Functional and evolutionary study of MLO gene family in the regulation of Sclerotinia stem rot resistance in Brassica napus L

Abstract Background Oilseed rape (Brassica napus L.) is known as one of the most important oilseed crops cultivated around the globe. However, in the humid season, it faced a severe challenge of Sclerotinia stem rot (SSR), a severely destructive disease caused by the fungus Sclerotinia sclerotiorum, which is negatively impact in terms of huge reduction in yield loss and annually. Like other field crops, in Brassica napus (B. napus) SSR resistance is quantitatively controlled, and there are few studies reported the effective major genes conferring SSR disease resistance so far. Thus, there is utmost need to design the studies at whole genome level to discover the promising major genes, which can further enhance the protective shield against SSR in B. napus. Results Here, we used a natural population of B. napus consisting of 222 accessions to perform a genome-wide association study (GWAS) to find the candidate genes conferencing the SSR resistance trait. Our results showed that a total of 2779265 SNP markers were identified, which harboring 51 candidate genes. Moreover, we found that candidate gene BnaA08g25340D (BnMLO2_2), a member of seven homolog genes of Arabidopsis Mildew Locus O 2 (MLO2), was closely associated with the SSR resistance. The natural SNP variations was predominantly identified in the promoter region of BnMLO2_2 and 3 haplotypes were found to be closely related to the differential expressions of BnMLO2_2 in the leaves of the B. napus population, which may responsible for the variations of SSR resistance observed in this population. Besides, we also noted that among seven members of MLO2, only BnMLO2_2 showed constitutive expression in the leaf tissues. In Arabidopsis, loss of function mutation in MLO2 displayed enhanced susceptibility to SSR, whereas, Arabidopsis plants overexpression of MLO2 genes showed significantly enhanced resistance to SSR disease. The mechanism underlying MLO2 in the regulation of SSR resistance was associated with the cell death. Lastly, syntenic analysis revealed 57, 23, and 28 homolog genes of 15 Arabidopsis MLOs in the genomes of B. napus, Brassica rapa (B. rapa), and Brassica oleracea (B. oleracea), respectively indicated an asymmetrical evolution between the A and C subgenomes of B. napus. Conclusion Present investigation revealed the roles of MLO2 gene in the regulation of SSR disease resistance. Therefore, natural variations in the promoter region of BnMLO2 gene can be utilized for improving the resistance to SSR disease in B. napus. MLO locus play antagonistic role against SSR and PM diseases.