Rice receptor kinase FLR7 regulates rhizosphere oxygen levels and enriches the dominant Anaeromyxobacter that improves submergence tolerance in rice.

Oxygen is one of the determinants of root microbiome formation. However, whether plants regulate rhizosphere oxygen levels to affect microbiota composition and the underlying molecular mechanisms remain elusive. The receptor-like kinase (RLK) family member FERONIA modulates the growth-defense tradeoff in Arabidopsis. Here, we established that rice FERONIA-like RLK 7 (FLR7) controls rhizosphere oxygen levels by methylene blue staining, oxygen flux, and potential measurements. The formation of oxygen-transporting aerenchyma in roots is negatively regulated by FLR7. We further characterized the root microbiota of 11 FLR mutants including flr7 and wild-type Nipponbare (Nip) grown in the field by 16S ribosomal RNA gene profiling and demonstrated that the 11 FLRs are involved in regulating rice root microbiome formation. The most abundant anaerobic-dependent genus Anaeromyxobacter in the Nip root microbiota was less abundant in the root microbiota of all these mutants, and this contributed the most to the community differences between most mutants and Nip. Metagenomic sequencing revealed that flr7 increases aerobic respiration and decreases anaerobic respiration in the root microbiome. Finally, we showed that a representative Anaeromyxobacter strain improved submergence tolerance in rice via FLR7. Collectively, our findings indicate that FLR7 mediates changes in rhizosphere oxygen levels and enriches the beneficial dominant genus Anaeromyxobacter and may provide insights for developing plant flood prevention strategies via the use of environment-specific functional soil microorganisms.

[Characterization Composition of Soluble Microbial Products in an Aerobic Granular Sludge System].

Using flocculent activated sludge as seed sludge to cultivate aerobic granular sludge in a SBR, the main objective of this study was focused on the accumulation, relative molecular mass distribution, and composition of soluble microbial products (SMP) in an aerobic granular sludge (AGS) system. SMP were predominant (71-85 mg·L-1) in the effluent of the AGS system. The formation of SMP was related to substrate utilization, biomass decay, and EPS hydrolysis. A relative molecular mass distribution analysis indicated that the majority of SMP, accounting for about 54.8%-71.7%, had Mr<3×103; whereas, the Mr>100×103 formed a small fraction, constituting only 9.3%-14.5%. Three-dimensional excitation emission matrix fluorescence spectra (3D-EEM) identified four peaks in SMP, belonging to aromatic protein-like, tryptophan protein-like, humic acid-like, and fulvic acid-like substances. Gas chromatography-mass spectrometry (GC-MS) analysis revealed that esters (39.0%), short chain alkanes (14.9%), alkenes (11.7%), and alcohols (7.6%) were the main compounds in SMP. Most notably, bis(2-ethylhexyl) phthalate, as one kind of ester, accounted for 32% of the identified SMP.