Identification of the Bru1 Genomic Region for Brown Rust Resistance and Functional Analysis of Candidate Resistance Genes in Sugarcane

Sugarcane brown rust, caused by Puccinia melanocephala H. Sydow & P. Sydow, is one of the most destructive fungal diseases in sugarcane industry, leading to a reduction in sucrose content by 10% to 36%. Previous studies revealed that the major gene(s) for brown rust resistance was located in the Bru1 genomic region. The cloning of crucial candidate genes and their functional investigation should provide important candidate gene resources for breeding new sugarcane cultivars resistant to brown rust.

In this study, the contig-level genomic information of sugarcane cultivar ROC22 was obtained by utilizing the PacBio Sequel Ⅱ sequencing platform, and the Bru1 region associated with brown rust candidate resistance gene was identified, annotated, cloned, and analyzed for tissue specificity, expression patterns in resistant and susceptible sugarcane cultivars, subcellular localization, and transient overexpression.

The results demonstrated that, firstly, using tightly associated molecular markers R12H16 and 9O20-F4 within the Bru1 region, a total of 33 genes were annotated from this region, and five candidate resistance genes (Brrg99, Brrg103, Brrg108, Brrg115, and Brrg116) were selected based on the typical/conserved domains of the resistance genes. Secondly, the full-length sequence cDNA sequence of the Brrg116 gene with an open reading frame of 729 bp and encoding 242 amino acid residues, was cloned from sugarcane cultivar ROC22. The gene sequence was aligned with the genomic databases of Saccharum spontaneum, S. officinarum, and the closely related diploid species sorghum. A high degree of sequence similarity was observed between S. spontaneum and S. officinarum, exceeding 98%. In contrast, its similarity with sorghum was 93.77%. Phylogenetic tree analysis suggests that this gene may originate from S. spontaneum species during sugarcane domestication. qRT-PCR analysis showed its constitutive expression in various tissues of sugarcane cultivars, particularly with the highest expression level in the +1 leaf, which was 5.2 times higher than in the bud. Furthermore, significantly differential expression of Brrg116 was observed at 6 h and 72 h post-inoculation with the brown rust pathogen in resistant and susceptible sugarcane cultivars. Subcellular localization analysis indicated that the encoded protein of this gene was located on the cell membrane. Finally, the Brrg116 gene was transiently overexpressed in Nicotiana benthamiana leaves, followed by inoculation with Fusarium solani var. coeruleum. The color of Nicotiana benthamiana leaves showed a gradual deepening phenotype by 3,3'-diaminobenzidine (DAB) staining. The genes related to hypersensitive response, salicylic acid, and ethylene synthesis had a sustained upregulation pattern, evidencing that the expression of these genes can enhance plant disease resistance.

Brrg116 was constitutively expressed in different sugarcane tissues and in response to brown rust pathogen infection, it showed a rapidly induced expression pattern in the resistant sugarcane cultivar. Overexpression of Brrg116 could trigger defense responses through hormone signaling pathways such as salicylic acid and ethylene. It is thus hypothesized that this gene may play a significant regulatory role in enhancing plant disease resistance.