Transcriptome analysis reveals genes potentially related to maize resistance to Rhizoctonia solani

Banded leaf and sheath blight (BLSB) is a devasting disease caused by the necrotrophic fungus Rhizoctonia solani that affects maize (Zea mays L.) fields worldwide, especially in China and Southeast Asia. Understanding how maize plants respond to R. solani infection is a key step towards controlling the spread of this fungal pathogen. In this study, we determined the transcriptome of maize plants infected by a low-virulence strain (LVS) and a high-virulence strain (HVS) of R. solani for 3 and 5 days by transcriptome deep-sequencing (RNA-seq). We identified 3,015 (for LVS infection) and 1,628 (for HVS infection) differentially expressed genes (DEGs). We confirmed the expression profiles of 10 randomly selected DEGs by quantitative reverse transcription PCR. We also performed a Gene Ontology (GO) enrichment analysis to establish which biological processes are associated with these DEGs, which revealed the enrichment of defense-related GO terms in LVS- and HVS-regulated genes. We selected 388 DEGs upregulated upon fungal infection as possible candidate genes. Among them, the overexpression of ZmNAC41 (encoding NAC transcription factor 41) or ZmBAK1 (encoding BRASSINOSTEROID INSENSITIVE 1-associated receptor kinase 1) in rice enhanced resistance to R. solani. In addition, overexpressing ZmBAK1 in rice also increased plant height, plant weight, thousand-grain weight, and grain length. The identification of 388 potential key maize genes related to resistance to R. solani provides significant insights into improving BLSB resistance.

Identification of virulence associated milRNAs and their bidirectional targets in Rhizoctonia solani and maize during infection

BACKGROUND

Anastomosis group 1 IA (AG1-IA) of Rhizoctonia solani is the major agent of banded leaf and sheath blight (BLSB) disease that causes severe yield loss in many worldwide crops. MicroRNAs (miRNAs) are ~ 22 nt non-coding RNAs that negatively regulate gene expression levels by mRNA degradation or translation inhibition. A better understanding of miRNA function during AG1-IA infection can expedite to elucidate the molecular mechanisms of fungi-host interactions.

RESULTS

In this study, we sequenced three small RNA libraries obtained from the mycelium of AG1-IA isolate, non-infected maize sheath and mixed maize sheath 3 days after inoculation. In total, 137 conserved and 34 novel microRNA-like small RNAs (milRNAs) were identified from the pathogen. Among these, one novel and 17 conserved milRNAs were identified as potential virulence-associated (VA) milRNAs. Subsequently, the prediction of target genes for these milRNAs was performed in both AG1-IA and maize, while functional annotation of these targets suggested a link to pathogenesis-related biological processes. Further, expression patterns of these virulence-associated milRNAs demonstrated that theyparticipate in the virulence of AG1-IA. Finally, regulation of one maize targeting gene, GRMZM2G412674 for Rhi-milRNA-9829-5p, was validated by dual-luciferase assay and identified to play a positive role in BLSB resistance in two maize mutants. These results suggest the global differentially expressed milRNAs of R. solani AG1-IA that participate in the regulation of target genes in both AG1-IA and maize to reinforce its pathogenicity.

CONCLUSIONS

Our data have provided a comprehensive overview of the VA-milRNAs of R. solani and identified that they are probably the virulence factors by directly interfered in host targeting genes. These results offer new insights on the molecular mechanisms of R.solani-maize interactions during the process of infection.

Functional analysis of the GRMZM2G174449 promoter to identify Rhizoctonia solani-inducible cis-elements in maize

BACKGROUND

Banded leaf and sheath blight (BLSB), caused by the necrotrophic fungus Rhizoctonia solani, is a highly devastating disease in most maize and rice growing areas of the world. However, the molecular mechanisms of perceiving pathogen signals are poorly understood in hosts.

RESULTS

Here, we identified a Rhizoctonia solani-inducible promoter pGRMZM2G174449 in maize. Deletion analysis showed that the -574 to -455 fragment was necessary for pGRMZM2G174449 in responding to R. solani and this fragment contained the unknown pathogen-inducible cis-elements according to the bioinformatics analysis. Furthermore, detailed quantitative assays showed that two cis-elements, GCTGA in the -567 to -563 region and TATAT in the -485 to -481 region, were specifically responsible for the R. solani-inducible activity. A series of point mutation analysis indicated that the two cis-elements have the conserved motifs of NHWGN and DWYWT, respectively.

CONCLUSION

Our results indicated that pGRMZM2G174449 is a good R. solani-inducible promoter suitable for genetic engineering of BLSB resistance. And NHWGN and DWYWT are two R. solani-inducible cis-elements that play important roles in pGRMZM2G174449 responding to R. solani.