Composition And Function Of Extracellular Polymeric Substances Produced By Trichoderma Afroharzianum Under Salt Stress

Extracellular polymeric substances?EPSs?are mainly comprised of polysaccharides and proteins that are secreted by microorganisms and attached to the cell wall surface.They play a crucial role in maintenance of cell morphology,secretion of extracellular enzymes and defense under biotic and abiotic stresses,making the cells adapt to the complex and extreme environment.The aim of this study was to undermine the effect of different concentrations of NaCl on the EPSs composition and function and the probable halotaolerance mechanism of two Trichoderma sp.ACCC 32524 and ACCC 32527 with different halotolerance.The main findings are:1.Strains ACCC 32524 and ACCC 32527 were identified as T.afroharzianum according to the phylogenetic analysis of ITS/RPB2/TEF1 genes.Their growth was inhibited by different sodium salt in the order of Na₃PO₄>NaCl>NaNO₃>NaSO₄>sodium lactate.With glucose as the carbon source,their main components of EPSs were polysaccharides.After incubation with 0.6 mol/L NaCl,the polysaccharide contents of ACCC 32527 EPSs increased by 1.1-fold in comparison to control,which was significantly higher than that of ACCC 32524.2.The T.afroharzianum EPSs consisted of two polysaccharides of different molecular weights?EPSs-1 and EPSs-2?.Along with the increase of NaCl concentration,the molecular weight of polysaccharides increased;the content of EPSs-1 decreased,while that of EPSs-2 increased;and the glucose content in EPSs-1 decreased,while galactose and mannose contents increased.3.FTIR spectrum analysis indicated that the T.afroharzianum EPSs had no adsorption to NaCl.Under NaCl stress,the antioxidant activities of EPSs polysaccharides of ACCC 32524 and ACCC32527 were different.Along with the increase of NaCl concentration,the scanveging ability of ACCC32524 on ABTS⁺·and oxygen free radical decreased,while that of ACCCC32527 increased.4.Differential transcriptomes and metabolomes of ACCC 32524 and ACCC 32527 with 0,0.4 and0.6 mol/L NaCl were analyzed,and a total of 44,594 unigenes and 336 metabolites were identified.Under NaCl stress,31 genes related to polysaccharide transport and synthesis?glycosyl transferase,polysaccharide deacetylase,etc?were up-regulated.In addition,genes related to osmotic adjustment?8and 7?,antioxidant enzymes?7 and 13?and cell wall and extracellular components?6 and 4?were identified in ACCC 32524 and ACCC 32527,and most of them were up-regulated.In comparison to ACCC 32524,more glucose-1-phosphate,the polysaccharide synthesis precursor,and important cell osmotic regulation substances?glycerol as well as amino acids and their derivatives?accumulated in ACCC 32527.5.Using fructose,xylose,glycerol,mannitol,or sucrose as the carbon source,ACCC 32524 and ACCC 32527 EPSs were mainly composed of proteins,followed by polysaccharides.Under NaCl stress,all carbon sources promoted the production of EPSs polysaccharides,with xylose?1.4-fold?as the best.With sucrose,both strains had the strongest scavenging ability to oxygen free radicals and ABTS⁺·.These carbon sources had no effect on the types of monosaccharides,but influenced their ratios.In summary,the present study analyzed the differences of EPSs composition,antioxidant activity,transcriptome and metabolism of two T.afroharzianum strains ACCC 32524 and ACCC 32527,identified the key genes and metabolites probably related to EPSs biosynthesis,and primarily revealed the theoretical basis of fungal defense against hypersaline environment.