Rice phyllospheric Pantoea spp. suppress blast and bacterial blight diseases

Rice is a major food crop in China as well as Asia, yet its production is threatened by microbial diseases including blast disease caused by fungal pathogen (Magnaporthe oryzae) and bacterial blight caused by several bacterial pathogens. To screen for bacterial microbiota associated with rice blast occurrence, and/or contributing to disease resistance, we performed microbiota analysis with rhizosphere soil, root, stem, and leaf samples of blast susceptible (CO39) and resistant (Y33R) rice grown in a blast disease nursery garden.

Our result showed no significant difference in microbiota of rhizosphere soil, root, or leaf between these two rice cultivars, but stem microbiota were significantly different. Pantoea spp. were enriched in stem of blast susceptible rice, suggesting that it may play a role after fungal infection. A total of 822 bacterial strains were isolated from the phyllospheric (including leaf and stem) samples of Y33R and CO39 rice. Based on 16S rRNA amplicon sequencing, and phylogenic analysis using 16S rRNA, gyrB, leuS, and rpoB gene sequences, the 3 isolated strains and 1 strain were identified as P. ananatis and P. dispersa, respectively. The strains A25-H1 and B10-A1 were selected for genome sequencing, and based on Average Nucleotide Identity (ANI) analysis, we confirmed that A25-H1 was P. ananatis and B10-A1 was P. dispersa. The P. ananatis consortium (A25-F1, A25-G1, and A25-H1 combination) A25-11 and P. dispersa strain B10-A1 displayed suppressive effect on blast disease when they were applied to the susceptible rice CO39. Although a P. ananatis strain SC7 has been reported to cause bacterial blight in rice, A25-11 or B10-A1 was non-pathogenic to rice under experimental conditions. Furthermore, they could also suppress bacterial blight caused by SC7 or Xanthomonas oryzae pv. oryzae strain Pxo99A. A25-11 and B10-A1 did not affect the growth of M. oryzae mycelia in confrontation culture analysis, but induced transcription of rice immunity genes and promoted ROS accumulation, suggesting that the biocontrol effect of A25-11 or B10-A1 may lie on immunity priming. We further showed that A25-11 and B10-A1 possessed growth promoting capacity including indole 3-acetic acid (IAA) production, phosphate solubilization, nitrogen fixation, and siderophore production. Under field condition, the consortium A25-11 and strain B10-A1 could effectively suppress leaf and panicle blast.

Overall, this study established a microbiome method for identifying the rice bacterial communities of agricultural significance, with capacity of rice disease management and/or growth promotion.