Comparative Genomics Reveals Host-Specific Adaptation of Pyricularia oryzae Strains Isolated from Rice and Barnyard Grass

Barnyard grass, a widespread and persistent weed in rice paddies, belongs to the same family as rice and may act as a bridge host for the rice blast fungus. This study utilized comparative genomics to analyze six <i>Pyricularia oryzae</i> strains isolated from barnyard grass (Baicao series) and rice (GDYJ7 and ZJX18), integrating pathogenicity assays, whole-genome sequencing, and functional annotation. Pathogenicity tests demonstrated host specificity, as Baicao series strains caused typical lesion symptoms on barnyard grass but not on rice leaves, while GDYJ7 and ZJX18 caused lesions mainly on rice. Genomic analyses indicated that Baicao series strains possessed larger genomes (41.04 Mb to 41.16 Mb) with a higher content of repetitive sequences (6.68% to 7.09%) compared to rice strains GDYJ7 and ZJX18 (38.69 Mb and 39.05 Mb; 3.66% and 3.71% repeats). Phylogenetic analysis confirmed that Baicao series strains represent a grass-infecting pathotype of <i>P. oryzae</i> species, as they were grouped with the established grass-isolated <i>P. oryzae</i> strains, while GDYJ7 and ZJX18 were grouped with rice-isolated <i>P. oryzae</i> strains. However, Baicao series, GDYJ7 and ZJX18 are all relatively distant from <i>P. grisea</i> species. PCR amplification revealed that Baicao series strains harbored significantly fewer avirulence genes (<i>Avr-Pib</i>, <i>Avr-Pizt</i>, <i>PWL3</i>) than GDYJ7 and ZJX18 (<i>Avr-Pib</i>, <i>Avr-Pizt</i>, <i>Avr-Pi9</i>, <i>Avr-Pik</i>, <i>PWL2</i>), with Baicao9 retaining only <i>Avr-Pib</i>. In summary, our results suggested that the genomic sequences of the barnyard grass-isolated strains serve as a valuable resource for the study of <i>P. oryzae</i> strains with differential host preference and provide novel insights into the evolution of pathogen genomes during host adaptation.