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Abstract: Sugarcane is an important commercial crop that is the primary source of sugar and the second-largest source of biofuel worldwide. India is the second-largest producer of sugarcane next to Brazil. The genus Saccharum consist of 6-37 species depending on the taxonomic interpretation. Sugarcane is now grown for fibre and energy, primarily for ethanol and electricity from bagasse and bio manure from the filter cake. In India, the major sugarcane producing states are Uttar Pradesh, Maharashtra, Tamil Nadu, Karnataka and Gujarat. Considering the current trend in sugarcane breeding, the primary concern corresponding to low yield are diseases caused by insects and other pathogens. Among all the conditions, red rot of sugarcane, also called the cancer of sugarcane, is a severe disease reported predominantly in India. The disease is caused by a fungus named Colletotrichum falcatum Went, perfect stage (Glomerella tucumanensis (Speg.) Arx and Muller). Since cultivated sugarcane was easily affected by this fungus, the new disease-resistant varieties were formed through interspecific hybridization, which failed at some intervals due to varietal breakdown. This led to severe epiphytotic red rot in India, in Uttar Pradesh and Bihar regions during the 1990s. The reason behind the continuous emergence of new virulent strains of C. falcatum is an emerging concern as they possess challenges to host resistance in newly developed varieties of sugarcane. Recently complete genome and transcriptions of C. falcatum were sequenced, and pathogenicity hotspot and some proteins have been identified, which will help in knowing the inheritance pattern of C. falcatum and even may help in breeding disease-resistant sugarcane varieties.
Sugarcane is an important commercial crop grown in tropical and sub-tropical regions. It contains high sugar content in its stalk. About 60% of white sugar is produced worldwide. The sugarcane biomass is used for biofuel ethanol, and sugarcane is also used in many pharmaceutical compounds and industrial products. The high sugar content and crop yield have turned sugarcane into a highly competitive source of sucrose for food, fibre, and ethanol production, becoming a crop with significant economic and social importance (Amalraj et al., 2010). The harvest is cultivated in more than 90 countries worldwide, the most critical areas being Brazil and India.
Red rot is a very devastating disease of sugarcane. Went first described it in 1893. Since then, it has caused epiphytotic in different countries. It is caused by a fungus Colletotrichum falcatum, an ascomycete and a hemibiotroph whose primary food source is glucose and fructose. It affects the stalk tissues of sugarcane and causes internal stalk tissues leading to poor sugar content and poor quality of yield.
Source of infection:
Red rot can infect mature stalks of sugarcane leaf midribs and cause deterioration of planting material, resulting in substantial losses in the crop, the infection of mid-rib, lamina, leaf sheath, and stalk, while in winter, air currents aid in the spread of the pathogen.
The conidia produced over the rind is washed away with water and cause infection through nodes.
Dissemination of inoculum employing wind appears more difficult because of the mucilaginous nature of the spore mass. But the formation of the disease in the upper stalks of the canes is caused by the dispersal of inoculation by wind dispersal
Environmental conditions prevailing during the winter season do not favour the fungus to infect the crop and may not pose any serious threat to infect the cane crop but leads to incipient infections in the nodal region (buds, buds scales, root primordia, etc.).
Such infections serve as primary infections when the cane is used as a seed. The borers cause secondary transmission of the pathogen.
Etiology of Colletotrichum falcatum:
· Division: Eumycota
· Subdivision: Deuteromycotina
· Class: Coelomycetes
· Order: Melanconiales
· Family: Melanconiaceae
· Genus: Colletotrichum
· Species: falcatum
The critical morphological identification features of C. falcatum fungus are its mycelium which is both intracellular and intercellular; asexual fruiting bodies are known as acervuli, often with setae (dark-pigmented, unbranched, thick-walled sterile hyphae usually pointed at the tip), having hyaline, linear, or club-shaped conidiophores producing elongated, single-celled, thin-walled, uninucleate, colourless, sickle-shaped (Falcate), slimy conidia having granular protoplasm with a large oil globule, thick-walled, greenish-black chlamydospores and the presence of appressoria (thick-walled swellings at the end of a hypha or germ tube helpful in attaching the fungus to the host.
C. falcatum, a facultative saprophyte, is known to produce a phytotoxic metabolite identified as an anthraquinone compound. It has been established that the toxic metabolite is host-specific and makes part of the disease symptoms (Sharma & Tamta, 2015)
General resistance:
All the plants have a certain amount of resistance. The plant gets affected when this resistance is broken down, which increases the suitability of the pathogen into the host, which is generally caused by the increase in the pathogen virulence. The presence of resistance gene in host and corresponding virulence gene in host and corresponding virulence gene in pathogen by flors gene hypothesis (Malathi & Viswanathan, 2012) there are two types of interaction:
Compatible interaction:
First is compatible interaction where r genes or proteins produced by the plant are mutated absent or suppressed, which leads to disease susceptibility.
Incompatible interaction:
The incompatible interaction is where the r gene or protein interacts with the Avr gene of the pathogen and does not allow the pathogen to affect the host by any means. It causes resistance.
It can be either by direct method or by guarding the approach.
According to the zig-zag model of plant-pathogen interaction, the induced defence consists of two layers; the first is known as pamp triggered immunity (PTI). In PTI, conserved molecules or structures of pathogens are perceived by plant pattern recognition receptors, followed by activation of defence responses. To circumvent the PTI, pathogens deliver effector proteins inside host cells, interfering with defence responses. Plants perceive effectors through resistance (R) genes and activate a more robust and faster defence response, termed effector-triggered immunity (ETI). When one or more pathogen effectors suppress PTI, pathogens successfully infect susceptible hosts, and in the absence of effective R proteins, ETI is overcome, eventually leading to effector-triggered susceptibility (ETS) (Biochemistry et al., 2013).
According to the Jones and Dangl model, multiple shifts between ETS and ETI occur because of the coevolution of effects in pathogens and corresponding R genes in plant hosts
We used the cultivator CO 7805 to show the differential response to the c. falcatum pathotypes. Two strains of c. falcatum CF 87012 and CF 94012. CF87012 is incompatible with sugarcane as it has its natural resistance and CF 94012 is compatible with sugarcane as the pathogen became more virulent (Viswanathan et al., 2016).
The genomic and transcriptomic result:
Twelve differentially expressed genes belonging to various functional categories were selected to study their expression in red rot resistant and susceptible cultivars after c. falcatum inoculation. In qRT- PCR. But the pattern of expression differed between the two cultivars. As we see, the resistant cultivar showed the highest induction over susceptible cultivar in all the genes studied(Sathyabhama et al., 2016).
Defense responses:
The molecular interaction between the plant and the pathogen is critical in elucidating plant defense responses. Initial recognition triggers a series of signal transduction cascades which play a vital role in modulating plant defense responses. At surface level, the plan identifies the pathogen through chitin elicitor binding protein which recognizes chitin. Many disease-resistant proteins are also released through the LRR_NBSCC, which is present in almost all resistant genes and produces the disease-resistant protein RPS1, RPS2, RPM1, and these have MAPK. The pathogen recognition elicits ROS accumulation, making it toxic for the colonized pathogen (Prathima et al., 2013). The up-regulation of cysteine protease precursor and a serine protease inhibitor in this study may be attributed to the coordinated and controlled regulation of ET and JA (jas amino acids signals), ABA<BRSK, respectively. The hypersensitive responses are also very helpful in killing the pathogen when it enters into the host cell, and secondary metabolites formed also help in the hypersensitive response (Malathi et al., 2002)
The Red rot disease is a severe pest that evolves with the host and shows coevolution. The Colletotrichum falcatum shows different responses due to the varietal difference of the crops.
References:
1. Amalraj, R. S., Selvaraj, N., Veluswamy, G. K., Ramanujan, R. P., Muthurajan, R., Palaniyandi, M., Agrawal, G. K., Rakwal, R., & Viswanathan, R. (2010). Sugarcane proteomics: establishing a protein extraction method for 2-DE in stalk tissues and initiation of sugarcane proteome reference map. Electrophoresis, 31(12), 1959–1974. https://doi.org/10.1002/elps.200900779
2. Biochemistry, A., Muthurajan, R., & Kumar, K. K. (2013). Differential Regulation of Defense-Related Gene Expression in Response to Red Rot Pathogen Colletotrichum falcatum Infection in Sugarcane Differential Regulation of Defense-Related Gene Expression in Response to Red Rot Pathogen Colletotrichum falcatum Infection in Sugarcane. July. https://doi.org/10.1007/s12010-013-0346-4
3. Malathi, P., & Viswanathan, R. (2012). Variation in Colletotrichum falcatum-Red Rot Pathogen of Sugarcane in Relation to Host Resistance. Sugar Tech 2012 14:2, 14(2), 181–187. https://doi.org/10.1007/S12355-012-0150-4
4. Malathi, P., Viswanathan, R., Padmanaban, P., Mohanraj, D., & Sunder, A. R. (2002). Compatibility of biocontrol agents with fungicides against red rot disease of sugarcane. Sugar Tech, 4(3–4), 131–136. https://doi.org/10.1007/BF02942694
5. Prathima, P. T., Raveendran, M., Kumar, K. K., Rahul, P. R., Kumar, V. G., Viswanathan, R., Sundar, A. R., Malathi, P., Sudhakar, D., & Balasubramaniam, P. (2013). Differential regulation of defense-related gene expression in response to red rot pathogen colletotrichum falcatum infection in sugarcane. Applied Biochemistry and Biotechnology, 171(2), 488–503. https://doi.org/10.1007/S12010-013-0346-4
6. Sathyabhama, M., Viswanathan, R., Malathi, P., & Sundar, A. R. (2016). Identification of Differentially Expressed Genes in Sugarcane During Pathogenesis of Colletotrichum falcatum by Suppression Subtractive Hybridization (SSH). Sugar Tech, 18(2), 176–183. https://doi.org/10.1007/S12355-014-0364-8
7. Sharma, R., & Tamta, S. (2015). A Review on Red Rot : The " Cancer " of Sugarcane Plant Pathology & Microbiology A Review on Red Rot : The “ Cancer ” of Sugarcane. January. https://doi.org/10.4172/2157-7471.S1-003
8. Viswanathan, R., Prasanth, C. N., Malathi, P., & Ramesh, A. (2016). Draft Genome Sequence of Colletotrichum falcatum – A Prelude on Screening of Red Rot Pathogen in Sugarcane. 4(10), 10–12. https://doi.org/10.7150/jgen.13585
About the Author
Author: Arunaa NG
Bio: A biotech enthusiast, a passionate reader and writer, ever ready to take up challenges and interested in neuroscience, behavioural science, genetics and molecular biology, an aspiring researcher in the field of medicine and science.
Editor: Himanshi Yadav
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