Write an essay on the natural disease resistance mechanisms of plants. This should include details of anatomical defences, pre-existing protection, inducible systems and systemic responses.
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 place in plants and hence are beneficial for these plants. However, not all microbes are advantageous some are pathogenic and cause diseases in plants. There are around 450 species of viruses that cause diseases in plants (Soosar et al., 2005). Apart from viruses, bacteria, fungi, and certain insect pests also tend to infect plants.
To fight these viruses, plants have developed certain processes which 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 defence systems of plants involve certain cellular pathways as well as physiological characteristics. One of these systems is RNA silencing which is used as a defence mechanism against the foreign nucleotides of viruses. However to have a good and strong defence response, several cellular process act in combination. (Soosar et al., 2005)
R genes are those genes which are involved in imparting resistance from various pathogens including viruses (Soosar et al., 2005). There are different R genes in plants which 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. At some instances, these R genes play 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 (Soosar et al., 2005). These different genes are involved in defence mechanisms against specific pathogenic viruses.
One example of such an R gene-mediated defence response is the one which is found in the tomato plant against Cladosporium fulvum which causes leaf mould disease in tomato. The defence, 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 phenomena 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 the tomato plant, 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.) which 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 defence 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 which 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. (Dixon 2001). Certain phenolic compounds also have antifungal effects and hence help in protecting the plants form fungus pathogens (Lattanzio et al., 2006). 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 (Lattanzio et al., 2006). These are needed by plants for pigmentation, reproduction, growth, and pathogenic resistance plus other functions as well. These act as UV sunscreens, signalling compounds and internal chemical messengers (Lattanzio et al., 2006).
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.