| As one of the environmental problems in the world,soil salinization severely restricts the development of agriculture and forestry.Robinia pseudoacacia is one of the tree species for afforestation in saline soil,and its salt-resistant mechanism is of great significance for the improvement and utilization of saline land.At present,the effect of arbuscular mycorrhizal fungi(AMF)on the salt-resistant mechanism of R.pseudoacacia has mostly focused on physiological,biochemical and related gene expression,thus,its salt-resistant mechanism still needs to be further explored.In this study,fine roots and rhizosphere soils were sampled in black locust(R.pseudoacacia)plantations in four saline sites(Chifeng and Dengkou in Inner Mongolia Autonomous Region,Pingluo in Ningxia Hui Autonomous Region,and Dunhuang in Gansu Province)to detect soil properties and AMF attributes including their colonization level,spore and hyphal density,and AMF community composition in the rhizosphere of R.pseudoacacia were tested by Illumina Miseq and their influencing factors were investigated.Based on the results of investigation,Rhizophagus intraradices,the dominant species in saline soils,was used to clarify the effects of AMF on litter decomposition of R.pseudoacacia under salt stress.Then the influences and mechanisms of adding litter and inoculation of AMF on the growth of R.pseudoacacia under salt stress were further studied.Finally,the mechanism of how AMF affecting salt tolerance of R.pseudoacacia were elucidated by using transcriptomics.The findings are as follows:1. AMF community composition and their affecting factors in the rhizosphere of R.pseudoacacia in saline soilResults showed the dominant AMF species of R.pseudoacacia rhizosphere in saline soils was R.intraradices,AMF attributes varied with sampling sites,and this variation was mostly attributed to changes in habitat characteristics and geographic distance.Habitat characteristics shape the root-and soil-inhabiting AMF communities to the same extent,whereas geographic distance seems to contribute more to root-inhabiting AMF assemblages.The level of root colonization was significantly correlated with the relative abundance of R.intraradices in roots,and the density of AMF spores and hyphae in soils were significantly affected by the relative abundance of Sclerocystis sinuosa in soils.2. Effects of AMF on litter decomposition of R.pseudoacaciaSalt stress reduced the decomposition rate and nutrient release rate in litter,inoculating with AMF helped release potassium and organic carbon in litter under salt stress,roots and hyphae accelerated the decomposition and nutrients release rate of litter.Different treatment has significant effect on bacterial and fungal community composition,salt stress significantly increased fungal community diversity in litter,AMF significantly increased bacterial and fungal community diversity,and bacterial and fungal community diversity significantly increased with the entry of AMF hyphae and roots.Redundancy analysis further showed that the bacterial and fungal community was significantly positively correlated with the accumulation of total potassium when roots showed in the decomposition boxes,and the bacterial and fungal community was significantly positively correlated with the accumulation of total nitrogen,total phosphorus,and carbon compounds when hyphae showed only.3. Effects of adding litter and inoculating AMF on the growth of R.pseudoacacia under salt stressSalt stress significantly reduced the growth status,photosynthetic and fluorescence parameters,and K~+content of R.pseudoacacia;litter and AMF separately significantly increased the growth status,net photosynthetic rate,maximum fluorescence efficiency,K~+content of R.pseudoacacia,and reduced Na~+content in leaves.Adding litter and inoculating AMF at the same time significantly increased the net photosynthetic rate and maximum fluorescence efficiency of R.pseudoacacia leaves under salt stress.Correlation analysis showed that the growth of R.pseudoacacia was significantly positively correlated with soil organic carbon,total nitrogen,and soluble total carbon content,and significantly negatively correlated with nitrate nitrogen,available phosphorus,and soluble total nitrogen content.This indicated that AMF and litter may improve salt-tolerant of R.pseudoacacia by improving photosynthesis ability,K~+/Na~+ratio and soil nutrient status.4. Effect of AMF on transcription level of R.pseudoacacia roots under salt stressA total of 531,509,730 sequences and 51,310 unigenes were obtained.Results of GO and KEGG functional cluster analysis for differential expressed genes showed that the roots of R.pseudoacacia responded to salt stress mainly through the process of hydrogen peroxide metabolism as well as flavonoid and flavonoid synthesis pathways;the adaptability of R.pseudoacacia to salt stress was regulated by transcription factors such as ERF/AP2,WRKY,and MYB.After inoculating with AMF,the identified differential expressed genes were mainly involved in biological regulation,response to stimulus,transport,and signaling pathways such as oxidative phosphorylation and protein processing.And AMF may improve the salt tolerance of R.pseudoacacia by up-regulating of the expression levels of nutrient transporter genes and antioxidant enzyme genes of R.pseudoacacia.This study investigated the community composition and influencing factors of AMF in the rhizosphere of R.pseudoacacia in saline soils,and explored the salt-resistant mechanisms of R.pseudoacacia through the aspects of nutriention mechanisn and molecular mechanism by inoculating the dominant species R.intraradices in saline soils,providing theoretical basis for the utilization and ecological improvement of saline soils. |
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