Rhodopseudomonas Palustris PSB06 Promotes Pepper Growth And Development In Seedling Stage By Regulating The Rhizosphere Microecological Environment

Pepper is economically one of the most important crops,with an annual planting area of more than 1.3 million hectare in China.Traditional methods to promote the pepper growth and increase pepper yield mainly focus on breeding,water and fertilizer management and improving cultivation techniques.However,large amounts of chemical fertilizers and pesticides cause great harm to the environment when used to improve pepper cultivation.Rhodopseudomonas palustris(R.palustris)PSB06,a species belonging to the purple photosynthetic bacteria,not only can control soil-borne diseases caused by different plant pathogens,but also improve crop yields and quality.However,there are less studies addressing the effect of photosynthetic bacteria in terms of promoting crop growth.In this study,we used the pepper variety Xiangyan 15(the largest pepper cultivar)as a model to study the correlation between the rhizosphere micro-ecological environment and the pepper growth and development under field and greenhouse conditions when R.palustris PSB06 was applied by using microbiome and metabolome sequencing technology.The results indicated that R.palustris PSB06 promotes pepper growth and development in seedling stage by regulating the rhizosphere micro-ecological environment.The main research results are as follows:1.Field experiment results showed that R.palustris PSB06 treatment significantly improved pepper yield and increased the total nitrogen content compared with CK treatment.Total nitrogen was one of the most important factors affecting the bacterial community.The R.palustris PSB06 treatment increased the bacterial alpha diversity in the rhizosphere soil and changed the community structure.Meanwhile,the relative abundance of Gaiella and Gp6 of eight positive genera in the three test groups was significantly higher than that in the CK treatment after the application of the strain,which were significantly correlated with the yield.It is hypothesized that these bacteria may play an important role in the yield improvement.2.The colonization results showed that R.palustris PSB06 colonized the plant in the root elongation zone.The R.palustris PSB06 treatment significantly increased the bacterial alpha diversity in the rhizosphere soil compared with CK treatment.The bacterial community structure was significantly different from that of the CK treatment,and the differences in community structure decreased compared with CK treatment.The dominant population changed significantly within 14 days.Azospira oryzae,Clostridium beijerinckii and Rhodopseudomonas palustris were the dominant species among the 37 OTUs participating in nitrogen fixation,with the relative abundance between 1% and 6%.They increased significantly and then decreased gradually,indicating that Azospira oryzae,Clostridium beijerinckii and R.palustris PSB06 may be involved in a cooperative promotion relationship.3.After root irrigation with R.palustris PSB06,the photosynthetic bacteria and nitrogenfixing bacteria alpha diversity in the rhizosphere gradually increased,and the community structure was significantly different from that of the CK treatment.The relative abundance of species changed significantly within 14 days,and the difference in community structure of photosynthetic bacteria and nitrogen-fixing bacteria in the rhizosphere and bulk soil gradually decreased.R.palustris,Novosphingobium subterraneum,Methylibium sp.pch-m and Rhodobacter sphaeroides were the four most abundant photosynthetic bacteria representatives.R.palustris,Rhodobacter?sp?AKP,Azospira?oryzae and Azonexus?hydrophilus were the four most abundant nitrogen fixing bacteria.R.palustris was the dominant bacterium in the purple photosynthetic bacteria and nitrogen-fixing functional bacterial groups.4.At the same time,we also analyzed the correlation between the application of R.palustris PSB06 and pepper growth and development.And we also conducted a preliminary study on the interaction between root exudates and rhizosphere microorganisms by combining the metagenomic and metabolome methods.The results indicated that the root irrigation with R.palustris PSB06 increased fresh weight,root length and stem length,which promoted the pepper growth and development at seedling stage.The plant endogenous hormone cytokinin and abscisic acid were significantly lower,and phosphate increased significantly which is involved in plant photosynthesis light reaction,The reason that R.palustris PSB06 might promote pepper seedling growth by adjusting slightly the content of plant endogenous hormones and promote plant light reaction process.Nine metabolites matched the gene pathway,eight compounds were screened out from the root exudates that were significantly up-regulated and five compounds were significantly down-regulated.The major metabolites oleic acid and 1,2,4-Benzenetriol were significantly up-regulated.A total of 14 species including Opitutus?terrae were found to be significantly up-regulated among the top 30 rhizosphere species,and there was a correlation between them.That indicates the root exudates continuously enter the soil,which changed the rhizosphere micro-ecological environment,including the microbial community structure and dominant populations.In conclusion,the R.palustris PSB06 promotes pepper growth and development in seedling stage by accelerating the evolution of the rhizosphere microbial community.On the one hand,R.palustris PSB06 improved the micro-ecological rhizosphere soil environment by increasing the alpha diversity of rhizosphere soil and affected the population changes of nitrogen-fixing bacteria,such as Azospira oryzae,to improve the nitrogen content in soil by biological nitrogen fixation.At the same time,pepper may secrete root exudates such as oleic acid or 1,2,4-Benzenetriol to affect the rhizosphere microorganism and trigger the response of rhizosphere microorganisms including Opitutus terrae and others,and then maintain the plantmicroorganism interactions to promotes pepper growth and development.It is also possible to promote the pepper growth and development by change endogenous hormones and upregulate positive phosphate content.This study provides a theoretical basis for the study of the growth promotion mechanism of R.palustris PSB06 on crops.