL genes were identified in the peanut genome, including 12 from cultivated peanut and 17 from wild species. These genes were distributed unevenly at the ends of different chromosomes. Evolution, gene structure, and motif analysis revealed that CBLL proteins were composed of five different evolutionary branches. Chromosome distribution pattern and synteny analysis strongly indicated that whole-genome duplication (allopolyploidization) contributed to the expansion of CBLL genes. Comparative genomics analysis showed that there were three common collinear CBLL gene pairs among peanut, Arabidopsis, grape, and soybean, but no collinear CBLL gene pairs between peanut and rice. The prediction results of cis-acting elements showed that AhCBLLs, AdCBLLs, and AiCBLLs contained different proportions of plant growth, abiotic stress, plant hormones, and light response elements. Spatial expression profiles revealed that almost all AhCBLLs had significantly higher expression in pods and seeds. All AhCBLLs could respond to heat stress, and some of them could be rapidly induced by cold, salt, submergence, heat and drought stress. Furthermore, one polymorphic site in AiCBLL7 was identified by association analysis which was closely associated with pod length (PL), pod width (PW), hundred pod weight (HPW) and hundred seed weight (HSW). The results of this study provide a foundation for further research on the function of the CBLL gene family in peanut.

Profiling Cystathionine β/γ-Lyase in Complex Biosamples Using Novel Activatable Fluorogens

Cystathionine lyase, the key enzyme in transsulfuration and reverse transsulfuration pathways, is involved in a wide array of physiological and pathophysiological processes in both mammals and nonmammals. Though the biological significance of the hydrogen sulfide/cystathionine lyase system in disease states is extensively discussed, the absence of molecular methods for direct monitoring of cystathionine lyase in complex biosamples renders the result unreliable and perplexing. Here, we present the first attempt at designing and developing effective activatable fluorescent probes for cystathionine lyase based on the naphthylamide scaffold. CBLP and CSEP were designed based on the catalytic preference of cystathionine β-lyase (CBL) and cystathionine γ-lyase (CSE). Briefly, incorporation of cysteine/homocysteine as the recognition moiety and a carbamate ethyl sulfide group as a self-immolated linker proved to be an effective strategy for cystathionine lyase fluorescence reporting. CBLP exhibits high selectivity and sensitivity in vitro in semiquantifying CBL levels in roots of wild-type Arabidopsis thaliana and cbl mutants (cbl knockout: SALK_014740C, overexpressed: OE-CBL). Meanwhile, CSEP successfully detected CSE levels in HCC-LM3 cells, zebrafish models, and upregulated CSE in frozen section slides from the liver tissue of cecal ligation and puncture (CLP)-induced septic rats, which was also validated by Western blotting and immunohistochemical analysis. In summary, the practical design strategy facilitates profiling of cystathionine lyase activity in biological processes. It may pave the way for the development of accurate and efficient methods for the direct estimation of cystathionine lyase.

Cystathionine beta-lyase is crucial for embryo patterning and the maintenance of root stem cell niche in Arabidopsis

The growth and development of roots in plants depends on the specification and maintenance of the root apical meristem. Here, we report the identification of CBL, a gene required for embryo and root development in Arabidopsis, and encodes cystathionine beta-lyase (CBL), which catalyzes the penultimate step in methionine (Met) biosynthesis, and which also led to the discovery of a previous unknown, but crucial, metabolic contribution by the Met biosynthesis pathway. CBL is expressed in embryos and shows quiescent center (QC)-enriched expression pattern in the root. cbl mutant has impaired embryo patterning, defective root stem cell niche, stunted root growth, and reduces accumulation of the root master regulators PLETHORA1 (PLT1) and PLT2. Furthermore, mutation in CBL severely decreases abundance of several PIN-FORMED (PIN) proteins and impairs auxin-responsive gene expression in the root tip. cbl seedlings also exhibit global reduction in histone H3 Lys-4 trimethylation (H3K4me3) and DNA methylation. Importantly, mutation in CBL reduces the abundance of H3K4me3 modification in PLT1/2 genes and downregulates their expression. Overexpression of PLT2 partially rescues cbl root meristem defect, suggesting that CBL acts in part through PLT1/2. Moreover, exogenous supplementation of Met also restores the impaired QC activity and the root growth defects of cbl. Taken together, our results highlight the unique role of CBL to maintain the root stem cell niche by cooperative actions between Met biosynthesis and epigenetic modification of key developmental regulators.

Alteration of S-adenosylhomocysteine levels affects lignin biosynthesis in switchgrass

Methionine (Met) synthesized from aspartate is a fundamental amino acid needed to produce S-adenosylmethionine (SAM) that is an important cofactor for the methylation of monolignols. As a competitive inhibitor of SAM-dependent methylation, the effect of S-adenosylhomocysteine (SAH) on lignin biosynthesis, however, is still largely unknown in plants. Expression levels of Cystathionine γ-synthase (PvCGS) and S-adenosylhomocysteine hydrolase 1 (PvSAHH1) were down-regulated by RNAi technology, respectively, in switchgrass, a dual-purpose forage and biofuel crop. The transgenic switchgrass lines were subjected to studying the impact of SAH on lignin biosynthesis. Our results showed that down-regulation of PvCGS in switchgrass altered the accumulation of aspartate-derived and aromatic amino acids, reduced the content of SAH, enhanced lignin biosynthesis and stunted plant growth. In contrast, down-regulation of PvSAHH1 raised SAH levels in switchgrass, impaired the biosynthesis of both guaiacyl and syringyl lignins and therefore significantly increased saccharification efficiency of cell walls. This work indicates that SAH plays a crucial role in monolignol methylation in switchgrass. Genetic regulation of either PvCGS or PvSAHH1 expression in switchgrass can change intracellular SAH contents and SAM to SAH ratios and therefore affect lignin biosynthesis. Thus, our study suggests that genes involved in Met metabolism are of interest as new valuable targets for cell wall bioengineering in future.

Increased β-cyanoalanine nitrilase activity improves cyanide tolerance and assimilation in Arabidopsis

Plants naturally produce cyanide (CN) which is maintained at low levels in their cells by a process of rapid assimilation. However, high concentrations of environmental CN associated with activities such as industrial pollution are toxic to plants. There is thus an interest in increasing the CN detoxification capacity of plants as a potential route to phytoremediation. Here, Arabidopsis seedlings overexpressing the Pseudomonas fluorescens β-cyanoalanine nitrilase pinA were compared with wild-type and a β-cyanoalanine nitrilase knockout line (ΔAtnit4) for growth in the presence of exogenous CN. After incubation with CN, +PfpinA seedlings had increased root length, increased fresh weight, and decreased leaf bleaching compared with wild-type, indicating increased CN tolerance. The increased tolerance was achieved without an increase in β-cyanoalanine synthase activity, the other enzyme in the cyanide assimilation pathway, suggesting that nitrilase activity is the limiting factor for cyanide detoxification. Labeling experiments with [¹³C]KCN demonstrated that the altered CN tolerance could be explained by differences in flux from CN to Asn caused by altered β-cyanoalanine nitrilase activity. Metabolite profiling after CN treatment provided new insight into downstream metabolism, revealing onward metabolism of Asn by the photorespiratory nitrogen cycle and accumulation of aromatic amino acids.

Dynamic proteomics emphasizes the importance of selective mRNA translation and protein turnover during Arabidopsis seed germination

During seed germination, the transition from a quiescent metabolic state in a dry mature seed to a proliferative metabolic state in a vigorous seedling is crucial for plant propagation as well as for optimizing crop yield. This work provides a detailed description of the dynamics of protein synthesis during the time course of germination, demonstrating that mRNA translation is both sequential and selective during this process. The complete inhibition of the germination process in the presence of the translation inhibitor cycloheximide established that mRNA translation is critical for Arabidopsis seed germination. However, the dynamics of protein turnover and the selectivity of protein synthesis (mRNA translation) during Arabidopsis seed germination have not been addressed yet. Based on our detailed knowledge of the Arabidopsis seed proteome, we have deepened our understanding of seed mRNA translation during germination by combining two-dimensional gel-based proteomics with dynamic radiolabeled proteomics using a radiolabeled amino acid precursor, namely [(35)S]-methionine, in order to highlight de novo protein synthesis, stability, and turnover. Our data confirm that during early imbibition, the Arabidopsis translatome keeps reflecting an embryonic maturation program until a certain developmental checkpoint. Furthermore, by dividing the seed germination time lapse into discrete time windows, we highlight precise and specific patterns of protein synthesis. These data refine and deepen our knowledge of the three classical phases of seed germination based on seed water uptake during imbibition and reveal that selective mRNA translation is a key feature of seed germination. Beyond the quantitative control of translational activity, both the selectivity of mRNA translation and protein turnover appear as specific regulatory systems, critical for timing the molecular events leading to successful germination and seedling establishment.

Why have no new herbicide modes of action appeared in recent years?

Herbicides with new modes of action are badly needed to manage the evolution of resistance of weeds to existing herbicides. Yet no major new mode of action has been introduced to the market place for about 20 years. There are probably several reasons for this. New potential products may have remained dormant owing to concerns that glyphosate-resistant (GR) crops have reduced the market for a new herbicide. The capture of a large fraction of the herbicide market by glyphosate with GR crops led to significantly diminished herbicide discovery efforts. Some of the reduced herbicide discovery research was also due to company consolidations and the availability of more generic herbicides. Another problem might be that the best herbicide molecular target sites may have already been discovered. However, target sites that are not utilized, for which there are inhibitors that are highly effective at killing plants, suggests that this is not true. Results of modern methods of target site discovery (e.g. gene knockout methods) are mostly not public, but there is no evidence of good herbicides with new target sites coming from these approaches. In summary, there are several reasons for a long dry period for new herbicide target sites; however, the relative magnitude of each is unclear. The economic stimulus to the herbicide industry caused by the evolution of herbicide-resistant weeds, especially GR weeds, may result in one or more new modes of action becoming available in the not too distant future.

Plastidic isoprenoid biosynthesis in tomato: physiological and molecular analysis in genotypes resistant and sensitive to drought stress

Isoprenoid compounds synthesised in the plastids are involved in plant response to water deficit. The functionality of the biosynthetic pathway of these compounds under drought stress has been analysed at the physiological and molecular levels in two related species of tomato (Solanum chilense and Solanum lycopersicum) that differ in their tolerance to abiotic challenge. Expression analysis of the genes encoding enzymes of these pathways (DXS, IPI, GGPPS, PSY1, NCED and HPT1) in plants at different RWC values shows significant differences for only GGPPS and HPT1, with higher expression in the tolerant S. chilense. Chlorophyll, carotenoids, α-tocopherol and ABA content was also determined in both species under different drought conditions. In agreement with HPT1 transcriptional activity, higher α-tocopherol content was observed in S. chilense than in S. lycopersicum, which correlates with a lower degree of lipoperoxidation in the former species. These results suggest that, in addition to lower stomatal conductance, α-tocopherol biosynthesis is part of the adaptation mechanisms of S. chilense to adverse environmental conditions.

Transcriptional profiles of drought-responsive genes in modulating transcription signal transduction, and biochemical pathways in tomato

To unravel the molecular mechanisms of drought responses in tomato, gene expression profiles of two drought-tolerant lines identified from a population of Solanum pennellii introgression lines, and the recurrent parent S. lycopersicum cv. M82, a drought-sensitive cultivar, were investigated under drought stress using tomato microarrays. Around 400 genes identified were responsive to drought stress only in the drought-tolerant lines. These changes in genes expression are most likely caused by the two inserted chromosome segments of S. pennellii, which possibly contain drought-tolerance quantitative trait loci (QTLs). Among these genes are a number of transcription factors and signalling proteins which could be global regulators involved in the tomato responses to drought stress. Genes involved in organism growth and development processes were also specifically regulated by drought stress, including those controlling cell wall structure, wax biosynthesis, and plant height. Moreover, key enzymes in the pathways of gluconeogenesis (fructose-bisphosphate aldolase), purine and pyrimidine nucleotide biosynthesis (adenylate kinase), tryptophan degradation (aldehyde oxidase), starch degradation (beta-amylase), methionine biosynthesis (cystathionine beta-lyase), and the removal of superoxide radicals (catalase) were also specifically affected by drought stress. These results indicated that tomato plants could adapt to water-deficit conditions through decreasing energy dissipation, increasing ATP energy provision, and reducing oxidative damage. The drought-responsive genes identified in this study could provide further information for understanding the mechanisms of drought tolerance in tomato.

Regulation of aspartate-derived amino acid homeostasis in potato plants (Solanum tuberosum L.) by expression of E. coli homoserine kinase

The availability of the carbon backbone O-phosphohomoserine (OPHS) is critical to methionine (met) and threonine (thr) synthesis. OPHS derives from homoserine and is formed by homoserine kinase (HSK). To clarify the function of HSK in cellular metabolism, the E. coli HSK ortholog thrB was expressed in potato plants targeting the EcHSK protein to chloroplasts and to the cytosol. Both approaches resulted in up to 11 times increased total HSK enzyme activity. Transgenic plants exhibited reduced homoserine levels while met and thr did not accumulate significantly. However, the precursor cysteine and upstream intermediates of met such as cystathionine and homocysteine did indicating an accelerated carbon flow towards the end products. Coincidently, plants with elevated cytosolic levels of EcHSK exhibited a reduction in transcript levels of the endogenous HSK, as well as of threonine synthase (TS), cystathionine beta-lyase (CbL), and met synthase (MS). In all plants, cystathionine gamma-synthase (CgS) expression remained relatively unchanged from wild type levels, while S-adenosylmethionine synthetase (SAMS) expression increased. Feeding studies with externally supplied homoserine fostered the synthesis of met and thr but the regulation of synthesis of both amino acids retained the wild type regulation pattern. The results indicate that excess of plastidial localised HSK activity does not influence the de novo synthesis of met and thr. However, expression of HSK in the cytosol resulted in the down-regulation of gene expression of pathway genes probably mediated via OPHS. We integrated these data in a novel working model describing the regulatory mechanism of met and thr homeostasis.

Identification of a novel gene (Hsdr4) involved in water-stress tolerance in wild barley

Drought is one of the most severe stresses limiting plant growth and yield. Genes involved in water stress tolerance of wild barley (Hordeum spontaneoum), the progenitor of cultivated barley, were investigated using genotypes contrasting in their response to water stress. Gene expression profiles of water-stress tolerant vs. water-stress sensitive wild barley genotypes, under severe dehydration stress applied at the seedling stage, were compared using cDNA-AFLP analysis. Of the 1100 transcript-derived fragments (TDFs) amplified about 70 displayed differential expression between control and stress conditions. Eleven of them showed clear difference (up- or down-regulation) between tolerant and susceptible genotypes. These TDFs were isolated, sequenced and tested by RT-PCR. The differential expression of seven TDFs was confirmed by RT-PCR, and TDF-4 was selected as a promising candidate gene for water-stress tolerance. The corresponding gene, designated Hsdr4 (Hordeum spontaneum dehydration-responsive), was sequenced and the transcribed and flanking regions were determined. The deduced amino acid sequence has similarity to the rice Rho-GTPase-activating protein-like with a Sec14 p-like lipid-binding domain. Analysis of Hsdr4 promoter region that was isolated by screening a barley BAC library, revealed a new putative miniature inverted-repeat transposable element (MITE), and several potential stress-related binding sites for transcription factors (MYC, MYB, LTRE, and GT-1), suggesting a role of the Hsdr4 gene in plant tolerance to dehydration stress. Furthermore, the Hsdr4 gene was mapped using wild barley mapping population to the long arm of chromosome 3H between markers EBmac541 and EBmag705, within a region that previously was shown to affect osmotic adaptation in barley.

Effect of sulfur availability on the integrity of amino acid biosynthesis in plants

Amino acid levels in plants are regulated by a complex interplay of regulatory circuits at the level of enzyme activities and gene expression. Despite the diversity of precursors involved in amino acid biosynthesis as providing the carbon backbones, the amino groups and, for the amino acids methionine and cysteine, the sulfhydryl group and despite the involvement of amino acids as substrates in various downstream metabolic processes, the plant usually manages to provide relatively constant levels of all amino acids. Here we collate data on how amino acid homeostasis is shifted upon depletion of one of the major biosynthetic constituents, i.e., sulfur. Arabidopsis thaliana seedlings exposed to sulfate starvation respond with a set of adaptation processes to achieve a new balance of amino acid metabolism. First, metabolites containing reduced sulfur (cysteine, glutathione, S-adenosylmethionine) are reduced leading to a number of downstream effects. Second, the relative excess accumulation of N over S triggers processes to dump nitrogen in asparagine, glutamine and further N-rich compounds like ureides. Third, the depletion of glutathione affects the redox and stress response system of the glutathione-ascorbate cycle. Thus, biosynthesis of aromatic compounds is triggered to compensate for this loss, leading to an increased flux and accumulation of aromatic amino acids, especially tryptophan. Despite sulfate starvation, the homeostasis is kept, though shifted to a new state. This adaptation process keeps the plant viable even under an adverse nutritional status.

The aspartic acid metabolic pathway, an exciting and essential pathway in plants

Aspartate is the common precursor of the essential amino acids lysine, threonine, methionine and isoleucine in higher plants. In addition, aspartate may also be converted to asparagine, in a potentially competing reaction. The latest information on the properties of the enzymes involved in the pathways and the genes that encode them is described. An understanding of the overall regulatory control of the flux through the pathways is undisputedly of great interest, since the nutritive value of all cereal and legume crops is reduced due to low concentrations of at least one of the aspartate-derived amino acids. We have reviewed the recent literature and discussed in this paper possible methods by which the concentrations of the limiting amino acids may be increased in the seeds.

Involvement of ethylene in stress-induced expression of the TLC1.1 retrotransposon from Lycopersicon chilense Dun

The TLC1 family is one of the four families of long terminal repeat (LTR) retrotransposons identified in the genome of Lycopersicon chilense. Here, we show that this family of retroelements is transcriptionally active and its expression is induced in response to diverse stress conditions such as wounding, protoplast preparation, and high salt concentrations. Several stress-associated signaling molecules, including ethylene, methyl jasmonate, salicylic acid, and 2,4-dichlorophenoxyacetic acid, are capable of inducing TLC1 family expression in vivo. A representative of this family, named TLC1.1, was isolated from a genomic library from L. chilense. Transient expression assays in leaf protoplasts and stably transformed tobacco (Nicotiana tabacum) plants demonstrate that the U3 domain of the 5'-LTR region of this element can drive stress-induced transcriptional activation of the beta-glucuronidase reporter gene. Two 57-bp tandem repeated sequences are found in this region, including an 8-bp motif, ATTTCAAA, previously identified as an ethylene-responsive element box in the promoter region of ethylene-induced genes. Expression analysis of wild-type LTR and single and double ethylene-responsive element box mutants fused to the beta-glucuronidase gene shows that these elements are required for ethylene-responsive gene expression in protoplasts and transgenic plants. We suggest that ethylene-dependent signaling is the main signaling pathway involved in the regulation of the expression of the TLC1.1 element from L. chilense.

Functional analysis of cystathionine gamma-synthase in genetically engineered potato plants

In plants, metabolic pathways leading to methionine (Met) and threonine diverge at the level of their common substrate, O-phosphohomoserine (OPHS). To investigate the regulation of this branch point, we engineered transgenic potato (Solanum tuberosum) plants affected in cystathionine gamma-synthase (CgS), the enzyme utilizing OPHS for the Met pathway. Plants overexpressing potato CgS exhibited either: (a) high transgene RNA levels and 2.7-fold elevated CgS activities but unchanged soluble Met levels, or (b) decreased transcript amounts and enzyme activities (down to 7% of wild-type levels). In leaf tissues, these cosuppression lines revealed a significant reduction of soluble Met and an accumulation of OPHS. Plants expressing CgS antisense constructs exhibited reductions in enzyme activity to as low as 19% of wild type. The metabolite contents of these lines were similar to those of the CgS cosuppression lines. Surprisingly, neither increased nor decreased CgS activity led to visible phenotypic alterations or significant changes in protein-bound Met levels in transgenic potato plants, indicating that metabolic flux to Met synthesis was not greatly affected. Furthermore, in vitro feeding experiments revealed that potato CgS is not subject to feedback regulation by Met, as reported for Arabidopsis. In conclusion, our results demonstrate that potato CgS catalyzes a near-equilibrium reaction and, more importantly, does not display features of a pathway-regulating enzyme. These results are inconsistent with the current hypothesis that CgS exerts major Met metabolic flux control in higher plants.

Co-ordination of leaf minor amino acid contents in crop species: significance and interpretation

The question of whether general control of amino acid synthesis exists in plants remains to be resolved. It is not known whether there is overall co-ordination of the biosynthesis of amino acids that are formed through distinct pathways. In this work, amino acid contents were measured in a large number of samples taken from wheat, potato and barley leaves under different photosynthetic conditions. The variability in total soluble amino acid contents between samples was approximately 6-fold in wheat and potato. Subtracting the major amino acids from the total soluble amino acids showed that the variability in summed minor amino acid contents was approximately 20-fold. This variability was not correlated with short-term changes in primary carbon and nitrogen metabolism, and only poorly correlated with total leaf amino acids. By contrast, striking linear relationships between the contents of most minor amino acids were observed, demonstrating that the contents of many minor amino acids vary in concert. These observations show that amino acid contents are co-ordinated across biosynthetic families. While these data might be interpreted as an indication of cross-pathway regulation of the expression of key biosynthetic enzymes, the impact of factors such as protein degradation and storage cannot be ignored.

Metabolic engineering of amino acids and storage proteins in plants

Amino acid pathways are important targets for plant metabolic engineering. Since plants represent the major global food supply, large efforts are devoted to increasing the content of "essential" amino acids, which are absolutely required in human foods and animal feeds. Engineering of amino acids is also undertaken to improve plant growth and stress tolerance. Many of the pathways of amino acid metabolism in plants have been elucidated, and genes encoding most of the enzymes are now available. The expression of recombinant genes in transgenic plants, coupled with genetic and biochemical approaches, has contributed significantly to the understanding of regulatory networks of the metabolism of amino acids and their incorporation into proteins. This knowledge is now being extensively applied to metabolic engineering of crops, and this is reflected by a large patent literature. The problems of engineering plant amino acid metabolism, and ways to solve them, are discussed using the essential amino acids lysine and methionine as examples.

Antisense inhibition of threonine synthase leads to high methionine content in transgenic potato plants

Methionine (Met) and threonine (Thr) are members of the aspartate family of amino acids. In plants, their biosynthetic pathways diverge at the level of O-phosphohomo-serine (Ser). The enzymes cystathionine gamma-synthase and Thr synthase (TS) compete for the common substrate O-phosphohomo-Ser with the notable feature that plant TS is activated through S-adenosyl-Met, a metabolite derived from Met. To investigate the regulation of this branch point, we engineered TS antisense potato (Solanum tuberosum cv Désirée) plants using the constitutive cauliflower mosaic virus 35S promoter. In leaf tissues, these transgenics exhibit a reduction of TS activity down to 6% of wild-type levels. Thr levels are reduced to 45% wild-type controls, whereas Met levels increase up to 239-fold depending on the transgenic line and environmental conditions. Increased levels of homo-Ser and homo-cysteine indicate increased carbon allocation into the aspartate pathway. In contrast to findings in Arabidopsis, increased Met content has no detectable effect on mRNA or protein levels or on the enzymatic activity of cystathionine gamma-synthase in potato. Tubers of TS antisense potato plants contain a Met level increased by a factor of 30 and no reduction in Thr. These plants offer a major biotechnological advance toward the development of crop plants with improved nutritional quality.