| The leaf spot caused by Phoma medicaginis is a major disease of alfalfa(Medicago sativa)in the world,which usually occurs together with the pea aphid(Acyrthosiphon pisum)on alfalfa under natural conditions.Arbuscular mycorrhizal(AM)fungus as a very important soil microorganisms can colonize plant roots in 80%of terrestrial plant species,and then form mutualistic symbiosis with the roots in natural and agricultural ecosystems.AM fungus generally confers growth and fitness benefits to host plants and can promote host plants nutrients uptake.AM fungus also plays important roles in plant defence stress including biotic and abiotic stress.The present study was designed to investigate whether AM fungus can regulate the response of alfalfa to P.medicaginis and A.pisum.At the same time,the effect of AM fungus on the interaction between the leaf spot and pea aphid was studied.The mechanism of AM fungus on regulating pea aphid and leaf spot was analyzed.The main findings are as follows:(1)AM fungus enhanced the overall health level of alfalfa and the resistance of alfalfa to leaf spot and pea aphid was improved.AM fungus significantly promoted the growth of alfalfa and the absorption of nutrients N and P,increased the biomass and number of branches.The treatment of pathogen and aphid significantly reduced the photosynthetic rate,stomatal conductance and transpiration rate of alfalfa.However,compared with NM treatment,photosynthetic rate,stomatal conductance,and transpiration rate were increased in AM alfalfa by 168.99%,102.09%,and84.32%,respectively,which make alfalfa healthier.(2)AM fungus ameliorated the effects of P.medicaginis infection on alfalfa by reducing leaf spot incidence and disease index by 39.48%and 56.18%,respectively.Inoculation with pathogen and AM fungus induced the activity of defence pathways,including peroxidase(POD),polyphenol oxidase(PPO)activities and jasmonic acid(JA)by 58.21%,49.64%and 38.18%.Compared with NM plants,salicylic acid(SA)concentration was significantly increased by 62.95%in AM alfalfa.Plants showed differential expression of P.medicaginis resistance-related genes,including genes belonging to PR proteins,chitinase activity,flavonoid biosynthesis,phenylpropanoid biosynthesis,glutathione metabolism,phenylalanine metabolism and photosynthesis.Inoculation with AM fungus led to changes in the expression of genes involved in PR proteins,chitinase activity,phenylalanine metabolism and photosynthesis.(3)AM fungus can regulate plant response to aphids by promoting growth and increasing plant POD and CAT activities and SA concentration after aphid infestation.POD activity was significantly increased in AM alfalfa by 70.00%.POD,CAT activities and SA concentration was significantly increased in mycorrhizal fungus-inoculated plants and aphid-infested plants by 63.67%,44.61%and 29.97%,respectively.Transcriptome analysis showed that AM fungus increased the expression of resistance-related genes,such as‘WRKY transcription factor'and‘Kunitz trypsin inhibitor'.Additionally,GO terms“chitinase activity”,“peroxidase activity”,“defense response”,and“response to biotic stimulus”and KEGG pathways“phenylpropanoid biosynthesis”and“phenylalanine metabolism”were significantly enriched in mycorrhizal fungus-inoculated plants and aphid-infested plants.(4)AM fungus significantly increased the trypsin inhibitor activity of alfalfa.After infection with pathogen,the trypsin inhibitor activity and total phenol content were significantly increased,which enhanced the resistance of alfalfa to subsequent aphids.In addition,the olfactory response results showed that 75.83%of the tested pea aphids made a selection behavior on the alfalfa flavor sources under different treatments.Compared with NM plants,AM plants significantly attracted pea aphids(?~2=5.714,df=1,P=0.0168).Compared with NM plants,NMP alfalfa had a significant avoidance effect on pea aphids(?~2=4.113,df=1,P=0.0426).However,there was no significant difference in the choice of aphids between AM and AMP alfalfa(?~2=0.6250,df=1,P=0.4292).When both NM and AM alfalfa were infected with pathogen,pea aphids preferentially selected AM-infected plants(?~2=4.222,df=1,P=0.0399).The results of volatile organic compounds showed that the treatment of pathogen could induce alfalf to produce benzaldehyde and linalool(avoidance effect on pea aphids),so that increased the resistance of alfalfa to subsequent pea aphid.Compared with NM plants,the contents of benzaldehyde and linalool in AM plants were significantly reduced after pathogen infection.The results were consistent with the results of pea aphid host selection experiments. |
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