谷子及其根际土壤微生物群落对铬胁迫的响应机制

Response mechanisms of millet and its rhizosphere soil microbial communities to chromium stress

重金属铬(Cr)污染对农田中农作物产生毒害作用并破坏土壤微生物群落稳态,但不同农作物及其根际土壤微生物群落对Cr胁迫的响应机制均有所差异。该研究在时间序列上分析Cr胁迫对谷子(正名:粱, Setaria italica)长势、谷子差异表达基因(DEGs)的功能途径及土壤微生物群落结构和功能等方面的影响,阐明谷子及土壤微生物群落的响应机制,为Cr胁迫下的谷子生长及污染土壤的生态修复治理提供理论依据。基于室内盆栽实验,以谷子幼苗和种植谷子的土壤为实验材料,在Cr胁迫前(CK)及胁迫后6 h和6 d (Cr_6h、Cr_6d)的时间序列上分别进行样本采集,同时测量幼苗生理性状指标及土壤理化指标。通过转录组分析,研究Cr胁迫时间序列上谷子幼苗基因表达及所富集的功能途径的变化趋势;通过高通量测序分析,研究Cr胁迫时间序列上土壤微生物群落结构、物种多样性、群落功能的动态变化过程及与土壤理化性质的相关性。结果发现:1)转录组分析结果表明Cr胁迫诱导基因表达上调(上调DEGs 54%);Gene Ontology (GO)富集分析表明DEGs在CK和Cr_6h、Cr_6h和Cr_6d样本对中与光合作用相关的基因的表达显著下调;在Cr_6h和Cr_6d样本对中与防御和损伤调控相关的基因的表达显著上调,细胞壁及细胞膜和细胞分裂相关基因的表达下调。2)高通量测序结果表明Cr胁迫时间序列上土壤细菌与真菌在门、属水平组成变化显著;细菌群落α多样性呈现出由应激到稳定的阶段性变化特征(CK、Cr_6h和Cr_6d的Shannon多样性指数分别为6.09、5.93和6.05;Simpson多样性指数分别为0.0068、0.0078和0.0068;Chao多样性指数分别为2818.49、2630.73和2 769.38),而真菌群落α多样性显著下降(Shannon-Wiener多样性指数分别为4.17、3.81和3.23);细菌与真菌群落的β多样性在Cr胁迫时间序列的分布差异显著。3)土壤理化性质与微生物群落相关性分析表明土壤理化因子与多种真菌群落显著相关,而与细菌群落的关联关系较弱。结果表明Cr胁迫在时间序列上通过降低叶绿素含量、光系统活性并影响类囊体等结构组分显著抑制了谷子幼苗的光合作用过程,通过下调细胞壁及微管相关组分基因表达抑制叶片细胞的增殖分化过程,但同时激活了植物防御系统以降低自身所受的毒害作用。同时土壤细菌与真菌群落通过群落组成结构及多样性的变化来适应Cr胁迫,二者在胁迫时间序列上的响应程度和策略均有所不同。

Aims Heavy metal chromium(Cr)contamination has toxic effects on crops and will disrupt soil microbial community homeostasis in agricultural soils.Nevertheless,the mechanisms underlying the responses of different crops and their rhizosphere soil microbial communities to Cr stress were different.By using time series data,this study analyzed the effects of Cr stress on‘Jingu 21’growth,functional pathways of differentially expressed genes(DEGs)in cereals and soil microbial community structure and function.Our objective was to elucidate the response mechanism of millet(Setaria italica)and soil microbial community,and provide a theoretical basis for the growth of cereals under Cr stress and the ecological restoration of the contaminated soil.Methods We collected millet seedlings and soil samples from the pot experiments with planted millet before(CK)and 6-hour and 6-day after Cr stress(Cr_6h,Cr_6d),and determined the physiological traits of seedlings and soil physicochemical properties.The gene expression and the functional pathways enriched in the seedlings were investigated by transcriptome analysis;the dynamics of microbial community composition,structure,diversity as well as function in time series and their correlations with soil physicochemical properties were studied by high-throughput sequencing analysis.Important findings 1)Transcriptome analysis showed that Cr stress induced up-regulation of gene expression(54%for up-regulated DEGs);GO enrichment analysis showed that DEGs significantly down-regulated the expression of photosynthesis-related genes in CK&Cr_6h,Cr_6h&Cr_6d samples,and also significantly up-regulated the expression of defense and damage regulation-related genes,and down-regulated the expression of cell wall and cell membrane and cell division-related genes in Cr_6h&Cr_6d samples.2)High-throughput sequencing revealed a significant change in the composition of soil bacterial and fungal communities at the phylum and genus level during the time series of Cr stress.Theαdiversity of bacterial communities showed a phase change from stress to stability(Shannon-Wiener diversity index for CK,Cr_6h,Cr_6d were 6.09,5.93,6.05,respectively.Simpson diversity index for CK,Cr_6h,Cr_6d were 0.0068,0.0078,0.0068,respectively;Chao diversity index for CK,Cr_6h,Cr_6d were 2818.49,2630.73,2769.38,respectively),while theαdiversity of fungal community decreased significantly(Shannon-Wiener diversity index for CK,Cr_6h,Cr_6d were 4.17,3.81,3.23,respectively).The distribution ofβdiversity of bacterial community in the Cr stress time series was significantly different from that of fungal community.3)Correlation analysis between soil physicochemical properties and microbial communities showed that soil physicochemical factors were significantly correlated with a variety of fungal flora,but weakly correlated with bacterial flora.Cr stress significantly inhibited the photosynthesis of millet seedlings by reducing chlorophyll content,photosystem activity and affecting structural components such as thylakoid,and inhibited the proliferation and differentiation of leaf cells by down-regulating the expression of cell wall and microtubule-related components.At the same time,Cr stress also activated the plant defense system to overcome toxicity.Meanwhile,soil bacterial and fungal communities adapted to Cr stress through changing community composition and diversity,and their response levels and strategies differed in the stress time series.