Journal of Crop Breeding

Extended Abstract
Background: Rice is considered one of the strategic crops that plays an important role in providing food security to more than 50% of the world's population, especially in Asia. Blast caused by Pyricularia oryzae, one of the most important diseases of rice, poses a serious threat to rice cultivation every year. Induction of resistance in rice via biotic and abiotic factors, including phosphite, as one of the safe and modern management tools, has been used globally as an environmentally friendly alternative, and this method activates the defense mechanism in plants and increases their ability to respond to pathogen invasion. In this study, therefore, the effect of potassium phosphite on the expression changes of several defense genes, including PAL, LOX, NPR1, PR1, and PR5 at the transcriptional level, was evaluated in response to the resistance of the Hashemi rice cultivar against the blast disease agent.
Methods: The Split-time experiment in a completely randomized design with three replications was used in this research. After planting the same seedlings in pots, potassium phosphite at a concentration of 2 g/L was used for the foliar spraying of 4-5-leaf seedlings, and sterile distilled water was used for the control treatment. After 2 days, the spore suspension of P. oryzae at a concentration of 5 × 10⁵ spores per ml with 0.05% Tween 20 was used to inoculate all plants. Leaf tissues of treated and control seedlings were sampled at four times: zero (before fungal inoculation), 48, 96, and 144 hours after pathogen inoculation, respectively. The severity of blast disease was recorded 10 days after inoculation. After extracting RNA from the samples and making cDNA, quantitative real-time PCR was used to investigate changes in the expression of defense genes, and the Ubiquitin gene was considered the housekeeping gene. The statistical analysis and the average comparison using Duncan’s test at a probability level of 0.01% were done using SAS 9.1 software. Graphs were drawn with Excel 2010 software.
Results: A significant difference was observed in the disease severity, with less infection in the seedlings treated with potassium phosphite than in the control plants. The evaluation results of the effect of potassium phosphite treatment on the expression changes in NPR1, PR1, PR5, LOX, and PAL genes indicated that the data were significantly different from each other compared to the control. The comparison of the means showed that the expression patterns of NPR1, PR5, and LOX genes increased at all times after pathogen inoculation in potassium phosphite treatment, but the maximum expression of these genes was observed 48 hours after pathogen inoculation. The transcript levels of NPR1, PR5, and LOX genes in potassium phosphite treatment at 48 hours after pathogen inoculation were 7.08, 4.2, and 7.28, respectively, with increased activity by 2.36, 2.83, and 5.83 times, respectively, at the same time compared to control plants. PR5 gene activity was significantly highest in both potassium phosphite and control plants 48 hours after inoculation, after which the gene expression decreased at 96 hours and increased again at 144 hours. However, the gene activity was significantly lower in the control treatment than in the potassium phosphite treatment. The expression level of the PR1 gene in plants treated with potassium phosphite and control showed the highest value 48 hours after inoculation, with increases of 1.65 and 1.4 times compared to time zero, respectively. The expression pattern of the PR1 gene showed an increase in control plants, and plants treated with potassium phosphite 48 hours after pathogen inoculation, and a decrease in expression was observed in the rest of the hours compared to time zero. Despite this, the gene expression level was higher in the potassium phosphite treatment than in the control at all times. The effect of potassium phosphite treatment was significant in increasing the expression level of the PAL gene. The PAL gene activity in the potassium phosphite treatment was evaluated as 0.58, 2.65, and 0.41 times at 48, 96, and 144 hours, respectively, compared to time zero, with the highest activity level of this gene observed 96 hours after pathogen inoculation. In the control treatment, however, an increase in PAL gene expression was observed 48 hours after pathogen inoculation.
Conclusion: The results indicate the effective role of potassium phosphite in increasing the expression of some defense genes in rice plants and consequently inducing plant resistance and protection against P. oryzae, preventing the spread of rice blast disease. Therefore, using potassium phosphite as a resistance inducer can help strengthen plant defense against subsequent attacks by pathogens.