Combined Analysis Of Transcriptome And Microbiome On The Mechanism Of Response Of Rhododendron Decorum Subsp.diaprepes Seedlings To Cadmium Stress

With the intensification of human activities,the heavy metal pollutions in soil have become a serious environmental problem,posing a huge threat to crop production,food security and human health.Cd has potential toxicity and high mobility in the environment,making it one of the most toxic and widely distributed pollutants among heavy metals.Rhododendron decorum subsp.diaprepes is a constructive species in the Baili Rhododendron National Forest Park,which has ornamental value and ecological restoration value.The degree of Cd pollution in this area is high and uneven.However,little is known about the mechanism of R.decorum subsp.diaprepes respond to different Cd concentrations.So,R.decorum subsp.diaprepes seedlings were used as experimental materials and combined with physiological,molecular biology,transcriptomics and microbiomics to explore the change of physiological characteristics,transcriptional level and the community structure of endophytic bacteria and fungi of R.decorum subsp.diaprepes seedlings under different Cd concentrations（0 m M,2.5 m M and 5 m M）that clarify the mechanism of R.decorum subsp.diaprepes seedlings responding to Cd stress.The research results were as follows:1.As the Cd concentration increased,the degree of leaf curling of R.decorum subsp.diaprepes seedlings increased.Cd stress significantly inhibited the plant height,stem length,leaf area,leaf number and chlorophyll content of them,and it was also increased the content of Cd,Pb,Na and decreased the content of Ca,Mg and Mn,while the content of Al,Fe,Cu and Zn first increased and then decreased.In addition,the content of MDA and H₂O₂ and the activity of SOD and CAT were also increased in R.decorum subsp.diaprepes seedlings.2.Through transcriptome analysis of leaves of R.decorum subsp.diaprepes seedlings under different concentrations of Cd stress.Compared with 0 m M Cd treatment,1655 and 2286（up-regulated 1378,down-regulated 908）differentially expressed genes（DEGs）（up-regulated1033,down-regulated 622）were identified under 2.5 m M and 5 m M Cd treatments,respectively.Among the three comparative groups of 0 m M vs2.5 m M,0 m M vs 5 m M and 2.5 m M vs 5 m M,there were 19 common DEGs,while 714,1270 and 84 unique DEGs,respectively.However,in the 0 m M vs 2.5 m M and 0 m M vs 5 m M,0 m M vs 5 m M and 2.5 m M vs5 m M,0 m M vs 2.5 m M and 2.5 m M vs 5 m M groups,there were 886,111 and 36 DEGs,respectively.These DEGs mainly participated in biological processes,such as cellular processes,metabolic processes,cells,cellular components,binding and catalytic activities.The KEGG enrichment pathway showed that compared with 0 m M Cd treatment,under 2.5 m M and 5 m M Cd treatment,the up-regulated pathways of their common enrichment included photosynthesis-antenna proteins,porphyrin and chlorophyll metabolism,bassinosteriod biosynthesis,ribosome,nitrogen metabolism and ABC transport metabolism pathways,and down-regulated pathways included plant-pathogen interaction and plant hormone signal transduction metabolism pathways.The pathways significantly up-regulated under 2.5 m M Cd treatment were plant-pathogen interaction,while the down-regulated pathways were zeatin biosynthesis,fatty acid elongation,phenylalanine metabolism,diterpenoid biosynthesis,glycine,serine and threonine metabolism.The pathways significantly up-regulated only under 5 m M Cd treatment were isoflavonoid biosynthesis,lysine biosynthesis,glycosphingolipid biosynthesis-ganglio series and other glycan degradation,while the down-regulated pathways were beta-Alanine metabolism,brassinosteroid biosynthesis,MAPK signaling pathways and so on.3.Under different Cd concentrations,the dominant phyla of endophytic bacteria and fungi in the leaves of R.decorum subsp.diaprepes seedlings mainly include Proteobacteria,Actinobacteriota,Firmicutes,Bacteroidota,Basidiomycota and Ascomycota.The dominant genera mainly included Duganella,Aeromicrobium,Sphingomonas,Malassezia,Vishniacozyma and Nigrospora.The results of LEf Se analysis showed that there were different species in the endophytic bacterial community,including Myxococcota,Alkaliflexus,Micromonospora,Mesorhizobium,Acetoanaerobium and Bosea under 0m M Cd treatment,Actinomadura and Kocuria were under 2.5 m M Cd,Limnobacter and Achromobacter were under 5 m M Cd treatment.The different species of endophytic fungal community were Vishniacozyma and Malassezia under 0 m M and 2.5 m M Cd treatments,respectively.The results of co-occurrence network found that Cd stress change the co-occurrence network structure of endophytic bacteria and fungi in the leaves of R.decorum subsp.diaprepes seedlings.In addition,the top four network nodes of endophytic bacteria under different Cd concentrations were Proteobacteria,Actinobacteriota,Firmicutes and Bacteroidota,respectively.Compared with 0 m M Cd treatment,Firmicutes had a greater contribution than Bacteroidota under 2.5 m M and 5 m M Cd treatment.Endophytic fungi network nodes were mainly composed of Basidiomycota and Ascomycota.