Regulation of maize lysine metabolism and endosperm protein synthesis by opaque and floury mutations

Analysis of nutritional composition in opaque2- and crtRB1-based single- and double-biofortified super sweet corn

Sweet corn has emerged as a favorite food item worldwide owing to its kernel sweetness. However, traditional sweet corn cultivars are poor in provitamin-A (proA) and essential amino acids, viz., lysine and tryptophan. So far, no sweet corn hybrid with high nutritional qualities has been commercialized elsewhere. Here, we analyzed accumulation of provitamin-A (proA), lysine, and tryptophan in a set of mutant versions of (i) crtRB1-, (ii) o2-, and (iii) crtRB1 + o2-based sweet corn inbreds and hybrids with (iv) traditional sweet corn (wild-type: O2 + CrtRB1). The crtRB1- and crtRB1 + o2-based genotypes possessed significantly higher proA (17.31 ppm) over traditional sweet corn (2.83 ppm), while o2- and crtRB1 + o2-based genotypes possessed significantly higher lysine (0.345%) and tryptophan (0.080%) over traditional sweet corn (lysine 0.169%, tryptophan 0.036%). Late sowing favored high kernel lysine, proA, and green cob yield among hybrids. Sweetness (17.87%) among the improved inbreds and hybrids was comparable to the original sweetcorn genotypes (17.84%). Among the four genotypic classes, crtRB1 + o2-based improved genotypes showed stronger association among traits over genotypes with o2 and crtRB1 genes alone. Significant association was observed among (i) proA and BC (r = 0.99), (ii) proA and BCX (r = 0.93), (iii) lysine and tryptophan (r = 0.99), and (iv) green cob yield with fodder yield (r = 0.73) in sweet corn hybrids. The study demonstrated that combining crtRB1 and o2 genes did not pose any negative impact on nutritional, yield, and agronomic performance. Sweet corn with crtRB1 + o2 assumes significance for alleviating malnutrition through sustainable and cost-effective approach.

Effect of multiple-frequency ultrasound-assisted transglutaminase dual modification on the structural, functional characteristics and application of Qingke protein

Qingke protein rich in restricted amino acids such as lysine, while the uncoordination of ratio of glutenin and gliadin in Qingke protein has a negative impact on its processing properties. In this study, the effect of multiple-frequency ultrasound combined with transglutaminase treatment on the functional and structural properties of Qingke protein and its application in noodle manufacture were investigated. The results showed that compared with the control, ultrasound-assisted transglutaminase dual modification significantly increased the water and oil holding capacity, apparent viscosity, foaming ability, and emulsifying activity index of Qingke protein, which exhibited a higher storage modulus G' (P < 0.05). Meanwhile, ultrasound combined with transglutaminase treatment enhanced the cross-linking degree of Qingke protein (P < 0.05), as shown by decreased free amino group and free sulfhydryl group contents, and increased disulfide bond content. Moreover, after the ultrasound-assisted transglutaminase dual modification treatment, the fluorescence intensity, the contents of α-helix and random coil in the secondary structure of Qingke protein significantly decreased, while the β-sheet content increased (P < 0.05) compared with control. SDS-PAGE results showed that the bands of Qingke protein treated by ultrasound combined with transglutaminase became unclear. Furthermore, the quality of Qingke noodles made with Qingke powder (140 g/kg dual modified Qingke protein mixed with 860 g/kg extracted Qingke starch) and wheat gluten 60-70 g/kg was similar to that of wheat noodles. In summary, multiple-frequency ultrasound combined with transglutaminase dual modification can significantly improve the physicochemical properties of Qingke protein and the modified Qingke proteins can be used as novel ingredients for Qingke noodles.

Expression profile of protein fractions in the developing kernel of normal, Opaque-2 and quality protein maize

Maize protein quality is determined by the composition of its endosperm proteins, which are classified as nutritionally poor zeins (prolamin and prolamin-like) and nutritionally rich non-zeins (albumin, globulin, glutelin-like, and glutelin). Protein quality is considerably higher in opaque-2 mutants due to increased content of non-zeins over zeins. However, the opaque-2 endosperm is soft, which leads to poor agronomic performance and post-harvest infestation. Endosperm modification of opaque-2 had led to the development of Quality Protein Maize (QPM), which has higher protein quality along with hard kernel endosperm. The present study was planned to analyze the expression dynamics of different protein fractions in the endospem of developing maize kernel in normal, opaque-2 and QPM in response to the introgression of endosperm modifiers. Results revealed that albumin and globulin content decreases, whereas, prolamin, prolamin-like, glutelin-like, and glutelin content increases with kernel maturity. It has been observed that opaque-2 mutation affects protein expression at initial stages, whereas, the effect of endosperm modifiers was observed at the intermediate and later stages of kernel development. It has also been noted that prolamin, glutelin, and glutelin-like fractions can be used as quick markers for quality assessment for differentiating QPM varieties, even at the immature stage of kernel development. Overall, the present study implicates the role of different protein fractions in developing and utilizing nutritionally improved maize varieties.

Comparative Transcriptome, Metabolome, and Ionome Analysis of Two Contrasting Common Bean Genotypes in Saline Conditions

Soil salinity is a major abiotic stress factor that limits agricultural productivity worldwide, and this problem is expected to grow in the future. Common bean is an important protein source in developing countries however highly susceptible to salt stress. To understand the underlying mechanism of salt stress responses, transcriptomics, metabolomics, and ion content analysis were performed on both salt-tolerant and susceptible common bean genotypes in saline conditions. Transcriptomics has demonstrated increased photosynthesis in saline conditions for tolerant genotype while the susceptible genotype acted in contrast. Transcriptome also displayed active carbon and amino-acid metabolism for the tolerant genotype. Analysis of metabolites with GC-MS demonstrated the boosted carbohydrate metabolism in the tolerant genotype with increased sugar content as well as better amino-acid metabolism. Accumulation of lysine, valine, and isoleucine in the roots of the susceptible genotype suggested a halted stress response. According to ion content comparison, the tolerant genotype managed to block accumulation of Na⁺ in the leaves while accumulating significantly less Na⁺ in the roots compared to susceptible genotype. K⁺ levels increased in the leaves of both genotype and the roots of the susceptible one but dropped in the roots of the tolerant genotype. Additionally, Zn⁺² and Mn⁺² levels were dropped in the tolerant roots, while Mo⁺² levels were significantly higher in all tissues in both control and saline conditions for tolerant genotype. The results of the presented study have demonstrated the differences in contrasting genotypes and thus provide valuable information on the pivotal molecular mechanisms underlying salt tolerance.

Protein, Phytate and Minerals in Grains of Commercial Cowpea Genotypes

The objective of this study was to investigate and characterize cowpea (Vigna unguiculata) genotypes for total grain protein content, storage protein fractions (globulin, albumin, prolamin, basic and acid glutelins), and phytate and minerals contents. Eighteen cowpea genotypes were selected. Total grain protein content varied from 21.4% to 29.2%, for BRS Marataoã and Paulistinha genotypes, respectively. The variation in the concentration of each protein fraction was significant (P<0.05) only for glutelins (basic and acid). The genotypes studied exhibited great similarity in the PAGE electrophoretic profile of the grain protein fractions and also in the mineral content. BRS Paraguaçu genotype exhibited higher Zn content than thegenotypes that have been previously recommended for this characteristic. The lowest phytate grain content was observed in four of the 18 genotypes studied, which also exhibited high protein contents. Although the results did not converge to the selection of a few genotypes, some specific differences were detected that which may be further explored. Considering total grain protein, mineral and phytate contents, the genotype Paulistinha revealed a better balance unveiling high grain total protein content, low grain phytate content and more homogeneous mineral composition.

Temporal profiling of essential amino acids in developing maize kernel of normal, opaque-2 and QPM germplasm

Maize, an important cereal crop, has a poor quality of endosperm protein due to the deficiency of essential amino acids, especially lysine and tryptophan. Discovery of mutants such as opaque-2 led to the development of nutritionally improved maize with a higher concentration of lysine and tryptophan. However, the pleiotropic effects associated with opaque-2 mutants necessitated the development of nutritionally improved hard kernel genotype, the present-day quality protein maize (QPM). The aim of present study was to analyze and compare the temporal profile of lysine and tryptophan in the developing maize kernel of normal, opaque-2 and QPM lines. A declining trend in protein along with tryptophan and lysine content was observed with increasing kernel maturity in the experimental genotypes. However, opaque-2 retained the maximum concentration of lysine (3.43) and tryptophan (1.09) at maturity as compared to QPM (lysine-3.05, tryptophan-0.99) and normal (lysine-1.99, tryptophan-0.45) lines. Opaque-2 mutation affects protein quality but has no effect on protein quantity. All maize types are nutritionally rich at early stages of kernel development indicating that early harvest for cattle feed would ensure a higher intake of lysine and tryptophan. Two promising lines (CML44 and HKI 1105) can be used for breeding high value corn for cattle feed or human food in order to fill the protein inadequacy gap. Variation in lysine and tryptophan content within QPM lines revealed that differential expression of endosperm modifiers with varying genetic background significantly affects nutritional quality, indicating that identification of alleles affecting amino acid composition can further facilitate QPM breeding program.

Lysine Catabolism Through the Saccharopine Pathway: Enzymes and Intermediates Involved in Plant Responses to Abiotic and Biotic Stress

The saccharopine pathway (SACPATH) involves the conversion of lysine into α-aminoadipate by three enzymatic reactions catalyzed by the bifunctional enzyme lysine-ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH) and the enzyme α-aminoadipate semialdehyde dehydrogenase (AASADH). The LKR domain condenses lysine and α-ketoglutarate into saccharopine, and the SDH domain hydrolyzes saccharopine to form glutamate and α-aminoadipate semialdehyde, the latter of which is oxidized to α-aminoadipate by AASADH. Glutamate can give rise to proline by the action of the enzymes Δ¹-pyrroline-5-carboxylate synthetase (P5CS) and Δ¹-pyrroline-5-carboxylate reductase (P5CR), while Δ¹-piperideine-6-carboxylate the cyclic form of α-aminoadipate semialdehyde can be used by P5CR to produce pipecolate. The production of proline and pipecolate by the SACPATH can help plants face the damage caused by osmotic, drought, and salt stress. AASADH is a versatile enzyme that converts an array of aldehydes into carboxylates, and thus, its induction within the SACPATH would help alleviate the toxic effects of these compounds produced under stressful conditions. Pipecolate is the priming agent of N-hydroxypipecolate (NHP), the effector of systemic acquired resistance (SAR). In this review, lysine catabolism through the SACPATH is discussed in the context of abiotic stress and its potential role in the induction of the biotic stress response.

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.

Cellular Response to the high protein digestibility/high-Lysine (hdhl) sorghum mutation

A high protein digestibility/high-lysine mutant P721Q (hdhl) with a multi-folded protein body morphology has been developed, with a 22kDa α-kafirin single point mutation having also been recently identified. Relatively little is known regarding the resulting cellular response in hdhl endosperm. The aim is to elucidate these biochemical changes. Two-dimentional gel electrophoresis showed an apparent increase of non-kafirin and a decrease in kafirins content in hdhl endosperm. Mass spectrometry data yielded the identity of differentially expressed non-kafirin proteins in hdhl, wild-type lines such as cytoskeleton and chaperones proteins, and also others involved in amino acids and carbohydrates biochemical synthesis pathways. Western blot analysis showed that chaperone proteins were more highly expressed in the hdhl than the wild-type sorghum and confirmed the non-kafirin proteins proteomic results. Two-dimentional gel electrophoresis showed that the γ-kafirin subunits content had decreased, and the 22kDa α-kafirin subunit was increased in hdhl without any apparent molecular mass change. The observed differential expression most likely led to proteins interactions between γ- and α-kafirin subunits in particular, which resulted in a kafirins packing differently to form the protein body's multi-folded morphology, while also improving its digestibility.

Lysine metabolism in antisense C-hordein barley grains

The grain proteins of barley are deficient in lysine and threonine due to their low concentrations in the major storage protein class, the hordeins, especially in the C-hordein subgroup. Previously produced antisense C-hordein transgenic barley lines have an improved amino acid composition, with increased lysine, methionine and threonine contents. The objective of the study was to investigate the possible changes in the regulation of key enzymes of the aspartate metabolic pathway and the contents of aspartate-derived amino acids in the nontransgenic line (Hordeum vulgare L. cv. Golden Promise) and five antisense C-hordein transgenic barley lines. Considering the amounts of soluble and protein-bound aspartate-derived amino acids together with the analysis of key enzymes of aspartate metabolic pathway, we suggest that the C-hordein suppression did not only alter the metabolism of at least one aspartate-derived amino acid (threonine), but major changes were also detected in the metabolism of lysine and methionine. Modifications in the activities and regulation of aspartate kinase, dihydrodipicolinate synthase and homoserine dehydrogenase were observed in most transgenic lines. Furthermore the activities of lysine α-ketoglutarate reductase and saccharopine dehydrogenase were also altered, although the extent varied among the transgenic lines.

Co-suppression of synthesis of major α-kafirin sub-class together with γ-kafirin-1 and γ-kafirin-2 required for substantially improved protein digestibility in transgenic sorghum

KEY MESSAGE

Co-suppressing major kafirin sub-classes is fundamental to improved protein digestibility and nutritional value of sorghum. The improvement is linked to an irregularly invaginated phenotype of protein bodies.

ABSTRACT

The combined suppression of only two genes, γ kafirin-1 (25 kDa) and γ-kafirin-2 (50 kDa), significantly increases sorghum kafirin in vitro digestibility. Co-suppression of a third gene, α-kafirin A1 (25 kDa), in addition to the two genes increases the digestibility further. The high-digestibility trait has previously only been obtained either through the co-suppression of six kafirin genes (α-A1, 25 kDa; α-B1, 19 kDa; α-B2, 22 kDa; γ-kaf1, 27 kDa; γ-kaf 2, 50 kDa; and δ-kaf 2, 18 kDa) or through random chemical-induced mutations (for example, the high protein digestibility mutant). We present further evidence that suppressing just three of these genes alters kafirin protein cross-linking and protein body microstructure to an irregularly invaginated phenotype. The irregular invaginations are consistent with high pepsin enzyme accessibility and hence high digestibility. The approach we adopted towards increasing sorghum protein digestibility appears to be an effective tool in improving the status of sorghum as a principal supplier of energy and protein in poor communities residing in marginal agro-ecological zones of Africa.

Mutational breeding and genetic engineering in the development of high grain protein content

Cereals are the most important crops in the world for both human consumption and animal feed. Improving their nutritional values, such as high protein content, will have significant implications, from establishing healthy lifestyles to helping remediate malnutrition problems worldwide. Besides providing a source of carbohydrate, grain is also a natural source of dietary fiber, vitamins, minerals, specific oils, and other disease-fighting phytocompounds. Even though cereal grains contain relatively little protein compared to legume seeds, they provide protein for the nutrition of humans and livestock that is about 3 times that of legumes. Most cereal seeds lack a few essential amino acids; therefore, they have imbalanced amino acid profiles. Lysine (Lys), threonine (Thr), methionine (Met), and tryptophan (Trp) are among the most critical and are a limiting factor in many grain crops for human nutrition. Tremendous research has been put into the efforts to improve these essential amino acids. Development of high protein content can be outlined in four different approaches through manipulating seed protein bodies, modulating certain biosynthetic pathways to overproduce essential and limiting amino acids, increasing nitrogen relocation to the grain through the introduction of transgenes, and exploiting new genetic variance. Various technologies have been employed to improve protein content including conventional and mutational breeding, genetic engineering, marker-assisted selection, and genomic analysis. Each approach involves a combination of these technologies. Advancements in nutrigenomics and nutrigenetics continue to improve public knowledge at a rapid pace on the importance of specific aspects of food nutrition for optimum fitness and health. An understanding of the molecular basis for human health and genetic predisposition to certain diseases through human genomes enables individuals to personalize their nutritional requirements. It is critically important, therefore, to improve grain protein quality. Highly nutritious grain can be tailored to functional foods to meet the needs for both specific individuals and human populations as a whole.

Cadaverine: a lysine catabolite involved in plant growth and development

The cadaverine (Cad) a diamine, imino compound produced as a lysine catabolite is also implicated in growth and development of plants depending on environmental condition. This lysine catabolism is catalyzed by lysine decarboxylase, which is developmentally regulated. However, the limited role of Cad in plants is reported, this review is tempted to focus the metabolism and its regulation, transport and responses, interaction and cross talks in higher plants. The Cad varied presence in plant parts/products suggests it as a potential candidate for taxonomic marker as well as for commercial exploitation along with growth and development.

High-lysine maize: the key discoveries that have made it possible

Forty-five years ago, a paper published by Mertz et al. (Science 145:279-280, 1964) initiated a revolution in the history of plant protein quality and affected dramatically the study of cereal crop storage proteins. The observation of the high lysine content of the endosperm of the opaque-2 (o2) maize mutant was a key factor in bringing about a new concept in the production of cereal seeds with a high nutritional value. It has been a long and very interesting road with astonishing results over these 45 years. We are now probably about to see the release of commercially engineered high-lysine maize lines. We have decided to pinpoint some key contributions to the science behind high-lysine plants and concentrated on the research done on maize, which is possibly the most complete and simple example to illustrate the advances achieved. However, studies on other plant species such as barley and model species such as tobacco are totally relevant and will be briefly addressed.

Genetic manipulation of lysine catabolism in maize kernels

In plants, lysine catabolism is thought to be controlled by a bifunctional enzyme, lysine ketoglutarate reductase/saccharopine dehydrogenase (LKR/SDH). Lysine is converted to saccharopine, through condensation with alpha-ketoglutarate, by LKR, and subsequently to glutamate and alpha-aminoadipate-delta-semialdehyde by SDH. To investigate lysine catabolism in maize kernels, we generated transgenic plants with suppressed LKR/SDH activity in either endosperm or embryo. We found that the suppression of LKR/SDH in endosperm induced an increase in free lysine in developing endosperm, which peaked at 32 days after pollination. At later stages of kernel development, most of the free lysine was found in the embryo along with an elevated level of saccharopine. By combining endosperm LKR/SDH suppression with embryo LKR/SDH suppression through crosses, the saccharopine level in embryo was reduced and resulted in higher lysine accumulation in mature kernels. These results reveal new insights into how free lysine level is regulated and distributed in developing maize kernels and demonstrate the possibility of engineering high lysine corn via the suppression of lysine catabolism.

Lysine biosynthesis and nitrogen metabolism in quinoa (Chenopodium quinoa): study of enzymes and nitrogen-containing compounds

Aspartate kinase (AK, EC 2.7.2.4), homoserine dehydrogenase (HSDH, EC 1.1.1.3) and dihydrodipicolinate synthase (DHDPS, EC 4.2.1.52) were isolated and partially purified from immature Chenopodium quinoa Willd seeds. Enzyme activities were studied in the presence of the aspartate-derived amino acids lysine, threonine and methionine and also the lysine analogue S-2-aminoethyl-l-cysteine (AEC), at 1 mM and 5 mM. The results confirmed the existence of, at least, two AK isoenzymes, one inhibited by lysine and the other inhibited by threonine, the latter being predominant in quinoa seeds. HSDH activity was also shown to be partially inhibited by threonine, whereas some of the activity was resistant to the inhibitory effect, indicating the presence of two isoenzymes, one resistant and another sensitive to threonine inhibition. Only one DHDPS isoenzyme highly sensitive to lysine inhibition was detected. The results suggest that the high concentration of lysine observed in quinoa seeds is possibly due to a combined effect of increased lysine synthesis and accumulation in the soluble form and/or as protein lysine. Nitrogen assimilation was also investigated and based on nitrate content, nitrate reductase activity, amino acid distribution and ureide content, the leaves were identified as the predominant site of nitrate reduction in this plant species. The amino acid profile analysis in leaves and roots also indicated an important role of soluble glutamine as a nitrogen transporting compound.

Modifying lysine biosynthesis and catabolism in corn with a single bifunctional expression/silencing transgene cassette

Although it is one of the major crops in the world, corn has poor nutritional quality for human and animal consumption due to its low lysine content. Here, we report a method of simultaneous expression of a deregulated lysine biosynthetic enzyme, CordapA, and reduction of a bifunctional lysine degradation enzyme, lysine-ketoglutarate reductase/saccharophine dehydrogenase (LKR/SDH), in transgenic corn plants by a single transgene cassette. This is accomplished by inserting an inverted-repeat sequence targeting the maize LKR/SDH gene into an intron of a transgene cassette that expresses CordapA. This combination of LKR/SDH silencing and CordapA expression led to the accumulation of free lysine to over 4000 p.p.m. in transgenic corn grain, compared to less than 100 p.p.m. in wild-type controls. This intron-embedded silencing cassette design reduces the number of transgene cassettes needed in transgenic approaches for manipulating metabolic pathways that sometimes require expression of one gene and silencing of another.

Fractionation of zein by size exclusion chromatography

Zein is a group of alcohol-soluble corn proteins, which consists of several individual proteins. A single-step gel filtration chromatography method was developed to fractionate individual zeins from ethanol extracts of whole corn. A Superdex prep 75 column was used with different mobile phases to fractionate the zeins, which were analyzed by SDS-PAGE and UV spectrophotometry. With 70% aqueous ethanol as the mobile phase, fractions containing a mixture of alpha-zein/beta-zein and alpha-zein/delta-zein were obtained. With ammonium bicarbonate added to the 70% ethanol mobile phase, it was possible to obtain beta-zein and delta-zein fractions devoid of other proteins. However, all fractions containing alpha-zein also contained minor amounts of delta-zein and/or beta-zein. Almost all fractions also contained non-protein impurities.

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.

Lysine catabolism, an effective versatile regulator of lysine level in plants

Lysine is a nutritionally important essential amino acid, whose synthesis in plants is strongly regulated by the rate of its synthesis. Yet, lysine level in plants is also finely controlled by a super-regulated catabolic pathway that catabolizes lysine into glutamate and acetyl Co-A. The first two enzymes of lysine catabolism are synthesized from a single LKR/SDH gene. Expression of this gene is subject to compound developmental, hormonal and stress-associated regulation. Moreover, the LKR/SDH gene of different plant species encodes up to three distinct polypeptides: (i) a bifunctional enzyme containing the linked lysine-ketoglutarate (LKR) and saccharopine dehydrogenase (SDH) whose LKR activity is regulated by its linked SDH enzyme; (ii) a monofunctional SDH encoded by an internal promoter, which is a part of the coding DNA region of the LKR/SDH gene; and (iii) a monofunctional, highly potent LKR that is formed by polyadenylation within an intron. LKR activity in the bifunctional LKR/SDH polypeptide is also post-translationally regulated by phosphorylation by casein kinase-2 (CK2), but the consequence of this regulation is still unknown. Why is lysine metabolism super-regulated by synthesis and catabolism? A hypothesis addressing this important question is presented, suggesting that lysine may serve as a regulator of plant growth and interaction with the environment.

Are high-lysine cereal crops still a challenge?

The essential amino acids lysine and threonine are synthesized in higher plants via a pathway starting with aspartate that also leads to the formation of methionine and isoleucine. Lysine is one of most limiting amino acids in plants consumed by humans and livestock. Recent genetic, molecular, and biochemical evidence suggests that lysine synthesis and catabolism are regulated by complex mechanisms. Early kinetic studies utilizing mutants and transgenic plants that over-accumulate lysine have indicated that the major step for the regulation of lysine biosynthesis is at the enzyme dihydrodipicolinate synthase. Despite this tight regulation, recent strong evidence indicates that lysine catabolism is also subject to control, particularly in cereal seeds. The challenge of producing crops with a high-lysine concentration in the seeds appeared to be in sight a few years ago. However, apart from the quality protein maize lines currently commercially available, the release of high-lysine crops has not yet occurred. We are left with the question, is the production of high-lysine crops still a challenge?

Effect of the opaque and floury mutations on the accumulation of dry matter and protein fractions in maize endosperm

Grains of nine opaque (o) and floury (fl) mutants of maize (Oh43o1, Oh43o2, B79o5, B37o7, W22o10, W22o11, W22o13, Oh43fl1 and Oh43fl2) were examined for the weight proportions of their component tissues and the content of eight nitrogen fractions in their endosperms. A linear regression was found connecting the amounts (mg per endosperm) of zeins and true proteins (crude proteins minus non-protein nitrogen) for the non-opaque2 mutants. The data points connecting zeins to true proteins present in the mature endosperms of six wild-type (+) inbred lines and their o2 versions were located outside (+) or within (o2) the 95% confidence range of the regression line. The data obtained from the developing and mature endosperms of the W22o7 inbred line (Di Fonzo et al., Plant Sci. Lett., 1979, 77) and the floury portion of mature endosperms of three other wild-type inbred lines fell practically on the regression line. The effects of genotype and environmental factors upon the relative accumulation rate of zeins were assessed from the present results and the data taken from the literature concerning the quantitative interdependence between zeins and true proteins in immature and mature endosperms.

Isolation and characterization of enzymes involved in lysine catabolism from sorghum seeds

Lysine is an essential amino acid synthesized in plants via the aspartic acid pathway. The catabolism of lysine is performed by the action of two consecutive enzymes, lysine 2-oxoglutarate reductase (LOR, EC 1.5.1.8) and saccharopine dehydrogenase (SDH, EC 1.5.1.9). The final soluble lysine concentration in cereal seeds is controlled by both synthesis and catabolism rates. The production and characterization of high-lysine plants species depends on knowledge of the regulatory aspects of lysine metabolism and manipulation of the key enzymes. We have for the first time isolated, partially purified, and characterized LOR and SDH from developing sorghum seeds, which exhibited low levels of activity. LOR and SDH were only located in the endosperm and were very unstable during the isolation and purification procedures. LOR and SDH exhibited some distinct properties when compared to the enzymes isolated from other plant species, including a low salt concentration required to elute the enzymes during anion-exchange chromatography and the presence of multimeric forms with distinct molecular masses.

Effects of enzymatic deamidation by protein-glutaminase on structure and functional properties of alpha-zein

The performance of novel protein-glutaminase (PG) purified from Chryseobacterium proteolyticumon alpha-zein was investigated. Highly insoluble alpha-zein was able to be deamidated to the extent of deamidation degree 62% by using 50 mM potassium phosphate (pH 8) containing 11.7% ethanol, at 40 degrees C for 137 h. Analysis by sodium dodecyl sulfate polyacrylamide-gel electrophoresis showed that deamidated and non-deamidated zeins have different mobilities. Results of circular dichroism spectra revealed the decline in alpha-helix contents of alpha-zein by deamidation. Besides, Fourier transform infrared spectroscopy analysis demonstrated alterations in the secondary structure of alpha-zein by deamidation. The assignment of the amide I region showed a remarkable decrease in antiparallel intermolecular beta-sheets (around 1690 cm(-1)) as an indication of the weakening aggregation ability of the deamidated molecules. Solubility and emulsification properties of alpha-zein, particularly at pH 7, were remarkably improved after the deamidation by PG. Gas chromatography and peroxide value studies pointed out that deamidated alpha-zein in powder form exhibited an inferior antioxidative property as compared with the non-deamidated one.

Regulation of lysine metabolism and endosperm protein synthesis by the opaque-5 and opaque-7 maize mutations

Two high lysine maize endosperm mutations, opaque-5 (o5) and opaque-7 (o7), were biochemically characterized for endosperm protein synthesis and lysine metabolism in immature seeds. Albumins, globulins, and glutelins, which have a high content of lysine, were shown to be increased in the mutants, whereas zeins, which contain trace concentrations of lysine, were reduced in relation to the wild-type lines B77xB79+ and B37+. These alterations in the storage protein fraction distribution possibly explain the increased concentration of lysine in the two mutants. Using two-dimensional polyacrylamide gel electrophoresis of proteins of mature grains, variable amounts of zein polypeptides were detected and considerable differences were noted between the four lines studied. The analysis of the enzymes involved in lysine metabolism indicated that both mutants have reduced lysine catabolism when compared to their respective wild types, thus allowing more lysine to be available for storage protein synthesis.