The Growth-Promoting Mechanism For Plant By Anoxygenic Phototrophic Bacteria

The use of chemical fertilizers and pesticides has resulted in increasingly serious problems of agricultural ecological environment pollution and food safety.Microbial fertilizer has been included in the national “13th Five-Year Plan” and entered the golden period of development,which will play a broader role in the new era of weight loss and efficiency.Anoxygenic phototrophic bacteria(APB)as microbial fertilizer(PSB agent)has been widely studied and applied.its market share has gradually increased.It has been reported that more attention has been paid to improving plant growth efficiency in agricultural production.The mechanism of plant growth promotion has been focused on nitrogen fixation and 5-aminolevulinic acid(ALA),while other mechanisms of growth promotion have not been thoroughly studied.For example,microorganisms have five IAA combinations,while APB has only been reported to have two synthetic pathways,and the studied strains were limited to individual strains.What growth-promoting mechanisms does APB have,and whether the growth-promoting mechanisms or pathways with different functions in the same species,as well as their production conditions and environmental impact factors? So far,the above problems are still unclear,which limits the innovation and rational application of its production technology.To comprehensively understand the plant growth-promoting mechanisms APB and whether these growth-promoting mechanisms are ubiquitous in APB,the study on the mechanism of APB promoting plant growth has been carried out systematically.The main results abtained are as follows:Firstly,in order to elucidate the mechanism of APB-induced plant growth,the distribution pattern of 99 APB-related functional genes was analyzed comprehensively and systematically at the genome level.The APB genome data were collected,and the key genes and enzymes in the biosynthetic pathway of microbial growth-promoting substances were reported as reference.Phosphorus-soluble(40.4%),nitrogenase(58.6%),IAA(23.2%),5-ALA(96.0%),ACC deaminase(6.1%),iron carrier(29.3%),?-aminobutyric acid(69.7%),extracellular polysaccharide(62.7%),nitric oxide(12.1%),spermidine(60.6%),anti-heavy metals(55.3%)and other key genes(numbers in parentheses represent the ratio of this gene to statistical strains)were analyzed and retrieved by bioinformatics methods.It indicated that APB had the ability to synthesize plant growth-promoting substances through these mechanisms: by dissolving phosphorus,nitrogen fixation,synthesizing IAA and ALA,and regulating ethylene synthesis to directly promote plant growth;by synthesizing iron carrier,?-aminobutyric acid,extracellular polysaccharide,nitric oxide,spermidine to improve plant resistance to indirectly promote plant growth;By synthesizing extracellular polysaccharides,passivating metal ions and synthesizing metallothionein,metal ions are adsorbed,fixed or transferred into microbial cells.The content of effective heavy metals is reduced and the absorption of heavy metals by plants is reduced.Under stress conditions,the toxicity of heavy metals to plants is reduced,and plant growth is indirectly promoted.This study provided a comprehensive and systematic theoretical basis for the effect of photosynthetic bacteria on plant growth.Six APB strains,which are widely used and preserved in our laboratory,were selected to determine their ability of nitrogen fixation,phosphorus dissolution and IAA synthesis.Pot experiments were carried out with highly active strains.The effects of fermentation broth,bacteria and their supernatants on the growth of Chinese cabbage were preliminarily observed in order to elucidate the plant growth promoting effect of APB.Preliminary results showed that IAA synthesis was detected in all six APBs,and nitrogen fixation and phosphorus dissolution were observed in all four purple non-sulfur bacteria.Rhodopseudomonas palustris CQV97 had the highest IAA yield(2.52 mg/L).A 21-day pot experiment was conducted with CQV97 strain.The results showed that the effects of fermentation broth,bacteria and their supernatant on plant growth(plant height,root length,soluble protein,soluble sugar and chlorophyll content)were different.The supernatant had a greater impact on plant height,chlorophyll content and soluble protein of Chinese cabbage,which increased by 21.8%,18.9% and 64.5%,respectively.The effects of bacteria on root length and soluble sugar of Chinese cabbage were greater,which were 30.8% and 65.55% higher than the control CK,respectively.The results showed that both bacteria and supernatant could promote plant growth and improve crop quality,while bacteria secreted a variety of different substances to promote plant growth.The optimum conditions for IAA synthesis from six APBs were further optimized;the effects of exogenous tryptophan on the growth of bacteria and IAA synthesis were discussed;and the IAA synthesis pathways and main influencing factors of six APB strains were explored through the detection of intermediate metabolites.The results showed that the synthesis pathway of IAA was affected by initial pH,light intensity,inoculation amount and sample size.The optimum conditions for IAA synthesis by the six APBs were quite different.Tryptophan promotes IAA synthesis but inhibits growth,suggesting that these six APBs may synthesize IAA through tryptophan-dependent pathway.All six APBs have three IAA synthesis pathways,i.e.,indole-3-acetamide(IAM),tryptamine(TAM)and indole-3-pyruvic acid(IPDC)pathways.Rhodopseudomonas palustris CQV97,CQV009 and Rhodobacter sphaeroides QS also had IAN metabolism pathways.The contribution of these four synthetic pathways to IAA synthesis was influenced by environmental factors,of which IAM always contributed the most.Three APBs with small difference in heavy metal key genes were selected to observe their effects on the growth and heavy metal accumulation of Brassica napus under heavy metal cadmium stress.The results showed that the growth of plants under cadmium stress increased with the increase of cadmium concentration.The stress of cadmium on plants could be significantly alleviated by the treatment of bacterial suspension of three strains,which was manifested in the increase of chlorophyll content,plant height and root length,and CQV97 had the best treatment effect.When the soil content contained 10.0mg/kg cadmium,the treatment effect was the most obvious,which was increased by 5.09 times,68.97% and 83.3% compared with CK,and the cadmium enrichment coefficient of plants was significantly reduced.The results showed that APB could repair and promote plant growth and cadmium pollution in stress environment,which was not only related to key genes,but also related to gene expression.Genome analysis plays an important role in understanding its mechanism of action,but does not represent the effectiveness of its practical application.We should evaluate its efficacy and mechanism from more perspectives,such as protein and metabolites.Collectively,APB could promote plant growth and remedy cadmium pollution.In this study,the potential of APB to synthesize growth-promoting substances was analyzed at genome level,and the mechanism of APB to promote plant growth was elucidated systematically.The results showed that APB could promote plant growth through the mechanisms of nitrogen fixation,phosphorus dissolution and IAA synthesis.Different APB species had different growth promotion mechanisms.Six IAA synthesis pathways and their influencing factors were explored and elucidated.The six APBs could synthesize IAA through three to four pathways.IAM was the main synthesis pathway.The effects of excellent acid,light intensity,inoculation amount,sample loading and initial pH on IAA synthesis were revealed.This study clarified the mechanism of APB promoting plant growth,and provided theoretical basis for further study of APB promoting plant growth ability and development of excellent APB microbial agents.