Threonine accumulation in the seeds of a barley mutant with an altered aspartate kinase

Lysine biofortification of crops to promote sustained human health in the 21st century

Crop biofortification is pivotal in preventing malnutrition, with lysine considered the main limiting essential amino acid (EAA) required to maintain human health. Lysine deficiency is predominant in developing countries where cereal crops are the staple food, highlighting the need for efforts aimed at enriching the staple diet through lysine biofortification. Successful modification of aspartate kinase (AK) and dihydrodipicolinate synthase (DHDPS) feedback inhibition has been used to enrich lysine in transgenic rice plants without yield penalty, while increases in the lysine content of quality protein maize have been achieved via marker-assisted selection. Here, we reviewed the lysine metabolic pathway and proposed the use of metabolic engineering targets as the preferred option for fortification of lysine in crops. Use of gene editing technologies to translate the findings and engineer lysine catabolism is thus a pioneering step forward.

Lysine biofortification in rice by modulating feedback inhibition of aspartate kinase and dihydrodipicolinate synthase

Lysine is the main limiting essential amino acid (EAA) in the rice seeds, which is a major energy and nutrition source for humans and livestock. In higher plants, the rate-limiting steps in lysine biosynthesis pathway are catalysed by two key enzymes, aspartate kinase (AK) and dihydrodipicolinate synthase (DHDPS), and both are extremely sensitive to feedback inhibition by lysine. In this study, two rice AK mutants (AK1 and AK2) and five DHDPS mutants (DHDPS1-DHDPS5), all single amino acid substitution, were constructed. Their protein sequences passed an allergic sequence-based homology alignment. Mutant proteins were recombinantly expressed in Escherichia coli, and all were insensitive to the lysine analog S-(2-aminoethyl)-l-cysteine (AEC) at concentrations up to 12 mm. The AK and DHDPS mutants were transformed into rice, and free lysine was elevated in mature seeds of transgenic plants, especially those expressing AK2 or DHDPS1, 6.6-fold and 21.7-fold higher than the wild-type (WT) rice, respectively. We then engineered 35A2D1L plants by simultaneously expressing modified AK2 and DHDPS1, and inhibiting rice LKR/SDH (lysine ketoglutaric acid reductase/saccharopine dehydropine dehydrogenase). Free lysine levels in two 35A2D1L transgenic lines were 58.5-fold and 39.2-fold higher than in WT and transgenic rice containing native AK and DHDPS, respectively. Total free amino acid and total protein content were also elevated in 35A2D1L transgenic rice. Additionally, agronomic performance analysis indicated that transgenic lines exhibited normal plant growth, development and seed appearance comparable to WT plants. Thus, AK and DHDPS mutants may be used to improve the nutritional quality of rice and other cereal grains.

Proteomic analysis of mature barley grains from C-hordein antisense lines

Hordeins are the major storage proteins in barley grains and are responsible for their low nutritional quality. Previously, antisense C-hordein barley lines were generated and were shown to contain a more balanced amino acid composition and an altered storage protein profile. In the present study, a proteomic approach that combined two-dimensional gel electrophoresis (2-DE) and mass spectrometry was used to (1) identify the changes in the protein profile of non-storage proteins (salt soluble fraction) in antisense C-hordein barley lines (L1, L2 and L3) and (2) map the differentially expressed proteins compared to the non-transgenic control line (Hordeum vulgare cv. Golden Promise). Moreover, the changes in the proteins were correlated with the more balanced amino acid composition of these lines, with special attention to the lysine content. The results showed that suppression of C-hordein expression does not exclusively affect hordein synthesis and accumulation. The more balanced amino acid composition observed in the transgenic lines L1, L2 and L3 was an indirect result of the profound alterations in the patterns of the non-storage proteins. The observed changes included up-regulated expression of the proteins involved in stress and detoxification (L1), defence (L2 and L3), and storage globulins (L1, L2 and L3). To a lesser extent, the proteins involved in grain metabolism were also changed. Thus, the increased essential amino acids content results from changes in distinct protein sources among the three antisense C-hordein lines analyzed, although the up-regulated expression of lysine-rich proteins was consistently observed in all lines.

Regulation of amino acid metabolic enzymes and transporters in plants

Amino acids play several critical roles in plants, from providing the building blocks of proteins to being essential metabolites interacting with many branches of metabolism. They are also important molecules that shuttle organic nitrogen through the plant. Because of this central role in nitrogen metabolism, amino acid biosynthesis, degradation, and transport are tightly regulated to meet demand in response to nitrogen and carbon availability. While much is known about the feedback regulation of the branched biosynthesis pathways by the amino acids themselves, the regulation mechanisms at the transcriptional, post-transcriptional, and protein levels remain to be identified. This review focuses mainly on the current state of our understanding of the regulation of the enzymes and transporters at the transcript level. Current results describing the effect of transcription factors and protein modifications lead to a fragmental picture that hints at multiple, complex levels of regulation that control and coordinate transport and enzyme activities. It also appears that amino acid metabolism, amino acid transport, and stress signal integration can influence each other in a so-far unpredictable fashion.

Fortifying plants with the essential amino acids lysine and methionine to improve nutritional quality

Humans, as well as farm animals, cannot synthesize a number of essential amino acids, which are critical for their survival. Hence, these organisms must obtain these essential amino acids from their diets. Cereal and legume crops, which represent the major food and feed sources for humans and livestock worldwide, possess limiting levels of some of these essential amino acids, particularly Lys and Met. Extensive efforts were made to fortify crop plants with these essential amino acids using traditional breeding and mutagenesis. However, aside from some results obtained with maize, none of these approaches was successful. Therefore, additional efforts using genetic engineering approaches concentrated on increasing the synthesis and reducing the catabolism of these essential amino acids and also on the expression of recombinant proteins enriched in them. In the present review, we discuss the basic biological aspects associated with the synthesis and accumulation of these amino acids in plants and also describe recent developments associated with the fortification of crop plants with essential amino acids by genetic engineering approaches.

Valine-Resistance, a Potential Marker in Plant Cell Genetics. I. Distinction between Two Types of Valine-Resistant Tobacco Mutants Isolated from Protoplast-Derived Cells

In previous experiments, seven lines of valine-resistant plants were regenerated from protoplast-derived haploid tobacco mesophyll cells which had been UV mutagenized and submitted to selection by toxic concentrations of valine. In this study we described the transmission of valine-resistance to progeny and a preliminary phenotypical and biochemical characterization of the resistant plants.-Two types were thus distinguished among the seven mutant lines. Valine-resistance of the mutants of the first type (three lines) was transmitted as a single Mendelian dominant character (Vr1), whereas valine-resistance of the second type (four lines) was transmitted as a digenic recessive character (vr2 and vr3). Allelism tests revealed that the four recessive mutant lines yielded resistant progeny when intercrossed and, therefore, bear recessive mutant alleles at the same two unlinked loci.-When cultured at a density of 100 cell/ml, protoplast-derived cells of mutants of the first type had a low level of resistance to valine, whereas protoplast-derived cells of mutants of the second type displayed a high level of resistance to valine and to other amino acids.-According to the results of (14)C-labelled amino acid uptake experiments, the amino acid resistance of mutants of the second type, but not valine-resistance of the first type, could be accounted for by reduced uptake of several amino acids. Possible uses of valine-resistance as a marker in plant cell genetics are discussed.

Non-antibiotic selection systems for soybean somatic embryos: the lysine analog aminoethyl-cysteine as a selection agent

BACKGROUND

In soybean somatic embryo transformation, the standard selection agent currently used is hygromycin. It may be preferable to avoid use of antibiotic resistance genes in foods. The objective of these experiments was to develop a selection system for producing transgenic soybean somatic embryos without the use of antibiotics such as hygromycin.

RESULTS

When tested against different alternate selection agents our studies show that 0.16 microg/mL glufosinate, 40 mg/L isopropylamine-glyphosate, 0.5 mg/mL (S-(2 aminoethyl)-L-cysteine) (AEC) and the acetolactate synthase (ALS) inhibitors Exceed and Synchrony both at 150 microg/mL inhibited soybean somatic embryo growth. Even at the concentration of 2 mg/mL, lysine+threonine (LT) were poor selection agents. The use of AEC may be preferable since it is a natural compound. Unlike the plant enzyme, dihydrodipicolinate synthase (DHPS) from E. coli is not feed-back inhibited by physiological concentrations of lysine. The dapA gene which codes for E. coli DHPS was expressed in soybean somatic embryos under the control of the CaMV 35S promoter. Following introduction of the construct into embryogenic tissue of soybean, transgenic events were recovered by incubating the tissue in liquid medium containing AEC at a concentration of 5 mM. Only transgenic soybeans were able to grow at this concentration of AEC; no escapes were observed.

CONCLUSION

Genetically engineered soybeans expressing a lysine insensitive DHPS gene can be selected with the non-antibiotic selection agent AEC. We also report here the inhibitory effects of glufosinate, (isopropylamine-glyphosate) (Roundup), AEC and the ALS inhibitors Exceed and Synchrony against different tissues of soybean.

Methods for selecting hypervirulent biocontrol agents of weeds: why and how

A considerable number of plant pathogens have been studied for their possible use in weed control. Some have proven virulent enough to control weed species and to compete commercially with chemical herbicides. However, most pathogens of weeds are not useful in their wild form because they are not sufficiently host-specific and/or virulent. The authors believe that these barriers can be overcome. The present research has focused on the inhibitory effects of certain amino acids on the growth and development of specific plants. Pathogens that overproduce these selected amino acids can be easily selected from a pool of spontaneous mutants. Such mutants can have increased pathogenicity to their target weed and enhanced field performance as biocontrol agents. Enhancement of biocontrol efficacy in three separate pathogen-host systems, two with Fusarium and one with Pseudomonas, has already been reported. It is proposed to use the same technology to enhance the biocontrol efficacy of the two species of Fusarium that are host-specific pathogens of the broomrape group of parasitic weeds. The stepwise approach outlined can lead to obtaining enhanced biocontrol agents capable of producing inhibitory levels of selected amino acids in situ. It is proposed that these approaches, in combination with other methods of virulence enhancement, will lead to sustainable systems of biological control of parasitic weeds.

The intragenic approach as a new extension to traditional plant breeding

The novel intragenic approach to genetic engineering improves existing varieties by eliminating undesirable features and activating dormant traits. It transforms plants with native expression cassettes to fine-tune the activity and/or tissue specificity of target genes. Any intragenic modification of traits could, at least in theory, also be accomplished by traditional breeding and transgenic modification. However, the new approach is unique in avoiding the transfer of unknown or foreign DNA. By consequently eliminating various potential risk factors, this method represents a relatively safe approach to crop improvement. Therefore, we argue that intragenic crops should be cleared through the regulatory process in a timely and cost-effective manner.

Aspartate kinase 2. A candidate gene of a quantitative trait locus influencing free amino acid content in maize endosperm

The maize (Zea mays) Oh545o2 inbred accumulates an exceptionally high level of free amino acids, especially lysine (Lys), threonine (Thr), methionine, and iso-leucine. In a cross between Oh545o2 and Oh51Ao2, we identified several quantitative trait loci linked with this phenotype. One of these is on the long arm of chromosome 2 and is linked with loci encoding aspartate (Asp) kinase 2 and Asp kinase (AK)-homoserine dehydrogenase (HSDH) 2. To investigate whether these enzymes can contribute to the high levels of Asp family amino acids, we measured their specific activity and feedback inhibition properties, as well as activities of several other key enzymes involved in Lys metabolism. We did not find a significant difference in total activity of dihydrodipicolinate synthase, HSDH, and Lys ketoglutarate reductase between these inbreds, and the feedback inhibition properties of HSDH and dihyrodipicolinate synthase by Lys and/or Thr were similar. The most significant difference we found between Oh545o2 and Oh51Ao2 is feedback inhibition of AK by Lys but not Thr. AK activity in Oh545o2 is less sensitive to Lys inhibition than that in Oh51Ao2, with a Lys I50 twice that of Oh51Ao2. AK activity in Oh545o2 endosperm is also higher than in Oh51Ao2 at 15 d after pollination, but not 20 d after pollination. The results indicate that the Lys-sensitive Asp kinase 2, rather than the Thr-sensitive AK-HSDH2, is the best candidate gene for the quantitative trait locus affecting free amino acid content in Oh545o2.

The biosynthesis and metabolism of the aspartate derived amino acids in higher plants

The essential amino acids lysine, threonine, methionine and isoleucine are synthesised in higher plants via a common pathway starting with aspartate. The regulation of the pathway is discussed in detail, and the properties of the key enzymes described. Recent data obtained from studies of regulation at the gene level and information derived from mutant and transgenic plants are also discussed. The herbicide target enzyme acetohydroxyacid synthase involved in the synthesis of the branched chain amino acids is reviewed.

The enzymology of lysine catabolism in rice seeds--isolation, characterization, and regulatory properties of a lysine 2-oxoglutarate reductase/saccharopine dehydrogenase bifunctional polypeptide

In plant, the catabolism of lysine has only been studied in some detail in maize. The enzymes lysine 2-oxoglutarate reductase (also known as lysine alpha-ketoglutarate reductase; LOR) and saccharopine dehydrogenase (SDH), which convert lysine into saccharopine, and saccharopine into glutamic acid and 2-aminoadipate 6-semialdehyde, respectively, were isolated from immature rice seeds and partially purified through a three-step purification procedure involving ammonium sulphate precipitation, and anion-exchange and gel-filtration chromatographies, leading to a final yield of 30% for LOR and 24% for SDH. The molecular masses estimated by gel-filtration chromatography on a Sephacryl S200 column and by native non-denaturing PAGE using Ferguson plots were 203 kDa for both enzymes by gel-filtration and 202 kDa for both enzymes by native non-denaturing PAGE. A second band of LOR and SDH activities on native gels was observed for both enzymes with an estimated molecular mass of 396 kDa, which indicated a multimeric structure. Kinetic studies were consistent with an ordered sequence mechanism for LOR, where 2-oxoglutarate is the first substrate and saccharopine is the last product. The results observed for the LOR/SDH activity ratios during purification, the copurification in all three steps, the molecular masses, the relative mobilities on native non-denaturing gels and the pI estimated for LOR and SDH suggest the existence of a bifunctional polypeptide containing LOR and SDH activities.

Nutritional aspects of cereal proteins and approaches to overcome their deficiencies

The inferior nutritional value of cereal protein is primarily because of the high content of the storage protein prolamin. These proteins are in general characterized by a very high content of proline and glutamine and a low content of lysine and other nutritionally essential amino acids. The cereals vary with respect to prolamin and lysine content. Rice and oats have a relatively low prolam in content, around 10% , and an acceptable lysine content. W heat and barley contain 40-45% prolamin and about 3.5% lysine, while maize and sorghum contain more than 50% prolamin in their seed protein, that results in a lysine content below 2.5% . Intensive screenings for changes in endosperm morphology, protein composition, or lysine content have led to the detection of a number of mutants with reduced prolamin content and increased content of lysine and other essential amino acids. These high-lysine and low-prolamin mutants have a considerably improved nutritional value of the seed protein. However, all the mutants found so far are also characterized by a reduced starch content and grain yield, and high-lysine varieties have only been grown to a very limited extent. The present main approaches studied to improve the nutritional value of cereal proteins are (i) the replacement of some of the prolamin with lysine-rich storage proteins present in low amounts in the seed, and (ii) the improvement of the nutritional value of prolam in by genetic engineering.

Engineering of the aspartate family biosynthetic pathway in barley (Hordeum vulgare L.) by transformation with heterologous genes encoding feed-back-insensitive aspartate kinase and dihydrodipicolinate synthase

In prokaryotes and plants the synthesis of the essential amino acids lysine and threonine is predominantly regulated by feed-back inhibition of aspartate kinase (AK) and dihydrodipicolinate synthase (DHPS). In order to modify the flux through the aspartate family pathway in barley and enhance the accumulation of the corresponding amino acids, we have generated transgenic barley plants that constitutively express mutant Escherichia coli genes encoding lysine feed-back insensitive forms of AK and DHPS. As a result, leaves of primary transformants (T0) exhibited a 14-fold increase of free lysine and an 8-fold increase in free methionine. In mature seeds of the DHPS transgenics, there was a 2-fold increase in free lysine, arginine and asparagine and a 50% reduction in free proline, while no changes were observed in the seeds of the two AK transgenic lines analysed. When compared to that of control seeds, no differences were observed in the composition of total amino acids. The introduced genes were inherited in the T1 generation where enzymic activities revealed a 2.3-fold increase of AK activity and a 4.0-9.5-fold increase for DHPS. T1 seeds of DHPS transformants showed the same changes in free amino acids as observed in T0 seeds. It is concluded that the aspartate family pathway may be genetically engineered by the introduction of genes coding for feed-back-insensitive enzymes, preferentially giving elevated levels of lysine and methionine.

Genetic and amino-acid analysis of two maize threonine-overproducing, lysine-insensitive aspartate kinase mutants

The aspartate-derived amino-acid pathway leads to the production of the essential amino-acids lysine, methionine, threonine and isoleucine. Aspartate kinase (AK) is the first enzyme in this pathway and exists in isoforms that are feedback inhibited by lysine and threonine. Two maize (Zea mays L.) threonine-overproducing, lysine-insensitive AK mutants (Ask1-LT19 and Ask2-LT20) were previously isolated. The present study was conducted to determine the map location of Ask2 and to examine the amino-acid profiles of the Ask mutants. The threonine-overproducing trait conferred by Ask2-LT20 was mapped to the long arm of chromosome 2. Both mutants exhibited increased free threonine concentrations (nmol/mg dry weight) over wild-type. The percent free threonine increased from approximately 2% in wild-type kernels to 37-54% of the total free amino-acid pool in homozygous mutant kernels. Free methionine concentrations also increased significantly in homozygous mutants. Free lysine concentrations were increased but to a much lesser extent than threonine or methionine. In contrast to previous studies, free aspartate concentrations were observed to decrease, indicating a possible limiting factor in threonine synthesis. Total (free plus protein-bound) amino-acid analyses demonstrated a consistent, significant increase in threonine, methionine and lysine concentrations in the homozygous mutants. Significant increases in protein-bound (total minus free) threonine, methionine and lysine were observed in the Ask mutants, indicating adequate protein sinks to incorporate the increased free amino-acid concentrations. Total amino-acid contents (nmol/kernel) were approximately the same for mutant and wild-type kernels. In five inbred lines both Ask mutations conferred the threonine-overproducing phenotype, indicating high expressivity in different genetic backgrounds. These analyses are discussed in the context of the regulation of the aspartate-derived amino-acid pathway.

Molecular analysis of the aspartate kinase-homoserine dehydrogenase gene from Arabidopsis thaliana

The gene encoding Arabidopsis thaliana aspartate kinase (ATP:L-aspartate 4-phosphotransferase, EC 2.7.2.4) was isolated from genomic DNA libraries using the carrot ak-hsdh gene as the hybridizing probe. Two genomic libraries from different A. thaliana races were screened independently with the ak probe and the hsdh probe. Nucleotide sequences of the A. thaliana overlapping clones were determined and encompassed 2 kb upstream of the coding region and 300 bp downstream. The corresponding cDNA was isolated from a cDNA library made from poly(A)(+)-mRNA extracted from cell suspension cultures. Sequence comparison between the Arabidopsis gene product and an AK-HSDH bifunctional enzyme from carrot and from the Escherichia coli thrA and metL genes shows 80%, 37.5% and 31.4% amino acid sequence identity, respectively. The A. thaliana ak-hsdh gene is proposed to be the plant thrA homologue coding for the AK isozyme feedback inhibited by threonine. The gene is present in A. thaliana in single copy and functional as evidenced by hybridization analyses. The apoprotein-coding region is interrupted by 15 introns ranging from 78 to 134 bp. An upstream chloroplast-targeting sequence with low sequence similarity with the carrot transit peptide was identified. A signal sequence is proposed starting from a functional ATG initiation codon to the first exon of the apoprotein. Two additional introns were identified: one in the 5' non-coding leader sequence and the other in the putative chloroplast targeting sequence. 5' sequence analysis revealed the presence of several possible promoter elements as well as conserved regulatory motifs. Among these, an Opaque2 and a yeast GCN4-like recognition element might be relevant for such a gene coding for an enzyme limiting the carbon-flux entry to the biosynthesis of several essential amino acids. 3' sequence analysis showed the occurrence of two polyadenylation signals upstream of the polyadenylation site. This work is the first report of the molecular cloning of a plant ak-hsdh genomic sequence. It describes a promoter element that may bring new insights to the regulation of the biosynthesis of the aspartate family of amino acids.

Regulation of lysine synthesis in transgenic potato plants expressing a bacterial dihydrodipicolinate synthase in their chloroplasts

The essential amino acid lysine is synthesized in higher plants by a complex pathway that is predominantly regulated by feedback inhibition of two enzymes, namely aspartate kinase (AK) and dihydrodipicolinate synthase (DHPS). Although DHPS is thought to play a major role in this regulation, the relative importance of AK is not known. In order to study this regulation, we have expressed in the chloroplasts of transgenic potato plants a DHPS derived from Escherichia coli at a level 50-fold above the endogenous DHPS. The bacterial enzyme is much less sensitive to lysine inhibition than its potato counterpart. DHPS activity in leaves, roots and tubers of the transgenic plants was considerably higher and more resistant to lysine inhibition than in control untransformed plants. Furthermore, this activity was accompanied by a significant increase in level of free lysine in all three tissues. Yet, the extent of lysine overproduction in potato leaves was significantly lower than that previously reported in leaves of transgenic plants expressing the same bacterial enzyme, suggesting that in potato, AK may also play a major regulatory role in lysine biosynthesis. Indeed, the elevated level of free lysine in the transgenic potato plants was shown to inhibit the lysine-sensitive AK activity in vivo. Our results support previous reports showing that DHPS is the major rate-limiting enzyme for lysine synthesis in higher plants, but they suggest that additional plant-specific regulatory factors are also involved.

Homoserine kinase and threonine synthase in methionine-overproducing soybean tissue cultures

To gain understanding of the regulation of methionine level in plants, we assayed homoserine kinase and threonine synthase in extracts of wild type and several methionine-overproducing soybean [Glycine max (L.) Merr.] callus lines. The specific activity of homoserine kinase was depressed by 45-73%, and that of threonine synthase by 26-43% in the high methionine lines. Cysteine inhibited threonine synthase in wild type and variant lines. Threonine synthase in two variant lines showed significantly less inhibition by cysteine and in one line was inhibited by threonine. Depressed threonine synthase activity may increase the availability of homoserine phosphate to the competing methionine biosynthetic pathway.

Altered feedback sensitivity of acetohydroxyacid synthase from valine-resistant mutants of tobacco (Nicotiana tabacum L.)

Acetohydroxyacid synthase (EC 4.1.3.18) has been extracted from leaves of three valine-resistant (Val(r)) tobacco (Nicotiana tabacum) mutants, and compared with the enzyme from the wild-type. The enzyme from all three mutants is appreciably less sensitive to inhibition by leucine and valine than the wild-type. Two of the mutants, Val(r)-1 and Val(r)-6, have very similar enzymes, which under all conditions are inhibited by less than half that found for the wild-type. The other mutant, Val(r)-7, has an enzyme that only displays appreciably different characteristics from the wild-type at high pyruvate or inhibitor concentrations. Enzyme from Val(r)-7 also has a higher apparent Km for pyruvate, threefold greater than the value determined for the wild-type and the other mutants. The sulphonylurea herbicides strongly inhibit the enzyme from all the lines, though the concentrations required for half-maximal inhibition of enzyme from Val(r)-1 and Val(r)-6 are higher than for Val(r)-7 or the wildtype. No evidence has been found for multiple isoforms of acetohydroxyacid synthase, and it is suggested that the valine-resistance of these mutant lines is the result of two different mutations affecting a single enzyme, possibly involving different subunits.

Progress in cellular engineering of plants: biochemical and genetic assessment of selectable markers from cultured cells

Recent availability of stable and well characterized selectable markers and ability to combine alien genomes parasexually have contributed to the development of molecular biology in higher plants, including gene expression and genetic manipulation.Several types of biochemical mutants (resistant to inhibitory concentrations of aminoacid(s) or aminoacid analogs as well as deficient for enzyme activity) have recently been isolated and characterized biochemically and genetically. Among them, mutants with alterations in the nitrogen and aminoacid metabolism, or in the activity of alcohol dehydrogenases are being used in the development of more efficient techniques of gene transfer.The manipulation of whole genomes by sexual or somatic cell fusion offers new potential in this field, but refinement of transfer techniques is desirable. The new set of selectable markers obtained through advanced cellular technology, as well as our ability to regenerate plants from manipulated cell lines are expected to play a major role in cellular engineering.

Lysine overproducer mutants with an altered dihydrodipicolinate synthase from protoplast culture of Nicotiana sylvestris (Spegazzini and Comes)

Two S-(2-aminoethyl)L-cysteine (AEC) resistant lines were isolated by screening mutagenized protoplasts from diploid N. sylvestris plants. Both lines accumulated free lysine at levels 10 to 20-fold higher than in controls. Lysine overproduction and AEC-resistance were also expressed in plants regenerated from the variant cultures. A feedback insensitive form of dihydrodipicolinate synthase (DHPS), the pathway specific control enzyme for lysine synthesis, was detected in callus cultures and leaf extracts from the resistant lines. Aspartate kinase (AK), the other key enzyme in the regulation of lysine biosynthesis, was unaltered in the mutants. Crosses with wild type plants indicated that the mutation conferring insensitivity to feedback in DHPS, with as result overproduction of lysine and resistance to AEC, was inherited as a single dominant nuclear gene.

The isolation and characterisation of a catalase-deficient mutant of barley (Hordeum vulgare L.)

A mutant line of barley, R(othamsted)-Pr 79/4, has been isolated which grows poorly in natural air, but normally in air enriched to 0.2% CO2. Analysis of the products of (14)CO2 fixation showed that there was no major block in photosynthetic or photorespiratory carbon metabolism in the mutant and that rates of CO2 fixation were only slightly lower than those measured in the wild type (c.v. Maris Mink). Leaves of the mutant line contained only 10% of the catalase (EC 1.11.1.6) activity found in the wild type; and the two major bands of catalase activity detected after starch-gel electrophoresis of extracts of normal leaves were missing from similar extracts of RPr 79/4. Peroxisomes isolated from mutant leaves contained negligible catalase activity, but normal levels of other enzymes involved in photorespiration. Genetic analysis has shown that the mutation is recessive and that both air-sensitivity and catalase-deficiency segregate together in F2 plants derived from a cross between the mutant and the cultivar Golden Promise. [1-(14)C]Glycollate was not converted to (14)CO2 faster in the mutant leaves than in the normal leaves. Thus there was no evidence that photorespiratory CO2 may be obtained by the chemical action of H2O2 on glyoxylate or hydroxypyruvate.

Feedback-insensitive aspartate kinase isoenzymes in barley mutants resistant to lysine plus threonine

The regulatory properties of aspartate kinase (EC 2.7.2.4) and homoserine dehydrogenase (EC 1.1.1.3) in two barley (Hordeum vulgare L.) mutants resistant to growth inhibition by lysine plus threonine, Rothamsted (R) 3004 and R3202, were compared with those in the normal, sensitive parent line cv. Bomi. Three forms of aspartate kinase (AKI, AKII, AKIII) were chromatographically separated and were considered to represent at least three independently regulated isoenzymes. Aspartate kinase I was inhibited by threonine; AKII and AKIII by lysine or lysine plus S-adenosylmethionine. The characteristics of AKI were unchanged in the mutants. Aspartate kinase II and AKIII from Bomi were both inhibited by lysine and by lysine plus S-adenosylmethionine. Aspartate kinase II from mutant R3202 was altered in its properties such that it was insensitive to lysine or lysine plus S-adenosylmethionine; AKII from mutant R3004 did not differ in its properties from AKII of Bomi. The concentration of lysine required to give half maximal inhibition of AKIII from R3004 was ten times that required for AKIII of Bomi; AKIII from R3202 did not differ from that of Bomi in this regard. There was no change in the properties of homoserine dehydrogenase of the mutants as compared with that of Bomi. We conclude that the lt1 and lt2 loci code for structural genes for lysine- and lysine plus S-adenosylmethionine-sensitive aspartate kinase isoenzymes. The mutant genes Lt1b and Lt2 in R3202 and R3004 respectively code for feedback-desensitized isoenzymes. The presence of one of these is sufficient to allow the synthesis of methionine to overcome the growth inhibition by lysine plus threonine.