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Wheat is the most widely grown crop throughout the world covering about 220 million hectares worldwide. Annually, approximately 725 metric tonnes are produced. The leaders in growing wheat are Russia, India, USA, Canada but wheat can be found across the world in every continent. It is also grown in Ireland but on a much smaller scale. Wheat is mainly used for human consumption but some is used for animal fodder. Wheat is mostly made into flour which in turn makes a wide variety of baked goods.
In recent years wheat production levels have not met the demand of a growing population. It is estimated by 2050 that there will be 9 billion people on the planet meaning wheat production will have to increase by 60%. To meet these demands, the annual wheat yield must rise. It is becoming a greater challenge to have a more efficient and sustainable wheat production.
Diseases have a direct implication on yield and quality of a wheat crop.
Wheat is a self-pollinating annual plant, belonging to the family Poaceae and genus Triticum. The two main types of commercial wheat are durum and bread wheats. Durum has 28 chromosomes and bread wheats have 42 chromosomes. There is still an importance to have wild wheat species as they contain useful agronomic traits to improve the quality of cultivated wheats. Improving agricultural practices and developing innovative cropping systems are a priority along with the need to enhance disease resistance against wheat.
Hybridization of wheat with grasses along with the cytogenetic manipulation of the hybrid material has been paramount in the improvement of wheat. Manipulation of pairing control mechanisms and induced translocations have been used to transfer into wheat plants, specific disease and pest resistance genes from annual members of the tribe Triticeae.
Using DNA markers helps to identify desirable genotypes more accurately and aids the gene transfer into wheat. The use of genetic engineering has produced better resistance in wheat plants to diseases but also has helped improve grain quality by modifying protein and starch profiles of the grain.
There is a continuous constraint on global wheat production posed by the disease Septoria tritici blotch. Despite resistant loci being identified, a commercially relevant STB-resistant wheat germplasm deficit remains. The issue is further compounded for wheat growers by the emergence of fungicide resistant strains of the causative pathogen Zymoseptoria tritici. Despite this issue, biotechnology-based research is providing new opportunities in the struggle against the disease. As the exome response of wheat to STB attack begins to be deciphered, genes intrinsic to resistant and susceptible phenotypes are identified. The application of genome-editing techniques, a growing appreciation of the complexity of wheats and the dynamism of Z. tritici genome, the generation of resulting STB-resistant wheat varieties will counter the prevalent threat of STB disease in wheat plants. (O’Driscoll et al., 2014).
The pathogenesis of the disease Septoria, consists of both a biotrophic phase and a necrotrophic phase. The host plant is infected by a pathogen by suppressing its immune response in the first stage of infection. Hemibiotrophic pathogens of the genus fusarium cause fusarium head blight and the necrotrophic parastagonosporanodorum is responsible for Septoria blotch in wheat. Cell wall degrading enzymes in plants promote infections by necrotrophic and hemibiotrophic pathogens. Trichothecenes and secondary fungal metabolites facilitate infections caused by fungi or the genus fusarium. There are no sources of complete resistance to these pathogens in wheat. Defence mechanisms in wheat are controlled by many genes encoding resistance traits. In the wheat genome, the characteristic features of loci responsible for resistance to pathogenic infections indicate that several dozen genes encode resistance to pathogens. The molecular interactions between wheat Z tritici, P nodorum and fusarium spp pathogens have been insufficiently investigated. Many studies focus on the mechanisms by which the hemibiotrophic Z tritici suppresses immune responses in plants and the role of mycotoxins and effector proteins in infections caused by P nodorum and Fusarium fungi. Effector proteins and trichothecne glycosylatation are involved in defense responses in wheat at a molecular level. Advances in molecular biology have produced interesting findings which could be researched as molecular interactions between wheat and fungal pathogens. A clustered regularly-interspaced short palindromic repeat associated system can be used to introduce targeted mutations into the wheat genome and confer resistance to selected fungal diseases including Septoria. Wheat pathogen interactions can be analysed by host induced gene silencing and spray induced gene silencing. These can develop new strategies to control fungal diseases. (Duba et al., 2018).
Yellow rust is a disease of wheat which occurs in cool and moist weather conditions while the crop is growing. Ireland has the perfect environment for yellow rust to cause damage to a wheat crop. It is caused by a fungus called Puccinia striiformis f. sp tritici (PST). The fungus is an obligate biotrophic parasite. It is a difficult fungus to culture on artificial media. PST is a macrocyclic, heteroecious fungus that requires primary and alternate host plants to complete its life cycle. The primary host plants would be grass or wheat. The alternate being Mahonia spp or Berberis. Urediniospores have the capacity for wind dispersal over long distances which could extend to hundreds of kilometres from its original infection site. (Chen W, et al.,2014)
Thinopyrum ponticum and Th.intermedium provide resistance against various wheat diseases including yellow rust. Many genes for enhancing disease resistance in wheat have been introduced from wheatgrass. The genes integrate as they are readily crossed with wheat. Genes for yellow rust have been transferred into wheat by producing chromosome translocations. These genes offer an opportunity to improve resistance of wheat to yellow rust and other diseases. Unfortunately, some of these genes have been extensively used to protect wheat from diseases, leaving the risk of new resistance to disease arising. (Li H and Wang X; 2009)
Near-isogenic thatcher lines carrying a gene for yellow rust resistance called Lr34 and other strains of seed displayed adult plant resistance to yellow rust was found. F2 and f3 populations from several inter crosses of wheats carrying Lr34 did not segregate for susceptibility to yellow rust indicating that at least one resistant gene was common in each parent. An evaluation of f3 lines for yellow rust from the crosses of yellow rust and leaf rust susceptible seed type Jupateco 73S with resistant Condor and Jupateco 73R showed linked segregation for the two diseases. Resistance was conferred by one partially dominant gene. Lr34 is located on the chromosome 7D which is linked to the Yr gene, because of this it is designated Yr18. Different varieties of wheat seed could be tested with genotypes for Yr18. (Singh, 2012)
Powdery mildew is a fungal leaf disease that reduces grain quality and yield in wheat varieties. It can be difficult to control once it is established. Powdery mildew is caused by the fungus Blumeria graminis f. sp. Tritici. The wheat thinopyrum intermedium gene addition disomic line germplasm SN6306 is an important source of genes for wheat resistance. SN6306 is highly resistant to mildew and to other wheat diseases. Knowledge of the resistant mechanism SN6306 still remains limited however. A study employed high throughput RNA sequencing based on next generation sequencing technology to obtain an overview of the transcriptome characteristics of SN6306 and Yannong 15 (YN15) its parent wheat during a powdery mildew infection. Varied expression level was seen from sequencing the generated 104773 unigenes. 9909 of these had a variation in expression level. Among the 9909 unigenes, 1678 unigenes expressed 0 reads in YN15. The expression levels in mildew inoculated SN6306 and YN15 of exactly 39 unigenes that showed none or considerably low reads in YN15 were validated to identify the genes involved in mildew resistance. 12 unigenes out of the 39 unigenes were upregulated in SN6306 by 3 to 45 times. These genes encoded synthase, kinase, signal transduction proteins and protease. These can play an important role in the resistance against mildew. To confirm whether the unigenes that showed that showed no reads in YN15 are unique to SN6306, eight unigenes were cloned and sequenced. From the results, it became clear that selected unigenes are more similar to SN6306 and TH. Intermedium than to the wheat cultivar YN15. The sequencing further confirmed that the unigenes showing no reads in YN15 are unique to SN6306 and are most likely derived from Th. Intermedium host. From the research, the genes from Th. Intermedium is most likely conferred the resistance of SN6306 to mildew. (Li Q, et al., 2016)
CRISPR-Cas9 technology enables geneticists to edit parts of a genome by adding, removing or altering sections of a DNA sequence. It is a versatile and precise method of genetic manipulation.
Sequence specific nucleases have been applied to engineer targeted modifications in polyploidy genomes. Simultaneous modification of multiple homoeoalles have not been reported. A transcription activator-like effector nuclease (TALEN) and regularly interspaced, short palindromic repeats and CRISPR Cas9 technologies in hexaploidy bread wheat was used to introduce targeted mutations in the 3 homoeolleles that encode mildew resistant proteins. Genetic redundancy has prevented evaluation of mutations of 3 mildew resistant alleles in wheat might confer resistance to powdery mildew. This trait is not found in natural populations. Talen induced mutations of all 3-mildew resistance homeologs in the same plant proved heritable broad-spectrum resistance to mildew. CRISPR-Cas9 technology was used to generate transgenic wheat plants carrying mutations in the mildew resistant protein allele. Feasibility of engineered targeted DNA insertion in bread wheat through nonhomologous end joining of the double strand breaks caused by transcription activator-like effector nuclease. (Wang Y, et al., 2014)
There is little information about the proteomic response of powdery mildew infection in resistant wheat. Quantitative proteomic analysis of a resistant wheat line N9134 was performed to explore the molecular mechanism of wheat defence against mildew. A total of 2182 proteins were quantified by iTRAQ at 24, 48 and 72-hour post inoculation with mildew by comparing leaf proteins of mildew inoculated N9134 with mock inoculated controls. Out of the 2182 proteins quantified, 394 showed differential accumulation. These differentially accumulated protein species mainly included pathogenesis related polypeptides, components involved in primary metabolic pathways and oxidative stress responsive proteins. KEGG enrichment analysis showed that phenylpropanoid biosynthesis, photosynthesis antenna proteins and phenylalanine metabolism were the key pathways in response to mildew infection. InterProScan 5 is a software package that allows protein and nucleic sequences to be scanned against interpro signatures. Gibbs Motif sampler is a software package for locating common elements in collections of biopolymer sequences. It can locate TFBS in unaligned DNA sequences. With the use of InterProScan 5 and Gibbs Motif Sampler, 394 DAPs were clustered into eight conserved motifs which shared histidine and leucine sites in the sequence motifs. A STRING database predicted eight separate protein-protein interaction networks. (Fu Y, et al.,2016)
In a report, it was found that the successful generation of a potentially valuable trait using genome editing technology in wheat provides germplasm for disease resistance breeding. Enhanced disease resistance plays a negative role in the defence response against powdery mildew in a plant called Arabidopsis thaliana. Arabidopsis thaliana is a small flowering plant used as a model organism in plant biology. The enhanced disease resistance (EDR) mutant does not show constitutively activated defence responses. This makes EDR an ideal target for approaches using new genome editing tools to improve resistance to powdery mildew. In the report, it was noted that wheat EDR from hexaploidy wheat was cloned and high similarity among the three homoelogs of EDR was found. Knock-down of wheat EDR by virus induced gene silencing or RNA interference enhanced resistance to powdery mildew, indicated that wheat EDR negatively regulates mildew resistance in wheat. Biotechnology methods were used to generate wheat EDR plants by simultaneous modification of the 3 homeologs of wheat EDR. There were no off-target mutations detected in the first EDR mutant plants. The wheat enhanced disease resistant plants were resistant to powdery mildew and did not show any mildew induced cell death. (Zhang Y, et al., 2017)
NAC transcription factors are transcriptional regulators in plants. They are involved in growth and development. They respond to abiotic and biotic stresses in plants. In a study, TaNAC6 was identified as a differentially expressed gene between 2 lines with broad spectrum resistance to powdery mildew. OEStpk-V, NAU9918 and corresponding susceptible isogenic lines SM-1 and Yangmai158 after mildew inoculation by transcriptome analysis expressed resistance. Three homoeologous genes of TaNAC6 were cloned and named as TaNAC6 A, B and D. A, B and D were subcellularly localized to the nucleus and displayed the transcriptional activation activity. A, B and D responded differently to pathogens and phytohormones. Transient over-expression of each of the TaNAC6s reduced the haustorium index of Yangmai158 and the stable transformation of TaNAC6 A enhanced its resistance to mildew. This implied that TaNAC6s play an important role in basal resistance. The silencing of TaNAC6s compromised the resistance of NAU9918 and OEStpk V suggesting that TaNAC6s play positive roles in broad spectrum resistance to mildew. JA induces TaNAC6s and the feedback regulates the JA pathway, leading to a better resistance to mildew. The role of TaNAC6s and their orthologous genes ATAF1 and HvNAC6 to mildew resistance implied that the NAC6 genes share a common signal pathway across species. (Zhou W et al., 2018)
The manipulation of genes stood out as the most common factor of enhancing disease resistance in wheat plants. The use of biotechnology led to the changing and mixing of specific genes in certain plants to reduce the chance of the plant being affected by a disease. Septoria is the disease which is causing most trouble for wheat growers. The pathogens responsible for causing the disease are difficult to avoid infection from. The changing of genomes to a certain extent is successful but a continuous development of the pathogen makes it harder for plants to be made resistant to the disease. The use of CRISPR-Cas9 technology and other biotechnologies makes it easier for scientist to manipulate DNA and to source the root of the infection problems. Disease resistant plants will be continued to be developed but the disease pathogens will continue to develop as well. There has been a considerable amount of research gone into the use of genetics to enhance disease resistance in wheat. It is a topic which is important as the world depends upon wheat for food. The research and work on improving wheat plants is global but so is the problem of disease affecting the growing of wheat. In some cases, people will look to wild strains of wheats and grasses to introduce new DNA into certified wheat seed. Despite all the information known, nature will bring more ideas and of course problems to wheat production.
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