This research illustrates different approaches of that have been taken to develop resistance against pathogens in plants. In this research, resistance against different pathogenic microorganisms will be addressed. The reliability of transgenic disease resistant plants is under close scrutiny following the negative implications of genetic modification. Nonetheless, this research emphasizes on the importance of crop protection against pathogens following the fact that use of chemical compounds to control pathogenic microorganisms is unsustainable and harmful to the environment. The pros and cons of transgenic disease resistance will be evaluated as well as the current situation regarding the development of transgenic crop protection.
Transgenic disease resistance, antimicrobial proteins, genetic modification
Chapter 1: Introduction
Plant disease resistance protects the plants against pathogens through either infection-induced responses of the immune system or mechanisms initiated by the plants immune system. Plants that exhibit little disease damage despite having substantial pathogen levels are regarded to as disease resistance. The effect of the disease pathogen, however, is determined by a 3-way interaction of the pathogen, the degree of response by the plant, and the prevailing environmental conditions. Mechanisms to engineer durable disease resistance in commercial plants has been hampered by the complexity of disease resistance signaling and the diversity of infection mechanisms of pathogens. Nevertheless, scientists have stepped up efforts and resources in ensuring that they can generate disease resistance plants, which aligned with the economic viability of modern agriculture. To fight pathogens effectively, scientists have been challenged to address different perspectives that regard the growth and development of plants. These perspectives entail understanding how plants relate with the environment, with respect to different pathogens (Campbell et al., 2002). Scientists, for instance, have discovered that different pathogens affect plants based on various environmental conditions. Implicitly, scientists require to alter the genetic structure of different plants based on regional aspects, which are affiliated to different climatic conditions. To achieve the objectives of developing disease resistance in plants, scientists can either: carry out a direct interference with the pathogenicity or inhibition of pathogen physiology, alter the naturally induced host defense mechanism, carry out pathogen mimicry, or pathogen-derived resistance (PDR).
Chapter 2: Various mechanisms of Crop Resistance in Plants
Different mechanisms have been devised to develop disease resistance in plants following several years of research and development on crop science. Although some mechanisms have been tested successfully, other mechanisms have been varying based on the mutation of different pathogens dynamics experience in various climates.
Disease outbreaks have been found as the core challenges of crop production across the world. Disease management, therefore, has become a core objective in attempts to increase production yields across the continent. Disease resistance in plants is enhanced crop resistance genes, which have the ability to detect pathogens and initiate the respective counter attacks against the pathogens. Although R-genes are widely used in developing disease resistance in plants, they have portrayed a varying degree of success following extensive changes in climatic conditions as well as the genetic structure in these plants. Nevertheless, scientists have established that taming the R-genes is manageable, owing to the advancement in bioinformatics, genomic, and molecular biology techniques.
2.1 Antimicrobial Agents
Antimicrobial resistance is the resistance of a microorganism to an antimicrobial drug that was originally effective in its treatment, mainly because of mutation or an alteration in the genetic structure of these organizations. Antimicrobial agents either kill microorganisms or inhibit their growth. They fall under different categories depending on the type of microorganisms they affect, for instance, antifungals act on fungi while antibacterial act on bacteria.
2.2 Antimicrobial Proteins
Antimicrobial proteins kill bacteria, fungi, and any foreign cellular microorganisms. Most eukaryotes depend upon microbial peptides for protection against pathogens by either disrupting the structure or functions of the microbial cell membranes. Since, antimicrobial proteins are readily found in insects, fungi, and animals, genetic engineering techniques have been successfully used to enhance resistance in most commercial crops such as apples (). Leonard, Evan, Hans (2011), assert that antimicrobial peptides (AMPs) have a significant contribution to the immune system of the plants, which enables the organism to prevent any attacks by microorganisms.
2.3 Antimicrobial Metabolites
Antimicrobial metabolites emanate from a process of cellular metabolism. The intermediates and products of metabolism are used to kill and prevent pathogens from interfering the physiological processes within these cells.
2.4 Detoxification of Toxins
Detoxification of toxins refers to the removal of toxins from infected plants once they have been infected by pathogens. Detoxification of toxins in plants is a short term and normally ineffective approach that is used to treat plants that have already been subdued by the pathogens. Additionally, these approaches are ineffective and costly and rarely used in the modern agriculture.
2.5 The Role of Reactive Oxygen Species in Plant Disease Resistance
Different pathogens use various mechanisms to penetrate plant cell contents following systematic mutative processes. Oxidative burst is a phenomenon characterized by the production of reactive oxygen species (ROS) following a continued assimilation of molecular oxygen. This process occurs when the host organisms interacts with the pathogen under specific physiological and environmental conditions. In most incidences of hormonal responses in plants, ROS functions as second messengers (). For instance, Joo et al. (2001) showed that ROS could be asymmetrically generated in roots and transported to regions of reduced growth through gravistimulation. Root curvature during tropisms was caused by inhibited growth in regions that lacked adequate nutrients and air.
2.6 System Acquired Resistance
System Acquired Resistance occurs whenever plants are infected with necrotizing pathogens, they gradually develop resistance to further pathogen attacks from a similar pathogen. This form of resistance occurs under special conditions especially when the plants grow under stable climatic conditions. During every subsequent attack, these plant will be primed to an effective response and the response is improved every other attack experienced during the growth and development process. Scientists have realized that by comprehending the mechanisms of system acquired resistance (SAR), they can develop techniques and antimicrobial agents that are effective and resistance to changing climatic conditions. The ability of plants to respond effectively to previous attacks was evidenced by Beauverie and Ray in the onset of twentieth century. After years of extensive responses, developments on the initial assertions indicated that a physiological and genetic process, plants become more adopted to consistent attacks from similar pathogens.
Chapter 3: Resistance Gene
According to Crute & Pink (1996), Selective breeding of crop species has significantly contributed to the development of knowledge on plant resistance pathogens. Isolation and sequencing of putatively interacting genes of plants and pathogens has facilitated the feasibility of attempts to understand the mechanism through which pathogen resistance is mediated in plants. The extensive approach to this knowledge is essential because it facilitates the formulation of techniques that could develop durable effects of disease resistance genes.
3.1 Resistance Gene Mediated Anti-pathogen
According to Campbell, Fitzgerald, & Ronald (2002), strategic measures require to be established through engineering pathogen resistance in crop plants to mitigate crop damage caused by pathogens. Traditional pathogen prevention approaches and use of chemical compounds have proved to be unsustainable; hence, advanced technological such as developing transgenic plants that express components of defense signaling pathways. RNA silencing of pathogens and plant transcript has also been successfully applied to reduce the destructive effects of pathogens.
3.2 The Main Class R Proteins
The main class of R genes comprises of a leucine rich repeat (LRR) domain and a nucleotide binding (NB) domain and a symbolically illustrated as (NB-LRR) R genes. The NB-LRR R genes are further categorized into coiled-coil (CC-NB-LRR) and toll interleukin 1 receptor (TIR-NB-LRR). During pathogen host-interaction, the R protein interacts with the avirulence gene (Avr gene) of the pathogen and in the process guards another R protein that detects the degradation process of the R gene by the Avr gene. The R protein further encodes the enzyme that degrades the toxins produced by the pathogen. This detection process enables the host (plant) to mount defense against the pathogen. In order to enhance resistance in their target crops, scientists obtain R genes from resistant organisms and transfer them into the target plant.
The R genes exist in the plant genome. Their major function is to convey resistance against pathogen by producing the R proteins. Resistance genes often do not act as a specific receptor for a specific molecule produced by the pathogen. Ellis & Jones (1998) carried out a study to illustrate the similarities in the amino acid sequence between plant disease resistances (R) proteins and animal proteins such as Apaf-1 and CED-4. According to the study, the two researchers found out that the two conceptual models could be effectively utilized in developing conceptual models that would form the basis for the study of resistance protein functions. Ellis & Jones (1998) obtained data from extensive DNA sequencing of resistance gene families, from which they verified that leucine-rich repeat motif is important in determining gene-for-gene specificity. Additionally, they concluded that the exchange of DNA sequences is a major contributor to R gene diversity.
3.3 Bacterial Resistance Gene
Bacterial resistance genes normally cause resistance against antibiotics following alteration of the genetic material. Bacterial resistance genes are the most adapted to various antibiotics, which makes it difficult for scientists to establish durable antibiotics to protect crops against bacteria.
3.4 Fungal Resistance Genes/Resistance Genes Oomycyte
Fungal diseases, which are also very common in plants are responsible for increased crop damage and reduced crop production. To enhance resistance against fungal attacks in plants, scientists insert antifungal genes to confer resistance against such pathogens. The most advanced genes include the cell wall-degrading enzymes, which produce fungal-resistant transgenic crop plants (Ceasar & Ignacimuthu, 2012).
3.5 Eggs Mildew-resistance Gene
Some insects lay their eggs on plant leaves, which interferes with effective respiration and photosynthesis. This kills the leaf and tampers with the effective development of the entire plant. Eggs mildew-resistance gene protects the leaf by secreting specific egg poisons while others attract predators to eat the eggs through developing special signals. According to Worrall, Holroyd, Moore, Glowacz, Croft, Yaylor, Paul, & Roberts (2012), treating seeds with activators of plant defense has been found to generate a long-lasting priming of resistance of plants against pests. This study was significant and influential in the development of disease resistance plants because of their long lasting effect on the resultant plants. Consequently, this approach would reduce future costs in the production of such crops. During this study, Warrall, (2012), resistance of tomato plants to arthropod herbivores and disease was measure on plants grown from plants that had been treated with jasmonic acid and beta-aminobutryric acid (BABA). The research indicated that plants grown from seeds that had been treated with jasmonic acid showed increased resistance against herbivory by spider mites, aphids, caterpillars, as well as necrotrophic fungal pathogen, Botritis cinerea (Warrall, 2012). Beta-aminobutryric acid treatment was found to provide a primed defense against powdery mildew disease.
3.6 Nematode Resistance Gene
Being the most prevalent microorganisms on earth, parasitic nematodes have transformed various plants into very resistant organisms. Plants protect themselves against these microorganisms through developing specific resistant genes. Scientist obtain these genes from resistant plants and use them to clone plants that are cultivated for commercial use. The nematode genes operate by initiate a death of cells of the host that are in proximity to the feeding cites of the endoplasmic worm.
3.8 R Gene with a wide range of Host Resistance
R genes have also been engineered through advanced research to provide resistance against a wide range of viral, bacterial, and fungal, among other causes. Developing R genes with a wide range of host resistance is important although, only a small minority of pathogenic microorganism can infect a wide range of plant species (Gururani et al., 2012).
According to Thakur and Sohal (2013), effectiveness in the utilization of fungicides, bactericides, and insecticides play a significant role in the disease control process. Although insecticides are being largely used in disease protection against crops, advanced scientific procedures have been established to modify the plant’s genome so as to prevent them against pathogens. Insecticides are chemical compounds that are toxic to pathogens and pathogen-inducing microorganisms. Thakur and Sohal (2013) assert that chemical compounds have long term harmful effects on other organisms or the general environment. Chemical compounds tend to alter the ecosystem because besides harmful pathogens, they tend to affect other organisms existing within the same environment with the target plants including humans. Less harmful approaches of disease protection are likely to improve crop production through providing effective genetic resistance against pathogens. In this case, Thakur and Sohal (2013) assert that elicitors can be used to effectively to activate chemical defense in plants against pathogens. This process entails the activation of various biosynthetic pathways n treated plants, which is characterized by the type of elicitors used in the process. The common chemical elicitors used in the activation of biosynthetic pathways include methyl salicylate, benzothiadiazole, benzoic acid, Chitosan, and Salycylic Acid.
Chapter 4: Transgenic Disease Resistant Plants
The current prevalence of transgenic crops across various countries is still faced by major challenges following the dynamics in environmental changes and resistances in most pathogens. According to Collinge, Jørgensen, Lund, and Lyngkjær (2010), most of these transgenic plants carry disease resistance traits, but only represent a small proportion of the plants in developed countries, especially North America. Although genetic engineering to develop these plants is on the rise, implementing these antimicrobials in all crops across the world is sophisticated and could even experience more challenges because all these genetically modified organisms must be approved by the relevant authorities to ensure they do not interfere with the effective traits that define these plants. According to Boyd, Ridout, O’Sullivan, Leach, and Leung (2013), there is a drastic growth in human population, which is entirely dependent on agriculture. Agriculture, however, experiences major drawbacks following the destructive destruction by pathogens. Boyd et al. (2013) asserts that for effectiveness in crop production, advancement in genetic engineering is inevitable. Transgenic resistance is advantageous because of its significant degree of protection against pathogens. Transgenic protection is also economic because transgenic disease resistant plants do not require constant protection against pathogens.
Transgenic Resistance against Virus
Sakaya et al. (2013) carried out a case study to find out how the destructive effect of viruses could be eradicated to increase production in rice farming. Use of genetic engineering has been initiated in the current attempts to eradicate viruses that are transmitted via insect vectors. Use of viral genes has been used to confer pathogen-derived resistance against crops. Additionally, some investigators have successfully used RNA interference (RNAi) (commonly known as RNA silencing) to confer resistance against plant viruses. In the case study, transgenic approaches were used to confer resistance against reoviruses and tenuiviruses in rice plants. Use of RNA silencing found to have an effective resistance against viruses (Karthikeyan et al., 2013).
Different transgenic approaches are very influential in enhancing effective resistance in plants amidst significant steps in genetic engineering. Genetic engineering has been effectively implemented in most crops to develop resistance which has protected them against infections by pathogens. Different strategic measures have been initiated through extensive crop improvement programs in the recent past because there is a significant degree of efficiency in the development approaches that have been initiated over the past decades. Currently, extensive measures are necessary because there is a significantly high demand of agricultural produce; hence, crop destruction should be reduced significantly. Advancement in genomic, bioinformatics, and molecular biology techniques have improved the efficiency and complexities of enhancing crop resistance among most crops. Continued research and develop in genetic engineering is thus projected to increase the percentage of transgenic disease resistant plants available for crop production. The research is further expected to reduce any side effects of gene alteration among most exotic crops.
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