Bacillus spp. can exert plant growth-promoting effects and biocontrol effects after effective colonization, and bacterial chemotaxis toward plant root exudates is the initial step to colonize. Under biotic stress, plants are able to alter their root exudates to attract or avoid different types of microbes. Hence, Bacillus chemotaxis toward root exudates after pathogen infection is crucial for exerting their beneficial effects. In this study, the Bacillus amyloliquefaciens OR2-30 strain, which exhibited greater chemotaxis ability toward maize root exudates after Fusarium graminearum infection, was screened from 156 rhizosphere microorganisms. The infected maize root exudates were further confirmed to improve the swarming and biofilm formation ability of the OR2-30 strain. Chemotaxis, swarming, and biofilm formation ability were able to influence bacterial colonization. Indeed, the the OR2-30 strain displayed more effective colonization ability in the maize rhizosphere after F. graminearum inoculation. Moreover, lipopeptides produced by OR2-30 were identified as iturins and responsible for suppressing F. graminearum growth. Further study showed that lipopeptides suppressed the growth of F. graminearum by inhibiting conidia formation and germination, inducing reactive oxygen species production and causing cell death in mycelium. Eventually, the OR2-30 strain increased maize resistance against F. graminearum. These results suggested that maize root exudates could recruit B. amyloliquefacines OR2-30 after F. graminearum infection, and that OR2-30 then suppresses the F. graminearum by producing lipopeptides, such as iturins, to protect maize.

Plant growth-promoting rhizobacteria (PGPR) are soil beneficial microorganisms that colonize plant roots for nutritional purposes and accordingly benefit plants by increasing plant growth or reducing disease. However, the mechanisms and pathways involved in the interactions between PGPR and plants remain unclear. In order to better understand these complex plant-PGPR interactions, changes in the transcriptome of the typical PGPR Bacillus subtilis in response to rice seedlings were analyzed. Microarray technology was used to study the global transcriptionl response of B. subtilis OKB105 to rice seedlings after an interaction period of 2 h. A total of 176 genes representing 3.8% of the B. subtilis strain OKB105 transcriptome showed significantly altered expression levels in response to rice seedlings. Among these, 52 were upregulated, the majority of which are involved in metabolism and transport of nutrients, and stress responses, including araA, ywkA, yfls, mtlA, ydgG et al. The 124 genes that were downregulated included cheV, fliL, spmA and tua, and these are involved in chemotaxis, motility, sporulation and teichuronic acid biosynthesis, respectively. We present a transcriptome analysis of the bacteria Bacillus subtilis OKB105 in response to rice seedings. Many of the 176 differentially expressed genes are likely to be involved in the interaction between Gram-positive bacteria and plants.

Bacillus velezensis FZB42 is able to activate induced systemic resistance (ISR) to enhance plant defense response against pathogen infections. Though the roles of microRNAs (miRNAs) in Bacillus-triggered ISR have been reported in Arabidopsis, the maize miRNAs responsible for the Bacillus-activated ISR process have not been discovered. To explore the maize miRNAs involved in ISR, maize miRNAs in response to FZB42 (ISR activating), FZB42△sfp△alss (deficient in triggering ISR), and a control for 12 h were sequenced. A total of 146 known miRNAs belonging to 30 miRNA families and 217 novel miRNAs were identified. Four miRNAs specifically repressed in FZB42-treatment were selected as candidate ISR-associated miRNAs. All of them contained at least one defense response-related cis-element, suggesting their potential roles in activating the ISR process. Interestingly, three of the four candidate ISR-associated miRNAs belong to the conserved miR169 family, which has previously been confirmed to play roles in abiotic stress response. Moreover, 52 mRNAs were predicted as potential targets of these candidate ISR-associated miRNAs through TargetFinder software and degradome sequencing. Gene Ontology (GO) and network analyses of target genes showed that these differentially expressed miRNA might participate in the ISR process by regulating nuclear factor Y transcription factor. This study is helpful in better understanding the regulatory roles of maize miRNAs in the Bacillus-activated ISR process.