Physiological Mechanisms Of Arbuscular Mycorrhizal Fungi (AMF) Enhancing Low Temperature Resilience In Maize

Low temperature is a major environmental factor,limiting crop productivity.The low temperature has many negative effects,such as decrease of photosynthetic capacity,damage of plasma membrane,imbalance of reactive oxygen species,inactivation of key enzymes,thereby impeding plant growth and development.Arbuscular mycorrhizal fungi(AMF),as an archaic kind of soil microorganism,distribute around the world,and are symbiotic with about 80%plant root system.Many studies have demonstrated that arbuscular mycorrhizal fungi-plant symbiosis is an effective strategy that this symbiosis enhances the low temperature tolerance.The physiological mechanisms to induce the low temperature tolerance by arbuscular mycorrhizal fungi,especially the genes expression and lipidomic reprogramming,still poorly understood.The physiological analysis,transcriptomics and lipidomics were used to further analyze physiological mechanisms of arbuscular mycorrhizal fungi enhancing low temperature tolerance in maize.These results contribute to exploring regulation mechanisms that arbuscular mycorrhizal fungi enhance low temperature tolerance of maize seedlings,and provide novel insights into discovering new cold-resistance genes,thereby increasing the maize productivity.(1)Under normal temperature control,mycorrhizal colonization of maize seedlings was 49.48%;mycorrhizal colonization of maize was 33.76%under low temperature.It was decreased by 33.76%(P<0.01).As compared to normal temperature control,aboveground fresh weight of maize seedlings was reduced by 18.97%(P<0.01)under low temperature,but aboveground dry weight was not affected by low temperature.However,inoculation with arbuscular mycorrhizal fungi significantly increased aboveground dry and fresh weight of maize by 23.40%and 16.41%than no arbuscular mycorrhizal fungi under low temperature,respectively.(2)As compared to normal temperature control,low temperature increased relative conductivity of maize leaves by 1.8-fold.Under low temperature,the relative conductivity of mycorrhizal maize leaves was decreased by 38.61%(P<0.05),compared with non-mycorrhizal maize.A similar trend was found in malonaldehyde concentration.Low temperature significantly decreased chlorophyll a,chlorophyll b,total chlorophyll concentration and maximum photochemical efficiency of PS Ⅱ(FV/FM)in maize leaves,but inoculation with arbuscular mycorrhizal fungi significantly increased chlorophyll a,chlorophyll b,total chlorophyll concentration by 44.46%(P<0.05),49.42%(P<0.01),45.96%(P<0.01)than no arbuscular mycorrhizal fungi,respectively.Meanwhile,it had no significant effect on FV/FM(P>0.05).Under low temperature,the soluble sugar concentration of maize was increased by 12.70%(P<0.05),and the starch concentration was decreased by 34.67%(P<0.01)as compared to the normal temperature control.The sucrose concentration was not affected by low temperature.Meanwhile,inoculation with arbuscular mycorrhizal fungi more significantly increased the soluble sugar concentration by 31.57%(P<0.05)under low temperature,but had no significant effects on starch concentration and sucrose concentration(P>0.05).(5)This experiment included four transcript libraries.Under low temperature,there are 702 differentially expressed genes(DEGs)between inoculation with arbuscular mycorrhizal fungi and no arbuscular mycorrhizal fungi.The upregulated DEGs were 196;the downregulated DEGs were 506.For GO analysis,the DEGs were enriched into biological processes(48.89%),cellular components(44.89%)and molecular functions(6.22%);For KEGG analysis,the relative genes enriched into phytohormone(i.e.,auxin,cytokinin,abscisic acid,gibberellin,ethylene,brassinolide and salicylic acid)signal transduction were differentially expressed,suggesting phytohormone signal transduction plays a critical role on enhancing low temperature tolerance of maize seedlings by inoculation with arbuscular mycorrhizal fungi.Inoculation with arbuscular mycorrhizal fungi had an effect on the expression of genes related to metabolism pathways,such as flavonoid biosynthesis,starch and sucrose metabolism,glycerophospholipid metabolism,indicating it is a key strategy for mycorrhizal maize in response to low temperature via changing gene expression of metabolism enzymes.(6)To study the role of lipidomic reprogramming in mycorrhizal maize in response to low temperature,the lipid composition and lipid abundance were determined via liquid chromatography-mass spectrometry in maize leaves.The lipid composition of maize leaves was identified as four classes,including fatty acyls,glycerolipids,glycerolphospholipids,sphingolipids,but different treatments had no significant effects on the lipid abundance.There are 19 differential lipid molecules between inoculation with arbuscular mycorrhizal fungi and no arbuscular mycorrhizal fungi under normal temperature controls,including increased abundance of 14 lipid molecules and decreased abundance of 5 lipid molecules;There are 10 differential lipid molecules between inoculation with arbuscular mycorrhizal fungi and no arbuscular mycorrhizal fungi under low temperature,including increased abundance of 8 lipid molecules and decreased abundance of 2 lipid molecules.Under low temperature,the abundance of CDP-diacylglycerols(44:0),monogalactosyl diacylglycerol(48:8),monogalactosyl diacylglycerol(48:9),digalactosyl diglyceride(36:6)in mycorrhizal maize leaves were significantly increased by 100%(P<0.01,P<0.05),the abundance of ceramides(36:0)were significantly increased by 44.09%(P<0.05),and the abundance of phosphatidylcholine(O-36:1)and phosphatidylglycerol(34:2)were significantly decreased by 100%,164%.These results showed inoculation with arbuscular mycorrhizal fungi contribute to increasing the stability and integrity of chloroplast membrane under low temperature,including thylakoid membrane,thereby maintaining normal function of oxygen-evolving complexes and photosynthetic electron transport.