Microbiome structure and response to watering in rhizosphere of Nitrosalsola vermiculata and surrounding bulk soil

Abstract

The plant rhizosphere microbiomes were thought to help the plant stands adverse condition. The study aims at deciphering signatures of rhizosphere soil microbiomes of the medicinal plant Nitrosalsola vermiculata and those of the surrounding bulk soil as well as to detect influence of watering in restructuring soil microbes that can improve the plant’s ability to tolerate drought stress. Amplicon sequencing of partial 16S rRNA gene indicated that alpha diversity indices are higher in rhizosphere than in bulk soils, while no distinctive differences were observed due to the watering. Relative abundance of phylum Cyanobacteria and its descendent unidentified genus is the highest among phyla and genera of bulk soil. Relative abundance of phyla Euryarchaeota, Chloroflexi, Actinobacteria, Proteobacteria, Bacteroidetes, Firmicutes, Acidobacteria and Gemmatimonadetes as well as genera Bacillus, Ammoniphilus, Sphingomonas, Microvirga, Pontibacter, Adhaeribacter and Arthrobacter was significantly higher in rhizosphere soil. The latter taxa were reported to act as plant growth-promoting bacteria (PGPB) through symbiotic associations. We speculate that relative abundance and mutual dominance of these taxa in rhizosphere of N. vermiculata were due to the intensity and type of plant root exudates. Other factors include soil pH where microbes favoring high soil pH can show better growth in rhizosphere soil. Also, co-existence of phyla that promote sustainability of cohabiting phyla in the rhizosphere and have high synergism prevalence in biofilm formation can be one extra factor. Quorum sensing (QS) also mediates bacterial population density in a given environment and elicit specific plant responses. The low abundance of Cyanobacteria in rhizosphere soil can be due to the inhibitory effect of highly abundant members of Firmicutes, especially those of genus Bacillus. The latter conclusion was confirmed by the occurrence of high expression rate of comQ gene triggering QS in genus Bacillus. Highly abundant microbes whose abundance was not changed due to watering are phyla Firmicutes, Proteobacteria, Chloroflexi and Cyanobacteria and their descendent genera Bacillus, Ammoniphilus, Sphingomonas, Microvirga and unidentified genus of Cyanobacteria. We speculate that non-responsive taxa to watering were drought tolerant and can help plants stand adverse conditions of water scarce. In conclusion, insights on the factors involved in shaping microbiome signatures and those eliciting differential plant responses to drought stress are raised and warrant further investigations.

Keywords

• Alpha and beta indices
• drought
• operational taxonomic unit (OTU)
• phylogenetic tree