Molecular pharming in cereal crops

There are many different agricultural expression systems that can be used for the large-scale production of recombinant proteins, but field-grown cereal crops are among the most attractive because recombinant proteins can be targeted to accumulate in the seed, and specifically in the endosperm, which has evolved naturally as a protein storage tissue. Within the developing endosperm, proteins are supplied with molecular chaperones and disulfide isomerases to facilitate folding and assembly, while the mature tissue is desiccated to prevent proteolytic degradation. Proteins expressed in cereal seeds can therefore remain stable for years in ambient conditions. Recent basic research has revealed a surprising diversity of protein targeting mechanisms in the endosperm, which can help to control post-translational modification and accumulation. Applied research and commercial development has seen several pharmaceutical proteins produced in cereals reach late stage preclinical development and the first clinical trials, with a number of companies now dedicated to developing cereal-based production platforms. In this review we discuss the basic science of molecular pharming in cereals, some of the lead product candidates, and challenges that remain to be addressed including the emerging regulatory framework for plant-made pharmaceuticals.     

Distribution of aphids in cereal crops

Cereal crops were examined weekly for aphids during 1969. Plants in twenty samples of row 0.3 m long were examined in a sheltered perimeter of a crop and along a transect 36.6 m into the crop. Aphids were usually first found within 1–4 weeks of the first alatae caught in a suction trap operating 12.2 m above ground. When first alatae caught in a suction trap operating 12.2 m above ground. When the first found from 10 to 27% of the 0.3 m lengths sampled contained aphids. Rhopalosiphum padi, first found late in May, were scarce (< 0.53/0.3 m) throughout June and July. Sitobium spp. and Metapolophium dirhodum, which appeared in mid-June, were more numerous than R. padi; most occurred during the second half of July, and populations decreased just before harvest in early August. Sitobium avenae was more abundant (max. 19.3/sample) than either S. fragariae (0.91) or M. dirhodum (2.51). More aphids occurred in oats (max. 52/0.3 m) during July than in wheat (45), and barley had fewer (6.8). S. avanae was more abundant than M. dirhodum in sheltered areas of barley and wheat, and in exposed areas of the same crop M. dirhodum was commonest. Along sheltered perimeters, the ratio of S. avenae to M. dirhodum was largest in barley (11:1), intermediate in oats (6:1) and smallest in wheat (3.7:1). Sitobium spp. were most numerous on the ears, when most M. dirhodum were on the leaves. Regression analyses of log. S² on log. m suggested that S. avenae was more evenly distributed within (36.6 m) the field (b = 1.056 + 0.109) than along the sheltered perimeter (b = 1.432 + 0.132), though it seemed similarly distributed along perimeters of barley, oats and wheat. The distributions of M. dirhodum and Sitobium spp. along sheltered perimeters of all crops were apparently similar.     

The molecular genetic mapping of cereal crops

The application of modern methods of genetic mapping using RFLP and PCR technologies allowed to advance essentially in construction of rye genome genetic maps and mapping of some morphological and breeding-valuable genes. Genetic mapping of cereal genomes, such as rye, wheat, maize and rice using common set of DNA-probes permitted to reveal considerable evolutionary conservation in gene organization and localization. This allows to use more effectively method of comparative mapping for fast localization and tagging of genes in genomes of less investigated species.     

Genome editing in cereal crops: an overview

Transgenic Research - Genome-editing technologies offer unprecedented opportunities for crop improvement with superior precision and speed. This review presents an analysis of the current state of...     

Future prospects for transgenic crops

Transgenic (genetically modified) crops are grown at present on more than 40 million hectares in 13 countries around the world. These `first generation' products principally comprise soybean, maize (corn) and cotton resistant to herbicides and/or insects. This review considers the wide range of `second generation' products under development and testing in many commercial and academic laboratories. Such products include examples for a variety of food, medical, veterinary and industrial purposes. In addition the review assesses the present state of public acceptance of transgenic products.     

Damage to cereal crops by larks in north-western Iraq

Two types of lark-caused damage occurred in cereal crops in north-western Iraq: grazing by skylarks and digging by calandra larks. The level of damage in large-scale cereal plantings was low but extensive damage, resulting in lowered grain yields, occurred in some experimental trials and made evaluation of cereal cultivars difficult. The amount of grazing varied amongst cultivars, differences being more pronounced with grazing than digging.     

Root system architecture: insights from Arabidopsis and cereal crops

Roots are important to plants for a wide variety of processes, including nutrient and water uptake, anchoring and mechanical support, storage functions, and as the major interface between the plant and various biotic and abiotic factors in the soil environment. Understanding the development and architecture of roots holds potential for the exploitation and manipulation of root characteristics to both increase food plant yield and optimize agricultural land use. This theme issue highlights the importance of investigating specific aspects of root architecture in both the model plant Arabidopsis thaliana and (cereal) crops, presents novel insights into elements that are currently hardly addressed and provides new tools and technologies to study various aspects of root system architecture. This introduction gives a broad overview of the importance of the root system and provides a snapshot of the molecular control mechanisms associated with root branching and responses to the environment in A. thaliana and cereal crops.     

Comparative analysis of multiple disease resistance in ryegrass and cereal crops

Ryegrass (Lolium spp.) is among the most important forage crops in Europe and Australia and is also a popular turfgrass in North America. Previous genetic analysis based on a three-generation interspecific (L. perenne x L. multiflorum) ryegrass population identified four quantitative trait loci (QTLs) for resistance to gray leaf spot (Magneporthe grisea) and four QTLs for resistance to crown rust (Puccinia coronata). The current analysis based on the same mapping population detected seven QTLs for resistance to leaf spot (Bipolaris sorokiniana) and one QTL for resistance to stem rust (Puccinia graminis) in ryegrass for the first time. Three QTLs for leaf spot resistance on linkage groups (LGs) 2 and 4 were in regions of conserved synteny to the positions of resistance to net blotch (Drechslera teres) in barley (Hordeum vulgare). One ryegrass genomic region spanning 19 cM on LG 4, which contained three QTLs for resistance to leaf spot, gray leaf spot, and stem rust, had a syntenic relationship with a segment of rice chromosome 3, which contained QTLs for resistance to multiple diseases. However, at the genome-wide comparison based on 72 common RFLP markers between ryegrass and cereals, coincidence of QTLs for disease resistance to similar fungal pathogens was not statistically significant.     

The toxicity of aphicide residues to beneficial invertebrates in cereal crops

The toxicity of dimethoate, deltamethrin and pirimicarb residues to Bembidion lampros and Coccinella septempunctata was evaluated by confining groups of insects to winter wheat foliage and soil for 24 h at different times after treatment in the field. Flag leaf residues were found to be more toxic than first leaf residues: soil residues were the least toxic with pirimicarb showing virtually no soil toxicity. In general, dimethoate and deltamethrin showed similar levels of foliar toxicity with flag leaf toxicity on the first day after treatment being in the range 60–80% for B. lampros; deltamethrin was however, less toxic than dimethoate at ground level. Both of these products were more toxic than pirimicarb. The long-term exposure of insects, surviving the 24 h bioassays, to treated soil at different times following application resulted in further mortality and provided estimates of the maximum levels of mortality that populations of predators might suffer migrating into the crop at different times following application. Dimethoate was shown to be particularly harmful at the current recommended field application rate and reduced doses were proposed to limit the severity of the initial effects.     

The effect of cereal break crops on barley mild mosaic virus

The effects of growing one, two or three years of resistant barley or winter wheat on barley mild mosaic virus were studied in experiments on two naturally-infested sites in Gloucestershire and Cambridgeshire. Disease incidence and yield of the susceptible cultivars Igri and Maris Otter following a three year break were not significantly different from those of control plots that had grown continuous susceptible barley. The effects of cropping sequence treatment on soil populations of the fungus vector, Polymyxa graminis, were assessed by estimating most probable numbers of propagules following bioassays. Variability was large and neither total numbers of propagules, nor those carrying virus, was significantly affected by the cropping treatments.     

Genetic control of different types of the aphid resistance in cereal crops

Inheritance of the two main types of the plant resistance to insects was investigated in the sorghum-greenbug (Schizaphis graminum Rond.) and wheat-bird cherry-oat aphid (Rhopalosiphon padi L.) interaction systems. The data obtained support the hypothesis that antixenosis (avoiding of the plant by the insect, given a choice) and antibiosis (adverse effect of the plant on the insect feeding on it) are pleiotropic manifestations of the same genes. This is confirmed by the following facts. (1) Identical patterns of segregation for antixenosis and antibiosis in different cases of sorghum resistance to the greenbug: monogenic control (gene Sgr4), digenic control (Sgr1, Sgr2 and Sgr7, Sgr8), and complementary action of the genes (Sgr9 and Sgr10). (2) Correlated changes in the levels of antibiosis and antixenosis during long-term reproduction of a greenbug clone on the resistant sorghum variety k-1206 (resistance controlled by one gene). (3) Simultaneous expression of antixenosis and antibiosis in F₃ wheat hybrid families to the bird cherry-oat aphid.     

Genetic enrichment of cereal crops via alien gene transfer: New challenges

Genetic improvement of crops has traditionally been achieved through sexual hybridization between related species, which has resulted in numerous cultivars with high yields and superior agronomic performance. Conventional plant breeding, sometimes combined with classical cytogenetic techniques, continues to be the main method of cereal crop improvement. More recently, through the introduction of new tools of biotechnology, crossing barriers have been overcome, and genes from unrelated sources have become available to be introduced asexually into plants. Cereal crops were initially difficult to genetically engineer, mainly due to their recalcitrance to in vitro regeneration and their resistance to Agrobacterium infection. Systematic screening of cultivars and explant tissues for regeneration potential, development of various DNA delivery methods and optimization of gene expression cassettes have produced transformation protocols for the major cereals, although some elite cultivars still remain recalcitrant to transformation. Most of the transgenic cereals developed for commercial purpose exhibit herbicide and/or insect resistance; traits that are often controlled by a single gene. In recent years, more complex traits, such as dough functionality in wheat and nutritional quality of rice have been improved by the use of biotechnology. The current challenges for genetic engineering of plants will be to understand and control factors causing transgene silencing, instability and rearrangement, which are often seen in transgenic plants and highly undesirable in lines to be used for crop development. Further improvement of current cereal cultivars is expected to benefit greatly from information emerging from the areas of genomics, proteomics and bioinformatics.     

Shoot apical meristem: A sustainable explant for genetic transformation of cereal crops

Summary Immature zygotic embryo has been the widely used explant source to develop embryogenic callus lines, cell suspensions and protoplasts for transformation of cereal crops including maize, wheat, rice, oat, barley, sorghum, and millet. However, the lack of competence of immature embryos in certain elite lines is still a barrier to rontine production of transgenic cereal crops in certain commercial cultivars. In addition, a great deal of effort is required to produce immature embryos, manipulate cultures, of immature embryos or their cell suspensions, and cryoperserve cultures for further use. In addition, undifferentiated cells may have reduced regenerability after a few months, of in vitro culture. Alternative explants and regeneration systems for efficient transformation of cereal crops are needed to avoid or reduce the above limitations. During the past decade, scientists have successfully manipulated the shoot apical meristerms from seedlings of maize oat, sorghum, millet, wheat, and barley in an effort to develop a less genetype-dependent and efficient cereal regneration system that can be maintained in vitro for long pertiods of time without the need for cryopreservation. Furthermore, apical mesistem regeneration systems were used to stably transform maize, wheat, rice, oat, barley, sorghum, and millet.     

Identification of selenosugars and other low-molecular weight selenium metabolites in high-selenium cereal crops

Several novel selenium containing compounds were characterized in staple crops (wheat, rice and maize) grown on soils naturally rich in selenium. A dedicated method based on the coupling of liquid chromatography with multiplexed detection (ICP-MS, ESI-Orbitrap MS(/MS)) was developed for the speciation of low-molecular weight (<5 kDa) selenium metabolites. Nine species present in different proportions as a function of the crop type were identified by cation-exchange HPLC-ESI-Orbitrap MS on the basis of the accurate molecular mass and MS/MS spectra. The natural origin of these species was then validated by varying extraction conditions and by using hydrophilic interaction LC (HILIC)-ESI-Orbitrap MS(/MS). Among the identified compounds, Se-containing monosaccharides (hexose moiety, m/z 317 and m/z 358) or Se-containing disaccharides (hexose-pentose moiety, m/z 407 and m/z 408) were the first selenosugars reported in edible plants. It is also the first report of the presence of 2,3-dihydroxypropionyl-selenolanthionine (m/z 345) in rice. Because these crops can be an important source of selenium in animal and human nutrition, the understanding of the origin and the fate of these species during metabolic processes will be of great interest.     

Striga infestation of cereal crops - an unsolved problem in resource limited agriculture

The parasitic weed Striga causes devastating losses in cereal yields in sub-Saharan Africa. The parasite lifecycle is intimately linked with its host via a complex interchange of signals. Understanding the molecular basis of these interactions and of host resistance to Striga is essential for the identification of genes for improving crop yield via biotechnological or marker assisted breeding strategies. Cloning and sequencing of ESTs from the 'model' parasite Triphysaria versicolor is facilitating the identification of parasitism genes. The identification of resistance to Striga in sorghum and rice germplasm is allowing molecular dissection of these traits using genomic platforms and quantitative trait loci (QTL) analysis. QTL underlying different resistance phenotypes have been identified and the use of advanced backcross populations is allowing the exploitation of sources of resistance in wild relatives of cereals.     

The effect of field margins on the yield of sugar beet and cereal crops

The effect of field margins on the yield of sugar beet, wheat and barley was studied on commercial farms and in a series of field experiments from 1992–1997. There was always a trend of increasing yield from the edge of the field to the centre, with a marked reduction around the ‘tramlines’ and the area where machinery turns. In the studies on commercial farms, headland yield loss varied widely. In sugar beet the headlands yielded 19–41% less than the centre, with a mean reduction of 26%. In cereals the range was 3–19%, with a mean loss of 7%. Headland yield reductions were generally smaller in the field experiments than those found on commercial farms. These headland effects did not move towards the centre of the field when grass margins were planted at the edge of the field; there was no significant effect on the yield of the adjacent crop. The presence of boundary trees had the greatest effect on yield: in the outer 9 m of the field, the area shaded by trees produced 4.4 t ha^-1 of wheat, and the area that was not shaded 8.1 t ha^-1. Turning of machinery also significantly reduced yield, while grazing by rabbits and hares surprisingly had no effect. Following the reform of the Common Agricultural Policy in 1992, the main effect of which was to change from a price support policy to direct payments to producers, farmers in the European Union who produce more than a specified tonnage of ‘eligible crops’ per year, are required to fallow a given percentage of their land (currently 5%), to qualify for Arable Area Payments. Growers can elect to fallow fields on a rotational basis, or permanently. Headland set-aside is a term used to describe strips of set-aside, a minimum of 20 m wide around the edges of fields. In these experiments, the headland effect did not extend beyond 20 m from the field edge. Therefore, particularly in fields with boundary hedges or trees, headland set-aside could effectively remove the poor-yielding area at the field margin.