Mechanisms of microbial community regulation by root exudate  of ryegrass under the stress of petroleum hydrocarbon pollution
In recent years, petroleum hydrocarbon pollution has become increasingly serious, and plant-microbe remediation technology has been widely concerned because of its somewhat green economy. Investigating the mechanisms of plant-microbe interactions in polluted environments can help to improve the application of plant-microbe remediation technology in practice. Root exudate has been proved to be a signal molecule for plants to " call for help" with microorganisms under adversity, therefore, the present study was based on petroleum hydrocarbon stressed ryegrass pot experiments to investigate the mechanism of rhizosphere microbial regulation by ryegrass root exudate. The main conclusions of this study are as follows:Petroleum hydrocarbon stress significantly altered the composition and structure of ryegrass rhizosphere secretions. A total of 22 differential metabolites were screened in this study. Matching the KEGG database according to the differential metabolites, it was found that the pathways of tyrosine metabolism and ABC transporter overlapped in the positive ion mass spectrometry mode, while the pathways of linoleic acid metabolism, naphthalene degradation, biosynthesis of plant secondary metabolites, and degradation of aromatic compounds overlapped in multiple groups in the anion mass spectrometry mode. Ryegrass root exudate under petroleum hydrocarbon stress can stimulate microbial nutrient uptake and promote microbial transfer to nutrients on the one hand, and promote petroleum hydrocarbon degradation on the other.Petroleum hydrocarbon stress significantly altered the microbial composition and metabolic functions of ryegrass rhizosphere microorganisms, as evidenced by the reduction of Alpha diversity, remodelling of rhizosphere microbial composition, increasing the relative abundance of the Proteobacteria, and decreasing the relative abundance of the Actinobacteria and Acidobacteria. Petroleum hydrocarbon stress significantly up-regulated the relative abundance of 27 functional genes, including PAH degradation, and Carbon fixation pathways in prokaryotes, which were mainly manifested in the regulation of the biological metabolism and degradation function of the rhizosphere community. Changes in the structure and composition of ryegrass rhizosphere plant beneficial bacteria (PBB) flora were similar to those of the rhizosphere microbiome, in addition to the promotion of soil C, N, and P cycling by petroleum hydrocarbon stress, which was associated with an increase in the predicted abundance of functional genes such as nifH, catalase, and pyrophosphatase. There was a significant increase in community dissimilarity of petroleum hydrocarbon degrading flora and a significant increase in the relative abundance of genes involved in alkane degradation and polycyclic aromatic hydrocarbon (PAH) degradation. This suggests that ryegrass can resist petroleum hydrocarbon stress by changing the structure and composition of the rhizosphere community, enhancing the metabolic pathways of functional genes, and promoting nutrient cycling.Differential metabolites of ryegrass roots under petroleum hydrocarbon stress were significantly correlated with the relative abundance of soil rhizosphere microorganisms, and the Mantel assay indicated that the compositions of plant beneficial bacteria and petroleum hydrocarbon degrading bacteria were regulated by differential metabolites in plant root exudate, and that the composition of the plant beneficial flora responded to the differential metabolites of the plant roots by a stronger mechanism than that of the petroleum hydrocarbon degrading bacteria. The results of the qualitative and quantitative chemotaxis validation tests showed that the recruitment of the signalling molecules to plant beneficial bacteria, especially to Rhodopseudomonas. sp, was stronger than that to petroleum hydrocarbon degrading bacteria.