Having a natural soil habitat, plants have evolved in an environment where there are many microorganisms. Most of these microbes help in the biosynthesis systems taking pace in plants and hence are beneficial for these plants.
However not all the microbes are advantageous some are pathogenic and cause diseases in plants. There are around 450 species of viruses that cause diseases in plants. Apart from viruses, bacteria, fungi, and certain insect pests also tend to infect plants.
To fight with these viruses, plants have developed certain processes that help them in prevention from the attack of these viruses. There are certain genes in the plants which help in developing resistance from these viruses and other pathogens as well.
The defense systems of plants involve certain cellular pathways as well as physiological characteristics. One of these systems is RNA silencing which is used as a defense mechanism against the foreign nucleotides of viruses. However to have a good and strong defense response, several cellular process act in combination
R genes are those genes that are involved in imparting resistance from various pathogens including viruses. There are different R genes in plants that are specific for different types of pathogens attacking the plant.
The underlying mechanism which is involved in the R gene is that these genes induce a hypersensitive response (HR) in the infected cells causing the programmed cell death of these cells and limiting the effect of the pathogen. In some instances.
These R genes play a role in systemic acquired resistance (SAR) other than PCD. The SAR is induced in the cells which are away from the site of infection, proteins from these R genes make these cells resistant to the infection from the attacking pathogen.
Some examples of R genes include N, Rx, Rx2, HRT, RCY1, Sw-5, Y-1, and Tm-2. These different genes are involved in defense mechanisms against specific pathogenic viruses. One example of such an R gene-mediated defense response is the one which is found in the tomato plant against Cladosporium fulvum which causes leaf mold disease in tomato.
The defense, as well as the disease, takes place under the action of gene pair, out of which, one is present in the host (R gene resistance gene) while the other is present in the pathogen (avirulence or Avr gene). The phenomenon underlying this combination is the fact that the plant which has this R gene would be resistant towards the pathogen which has the Avr gene.
When Cladosporium fulvum attacks tomato plants, the host plant recognizes the Microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs) which then mediate MAMP-triggered immunity (MTI) response.
The pathogen produces specific products (chitinases, proteases, etc.) that suppress the MTI response of the plant which is known as Effector Triggered Susceptibility. In response to ETS, plants that have developed an R gene induces Effector Triggered Immunity (ETI) leading to Hypersensitive Response or HR.
Two major classes of proteins that are involved in defense mechanisms are Nucleotide-binding leucine-rich repeat (NB-LRR) and C-terminal kinase domain (receptor-like kinase RL-K) (Fenyk et al., 2016). Furthermore, plants produce certain chemicals that we know as natural products that are involved in the resistance mechanisms against pathogenic microbes.
There are particular classes of compounds that are known to have antimicrobial effects and are produced by plants. These include terpenoids, ben-zoxazinone, and flavonoid/isoflavonoid, etc. Certain phenolic compounds also have antifungal effects and hence help in protecting the plants from fungus pathogens.
Now phenolics are natural compounds of plants that are not specifically released as a response to pathogens but have added benefits. Phenolics are secondary metabolites that are produced from shikimate-phenylpropanoids-flavonoids pathways.
These are needed by plants for pigmentation, reproduction, growth, and pathogenic resistance plus other functions as well. These act as UV sunscreens, signaling compounds, and internal chemical messengers.
Dixon, R.A., 2001. Natural products and plant disease resistance. Nature, 411(6839), pp.843-847.
Fenyk, S., Dixon, C.H., Gittens, W.H., Townsend, P.D., Sharples, G.J., Pålsson, L.O., Takken, F.L. and Cann, M.J., 2016. The Tomato Nucleotide-binding Leucine-rich Repeat Immune Receptor I-2 Couples DNA-binding to Nucleotide-binding Domain Nucleotide Exchange. Journal of Biological Chemistry, 291(3), pp.1137-1147.
Lattanzio, V., Lattanzio, V.M., and Cardinali, A., 2006. Role of phenolics in the resistance mechanisms of plants against fungal pathogens and insects. Phytochemistry: Advances in research, 661, pp.23-67.
Soosaar, J.L., Burch-Smith, T.M., and Dinesh-Kumar, S.P., 2005. Mechanisms of plant resistance to viruses. Nature Reviews Microbiology, 3(10), pp.789-798.
Ralstonia solanacearum is a bacterial plant pathogen that causes a lot of destruction in the kingdom Plantae in the whole world. These have a variety of strains that are specific in different regions of the world with particular adaptations
These can infect around 200 plants from almost 50 families which defines the broad range of this pathogen. It causes diseases like Moko banana disease, potato brown rot, and bacterial wilt of eggplant, tomato, and tobacco.
It is present in the soil and infects plants through the open wounds or cracks present in the roots usually. It colonizes first in the cortex cells of the root and then attacks the xylem tissue. Entering into the plant vascular system then allows it to attack the aerial parts of the plants including stem and leaves.
It multiplies rapidly once it is in the vascular system of the plant resulting in the wilting of plants and ultimately its death. Pili of type IV is known as the main virulence factors involved in the pathogenesis of this bacterium in potato plants.
The synthesis of these pili is under the control of tad genes and silencing or impairment of these genes causes the bacteria to lose virulence in potato. It has a broad phenotype which has led to the successful evolution of these strains and hence an attack many plant species.
Its wide host range among the economically important plant-like potato makes this pathogen and its control very significant. Having a wide geographical range, this pathogen causes a loss of around 1 billion dollars each year affecting the potato crops around the globe. It can also spread as an endemic making it all the way more destructive.
Elphinstone, J.G., Allen, C., Prior, P. and Hayward, A.C., 2005. The current bacterial wilt situation: a global overview. Bacterial wilt disease and the Ralstonia solanacearum species complex, pp.9-28.
Genin, S., 2010. Molecular traits controlling host range and adaptation to plants in Ralstonia solanacearum. New Phytologist, 187(4), pp.920-928.
Genin, S., and Denny, T.P., 2012. Pathogenomics of the Ralstonia solanacearum species complex. Annual review of phytopathology, 50, pp.67-89.
Mansfield, J., Genin, S., Magori, S., Citovsky, V., Sriariyanum, M., Ronald, P., Dow, M.A.X., Verdier, V., Beer, S.V., Machado, M.A. and Toth, I.A.N., 2012. Top 10 plant pathogenic bacteria in molecular plant pathology. Molecular plant pathology, 13(6), pp.614-629.
Wairua, C.K., Van der Waals, J.E., Van Schalkwyk, A., and Theron, J., 2012. Ralstonia solanacearum needs Flp pili for virulence on potato. Molecular Plant-Microbe Interactions, 25(4), pp.546-556.
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