A delicate balance of several factors such as the host, invading pathogen and the environment decide the resistance or susceptibility reaction (Chowdhury et al., 2014; Ray et al., 2015). There are very fewer report has been found which describe the host pathogen interaction in rice –sheath blight and the behavior of pathogen toward susceptible and resistant varieties (Zvirin et al., 2010; Ray et al., 2015). In this study characterization of early infection and establishment of the pathogen Rhizoctonia solani was done to see the interaction behaviour of a virulent isolate of Rhizoctonia solani on seven rice varieties. Whole plant assay on artificial inoculation of Rhizoctonia solani showed that susceptible genotypes are affected more with larger lesion height and shorter latent period while genotypes with resistant reaction have longer latent period and smaller lesion length. It is well established that pathogen can detect susceptible host and has a clear preference for them, possibly because of exudate production by susceptible host or inhibitors from resistant or tolerant host.
Different genotypes can have variable biochemical compositions for the same parameter analysis. (Khalid and Hameed, 2017). Plants defend themselves against invading pathogen by the response of complex antioxidant defence system and defence response pathways (Zhou et al., 2012). The activity of defense enzymes got elicitated significantly in all rice varieties but it was highest for the resistant Tetep and least for highly susceptible Jyoti. A higher induction of defense enzyme activities helps the host in combating against the invading pathogen more effectively. It has been established that PPO-generated highly reactive quinones that covalently modify and cross-link a variety of cellular constituents, including proteins, and modify dietary proteins by reacting with amino, sulphydryl, phenolic, and imidazole groups (Yoruk and Marshall, 2003). A number of studies have correlated the PPO activity and induction of resistance in plants (Anushree et al., 2016, Liu et al., 2019). In the present study, highest PPO activity was found to be associated with resistant variety indicated its role in effective defense response. POD has been linked to many physiological processes, including ethylene biogenesis, cell development, cell integrity maintenance, injury response, and disease resistance (Abeles and Biles, 1991). Many studies reported a link between Peroxidase activity during host-pathogen interaction and their link to pathogen infection (Young et al., 1995, Saikia et al., 2004). Earlier studies on POD activity indicated that the suppression of POD leads to weakening defense mechanisms in Rhizoctonia solani-inoculated rice plants. This helps in the further spread of the pathogen and eventually expression of severe sheath blight symptoms (Chattopadhyay and Bera, 1979; Mondal et al, 2012). Variable POD activity in different genotypes indicated that POD activity differing in different genotypes of the same crop and correlated with resistance against pathogens. After reaching to peak, suppression of POD activity indicated the weakening of plant defense mechanism, which helped the pathogen to spread further and eventually expressed sheath blight symptoms. The first line of defense against reactive oxygen species (ROS) is SOD. SOD iso-enzymes play critical roles in protecting cells from the toxic effects of superoxide radicals produced at various cellular loci (Halliwell and Gutteridge, 2000). Increased SOD production reduces the load of ROS within cells and mitigates the burden of oxidative stress while maintaining their growth and development under stressful conditions (Khalid and Hameed, 2017; Singh et al., 2020). Similar to earlier study in current study, higher SOD activity reflected by resistant genotypes indicated that SOD activity plays crucial role in rice plant defense against sheath blight. The enhanced activities of SOD helped in the scavenge the reactive oxygen derivatives. Initially low SOD activity appeared because of no infection in plants. Inoculation of pathogen induces ROS production in the plants, which increases in SOD activity as they act hand to hand to scavenge ROS. An increase in SOD activity indicates plant are becoming more susceptible to disease because of higher ROS production which attacks host integrity and results successful disease development. After a peak activity, decline in SOD activity was seen however, SOD activity at 7dpi and 9dpi was found significantly higher than initial activity at 0dpi because development of disease will damage host tissue which resulted in the production of ROS to combat which SOD activity will be there. Phenylalanine ammonia lyase (PAL) plays a key role in the biosynthesis of various defense chemicals in phenylpropanoid metabolism. It is the first step in the biosynthesis of phenolic compounds. Accumulation of phenols that act as phytoalexins is considered the primary inducible response in plants against many biotic and abiotic stresses (Daayf et al., 1997). Earlier reports in conformity of the present study showed decline in the PAL activity on successful infection of Rhizoctonia solani in rice. A significant decrease in PAL activity level suggested the role of this enzyme in susceptible host– pathogenic interaction (Mondal et al., 2012). In this study a sharp decline appeared in PAL activity after reaching to a peak in seven genotypes indicating reduction in PAL activity resulting in the susceptibility of rice plant toward sheath blight pathogen. In this study plants under control conditions didn’t show elicitation of defense enzymes, because of healthy condition of plants, while in inoculated plants, the host perceives pathogen presence and elicits enzymatic response for defending plants against pathogen. Genotypes ranked high in biochemical parameters can be employed in the breeding programme.
In case of Rhizoctonia solani, infection cushion and appressoria are important infectious structures and plays an important role in disease development. Earlier studies reported diverse ways of pathogen invasion, i.e. either hypha at the base of infection cushion or infection hypha developed from appressorium or through stomata (Zheng and Wang, 2011). An earlier study also reported sparse hyphal growth of Rhizoctonia solani with patches of bunchy growth frequently in tolerant rice genotype while vigorous and intimate growth was seen in the susceptible genotype (Matsurra k., 1986). Reason for a sparser and bunchy growth on resistant host is due to restriction posed by host on penetration and invasion of Rhizoctonia solani. To overcome the restricted entry of pathogen bunches formed by pathogen to exert more pressure on host surface for their penetration. Rhizoctonia solani forms a typical infection cushion at the pre-penetration stage and infection cushion formed by aggregation of branched hyphal tips (Kumar et al., 2003). In the current study, two types of infection structures i.e. lobate appressoria and infection cushion appeared in Jyoti while only infection cushion appeared in Tetep indicating aggressiveness for the susceptible host. Plant exudates could also one of the reasons for differential growth toward two genotypes. Pattern recognition receptors on plant surfaces perceive pathogen/ damage associated molecular patterns. Earlier studies reported that on attack of microbes, stomatal closure is a typical defense response by plants (Gahir et al., 2021). Stomatal closure process start by sensing the abiotic or biotic stress. In this study closure of stomata in Tetep indicated perceiving of pathogen presence and respond toward it but in Jyoti no such defense response has been seen due to failure of perception of pathogen. In a recent study it was found that the closure of stomata reduces CO2 content in plant resulted in lower sugar levels. Pathogens require sugar for growth and infection and lower sugars impede the growth of pathogen (Huai et al., 2020). Differential behaviour of hypha toward stomata in susceptible and resistant rice varieties has been observed first time in rice – Rhizoctonia solani interaction.
Host pathogen interaction studies showed that pathogen has a prominent preference for the susceptible host. Resistance host has several factors (enzymatic, stomatal closure and surface) impeding the growth of the pathogen. Infection of pathogen changes the activities of self-defence enzymes which are closely related to plant resistance. Enzyme activities increased sharply in resistant cultivars and in susceptible varieties slight increase were found. Defense enzymes play vital role in defense and the speed up the intensity with which these enzymes triggered is strongly connected with degree of resistance. No significant change in enzyme activities were found in control plants.