Journal of Crop Breeding

Extended Abstract

Introduction and Objective: 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 Pyricolaria 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 its ability to respond to pathogen invasion. Therefore, in this study, 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 Hashemi rice cultivar againt blast disease agent.

Material and Methods: The Split-time experiment in a completely randomized design was  used in this research with three in vivo. After planting the same seedlings in pots, potassium phosphite at a concentration of two grams per liter was used for foliar spraying of 4-5 leaf seedlings, and sterile distilled water was used for the control treatment. After two 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 tissue sampling of treated and control seedlings was performed 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. In order to investigate the changes in the expression of defense genes, after extracting RNA from the samples and making cDNA, Quantitative real-time PCR was used and the Ubiquitin gene was considered as the housekeeping gene. The statistical analysis and the average comparison using Duncan test at a probability level of 0.01% were done using SAS 9.1 software, and graphs were drawn by Excel 2010 software.

Results: The results showed that there is a significant difference at the disease severity and less infection in treated seedlings  with potassium phosphite compared to the control plants. The results of evaluating the effect of potassium phosphite treatment on the expression changes of NPR1, PR1, PR5, LOX and PAL genes compared to the control indicated that the data were significantly different from each other. Comparison of the means showed that the expression pattern 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, that it has increased its activity compared to control plants at the same time, by 2.36, 2.83 and 5.83 times respectively. PR5 gene activity was significantly highest at 48 hours after inoculation in both potassium phosphite and control treatments plants, then gene expression decreased at 96 hours and increased again at 144 hours, but the gene activity in the control treatment was significantly lower than that in the potassium phosphite treatment. The expression level of PR1 gene in plants treated with potassium phosphite and control showed the highest value at 48 hours after inoculation, with an increase of 1.65 and 1.4 times compared to time zero time, respectively. In the expression pattern of PR1 gene showed an increase in control plants and plants treated with potassium phosphite at 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 in potassium phosphite treatment was higher than the control at all times. The effect of potassium phosphite treatment was significant in increasing the expression level of PAL gene. The activity of PAL gene in potassium phosphite treatment was evaluated at 48, 96 and 144 hours as 0.58, 2.65 and 0.41 times compared to time zero, respectively, with the highest level of activity of this gene being at 96 hours after pathogen inoculation. However, in the control treatment, an increase in PAL gene expression was observed at 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, and prevents the spread of rice blast disease. Therefore, using potassium phosphite as a resistance inducer can help strengthen plant defense against subsequent attacks by pathogens.