Investigation Of Response Mechanisms Of Millet And Soil Microbial Communities To Chromium Stress

In Chinese agricultural soils,heavy metal chromium(Cr)contamination exerts harmful effects on millet and disturbs the homeostasis of the soil microbial community.However,different strains of millet and different soil microbial populations have different mechanisms for resistance to Cr.In this study,the phenotypic and genotypic alterations of millet seedlings(Setaria italica)in habitats contaminated with Cr,such as physiological indicators including root and stem length,and functional pathways of differentially expressed genes were examined.To shed light on the resistance mechanisms of millet and microbial communities under Cr stress,we also investigated at trends in soil microbial community structure,construction,and function.This study will provide theoretical support for millet and microbial community under Cr stress,as well as for the ecological restoration of contaminated soil.Based on the indoor pot experiment,samples were collected from millet seedlings and soil before(CK)and 6 hours and 6 days after Cr stress(Cr＿6 h,Cr＿6d).In Cr contaminated habitats,the phenotypic changes,gene expression regulation and the trends of the enriched functional pathways of millet were investigated through the measurement of millet physiological indicators and transcriptomic analysis.By high-throughput sequencing analysis,the composition,diversity,and community assembly processes of soil’s microbial communities were investigated,and the relationship with the soil’s physicochemical attributes was examined.The changes in the expression abundance and diversity of carbon and nitrogen cycle functional genes(AOA-amo A,AOB-amo A,nar G,nir K,nif H,pmo A,mcr A)were analyzed through fluorescence quantitative PCR(q PCR).The results indicated that:(1)Millet phenotype chlorophyll and nitrogen significantly decreased by23.66 % and 17.36 %.Stem length,dry weight and fresh weight of Cr＿6 d plants significantly increased by 40.04 %,47.06 % and 28.57 %,respectively.Millet gene expression: transcriptomic analysis showed that Cr stress induced up-regulation of gene expression.According to GO and KEGG enrichment analyses,Cr suppressed the expression of genes linked to grain cell walls,cell membranes,cell division,and photosynthesis.But it also increased the expression of genes for DNA replication and repair,plant defense mechanisms,and hormone signal transduction to keep the grain genome stable and make it more resistant to Cr stress.(2)Microbial community structure and diversity: high-throughput sequencing results showed significant changes in community composition structure at the level of phylum and genus in Cr contaminated habitats.While the diversity of fungal communities dramatically decreased,that of bacterial communities displayed a phase transition characteristic from stress to stability(Shannon index: 4.17,3.81,3.23;Simpson index: 0.0068,0.0078,0.0068;CHAO index: 2818.49,2630.73,2769.38).The β diversity of bacterial and fungal communities also differed significantly in the distribution of Cr contaminated habitats.Microbial community construction: bacterial community assembly changed from stochastic to deterministic process dominated,and fungal community was continuously dominated by stochastic process.In addition the correlation analysis between soil physicochemical properties and microbial communities showed that soil physicochemical properties were significantly related with fungal communities,while the correlation with bacterial communities was weak.(3)The qPCR results indicated that nitrogen cycling functional genes(AOA-amo A and AOB-amo A)showed a substantial drop followed by an increase(P < 0.05);there was no significant change in diversity.The results of RDA and Mantel analysis showed that soil physicochemical properties of N(r = 0.377,P =0.03),P(r = 0.323,P = 0.041),K(r = 0.397,P = 0.016),EC(r = 0.434,P =0.014)were significantly correlated with carbon and nitrogen cycle functional genes.The results of Partial Least Squares Structural Equation Modeling(PLS-EM)analysis showed that Cr had a direct inhibitory effect on microbial communities and carbon and nitrogen cycle functional genes,and indirectly inhibited millet biomass through soil physicochemical indicators.Millet has a direct inhibitory effect on microbial abundance,the effect of soil physicochemical indicators on functional genes was stronger than that of microbial communities.Thus,Cr stress mostly affected millet’s phenotype,inhibited photosynthesis and cell proliferation and differentiation of millet seedlings.But stimulated the expression of genes such as plant defense system to resist Cr toxicity.Bacteria and fungi in soil had different response characteristics and community assembly driving patterns,and accelerated the expression of functional genes of carbon and nitrogen cycle to survive.This study is of great practical significance to infer the processes of resistance mechanism,microbial community structure and functional response mechanism of millet in heavy metal contaminated farmland soils.