| Lignin is one of the main components of the plant cell wall.As a physical antibacterial substance in plant cells,it can form a physical barrier to enhance cell protection after accumulation.Previous studies have shown that lignin can improve plants resistance to pathogens and effectively prevent pathogenic infection.Herbaspirillum huttitense 5-28（H.5-28）is a strain of Herbaspirillum isolated and identified by our lab.Based on our previous studies,we found that H.5-28 could enhance the disease resistance of Arabidopsis thaliana by regulating the synthesis of related metabolites in the phenylalanine metabolism pathway.However,its specific resistance mechanism is not precise.And it is not clear whether the strain is universal or not.In this study,Arabidopsis thaliana,Nicotiana tabacum L.,and Solarium lycopersicum plants were inoculated with H.5-28 and then the pathogen Pseudomonas Pseudomonas syringae DC3000（Pst DC3000）to explore the mechanism of improving plant disease resistance by H.5-28.The main results are as follows:1.After different treatments of Arabidopsis thaliana,Nicotiana tabacum L.,and Solanum lycopersicum,the phenotypic difference of plants were observed and recorded,and their biomass was performed.The results showed that:compared with the plants inoculated with Pst DC3000,the disease degree and biomass of the plants treated with H.5-28 and Pst DC3000 were significantly alleviated.Among them,the leaf area,fresh weight,and root number of Arabidopsis thaliana were increased by 1.28,8.1,and 4.86 times after co-treatment with H.5-28 and Pst DC3000,respectively.The plant height,leaf diameter,wet weight,stem diameter,and root weight of Nicotiana tabacum L.were increased by 1.73 times,1.25 times,1.34 times,1.28 times,and 1.76 times,respectively.The plant height,fresh weight,stem diameter,root weight,and root length of Solanum lycopersicum were increased by 1.73 times,2.15 times,1.26 times,2.21 times,and 1.43 times,respectively.The above results revealed that H.5-28 could promote biomass accumulation among these three species and enhance disease resistance.It is preliminarily proved that the disease resistance of H.5-28 is universal.2.Based on the previous research results of our lab,the related genes involved in the biosynthesis of flavonoid and phenylpropane lignin were screened out,and the expression patterns of them and the contents of related compounds were further analyzed.The results showed that seven genes were up-regulated in the biosynthesis pathway of flavonoids in Arabidopsis thaliana treated with H.5-28 and Pst DC3000 compared to the Arabidopsis thaliana inoculated only with Pst DC3000.The expression levels of 14 genes in the phenylpropanoid lignin synthesis pathway were up-regulated,and the total flavonoid content had no significant difference.The total lignin and monomer lignin（H,S,G）contents increased significantly.The contents of total lignin,H-lignin,S-lignin and G-lignin were increased by 1.56 times,1.72 times,1.48 times and 1.44 times.It is indicated that H.5-28 might induce the disease resistance of Arabidopsis thaliana by regulating the expression of genes involved in lignin biosynthesis,which further increased the contents of total lignin and monomer lignin in plants and then enhanced the disease resistance of Arabidopsis thaliana.3.To explore the mechanism of disease resistance induced by H.5-28 in Nicotiana tabacum L.and Solanum lycopersicum.We further detected the expression patterns of essential enzyme genes in the lignin biosynthesis pathway and the contents of total lignin and monomer lignin（H,S,G）in the two species.The results showed that 54 genes were up-regulated and the contents of total lignin and monomer lignin（H,S,G）in Nicotiana tabacum L.treated with H.5-28 and Pst DC3000 were significantly increased compared with them only inoculated with PstDC3000.Compared with Solanum lycopersicum inoculated with Pst DC3000 only,the expression of 9 genes in Solanum lycopersicum treated with H.5-28 and Pst DC3000 was up-regulated,the contents of total lignin and monomer lignin（H,S,G）were also significantly increased.The above results revealed that H.5-28 might increase the contents of total lignin and monomer lignin in Nicotiana tabacum L.and Solanum lycopersicum by regulating the expression of related genes in the lignin biosynthesis pathway,and thus enhanced the disease resistance in plant.4.Transcriptome data showed that the essential enzyme genes in the lignin biosynthesis pathway of Arabidopsis thaliana had time-specific expression characteristics after treatment with H.5-28.Compared with the control group,there was no significant difference in the expression of those genes after treatment with H.5-28 for one day;however,after treatment with H.5-28 for three days,the expression levels of AT5G05340,AT5G06730,and AT3G43670 genes involved in lignin biosynthesis of Arabidopsis thaliana were significantly up-regulated.After treated with H.5-28 for three days and then inoculated with Pst DC3000 for one day,the expression of essential enzyme genes did not change significantly in Arabidopsis thaliana.Moreover,two days later,the expression of 9 enzyme genes was significantly up-regulated.Therefore,after induction by H.5-28,we speculated that the expression patterns of essential enzyme genes in the lignin biosynthesis pathway could be regulated in Arabidopsis thaliana whether inoculation with Pst DC3000 or not,on the third day and the second day,respectively,which increased the accumulation of lignin and other substances in the body,and finally improved the disease resistance of the plant.In conclusion,this study confirmed that H.5-28 could enhance plant disease resistance by regulating the phenylpropane lignin biosynthesis.Our results could provide a scientific reference for further developing and utilizing H.5-28. |
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