Trancriptome Analysis Of Rice Cultivar CO39 And Its Five Near-Isogenic Lines In Response To Blast Fungus Infection

Rice production is closely related to the survival of millions of people. Rice blast caused by Magnaporthe oryzae is a destructive fungal disease, which is a common disease in the temperate, tropical and subtropical rice growing regions. In the past few decades, many researches had been carried out to understand the genetic basis of rice response to fungus stress. However, examples of rice cultivars with durable resistance are rare, partly due to our limited knowledge of resistance mechanisms.In the present study, a blast-susceptible rice cultivar CO39(Oryza sativa L. ssp. indica) and its five blast-resistant Near Isogenic Lines (NILs), named CN-1, CN-2, CN-3, CN-4a and CN-4b, were employed as materials. RNA Sequencing (RNA-Seq) techniques was used to investigate molecular responses of rice to blast fungus infection. The results were shown as the following.1. An average of 12 million high-quality reads were obtained for each of the tested rice genotype. The reads were assembled and genes responsive to Magnaporthe oryzae fungus strain GUY11 treatment were examined, GUY11 is a hemibiotrophic ascomyceteous fungus which causes rice blast disease. The blast-resistant rice genotypes had higher number of up-regulated genes (average of 512) than that of blast-susceptible rice genotype CO39 at 24 hours post GUY11 inoculation (24hpi), indicating that 24 hpi is a key inoculation time point that many genes needed to be up-regulated to induce the early gene-dependent defence reactions in blast resistant rice genotypes. This were consistent with the phenotype of CO39 and CN-4b under GUY11 stress. However, at the later stage (48hpi), the blast-resistant NILs and susceptible parent line CO39 seemed to have some common reaction because of more number of common DEGs than at the early stage (24hpi). Therefore, molecular analyses at 24hpi in a resistant genotype should highlight defence pathways activated during the resistance response.2. Analysis on the GO term enrichment showed that the up-regulated genes in the 5 NILs under blast fungus stress shared 16 GO terms annotations, including endogenous stimulus, secondary metabolic process, stress responses, photosynthesis process. Genotype CN-4b showed most number of isoforms genes,82, than any other partially resistant NILs. Furthermore, some interesting up-regulated proteins were identified in the GUY11-resistant genotype, CN-4b, such as LOC_Os04g56910.3. In terms of the key protein families, we found no induction of WAK genes in CO39 under blast infection, but the differential expression of WAK genes could be detected in all the 5 NILs. This might be one of the factors contributing to the ability of high susceptible to blast infection in CO39. The induction of WRKY family genes were detected in CO39 and its 5 NILs. However, some specific WRKY genes were up-regulated in the 5 NILs to response to blast fungus stress. Compared to CO39, CN-4b had an up-regulated gene encoding WRKY 69, which was previously found to be modulated in rice when rice plants were attacked with pathogen. The further analysis on the function module showed that WRKY genes in the 5 NILs were connected in the module to response to biotic stress. However, a WRKY gene in CO39 induced by GUY11 stress interacted with CPN1 gene, resulting in the defense ability of CO39 suppressed by fungus.In summary, different number and functional module of DEGs contribute to the different ability of rice genotypes to avoid blast fungus infection. This result provide new insight into the response of rice plants to blast fungus infection.