Mechanisms For Phomopsis Liquidambari B3-Promoted Aerenchyma Formation And Improved Metaboilic Level In Rice

The relationships between the fungal endophyte and host plants are generally considered to be mutually beneficial symbiosis.The establishment of endophyte-plants symbiosis triggers a series of physiological and biochemical responses,including the improvement of metabolic level,nutrient absorption,and the resistance to abiotic stress.The endophytic fungus Phomopsis liquidambari B3 is a broad-spectrum endophytic fungus isolated from the inner bark of Bischofia polycarpa in our previous studies.It could improve nitrification rates,alter the abundance and composition of ammonia-oxidizers in the rhizosphere soil,and promote nitrogen(N)uptake of rice.Aerenchyma is related to oxygen(O2)transport efficiency and contributes to enhanced rhizospheric nitrification under flooding conditions.However,whether and how P.liquidambari affects aerenchyma formation is largely unknown.Given the importance of plant physiology and hormones signaling in aerenchyma formation,we aim to reveal the effects of P.liquidambari on aerenchyma formation and the mechanisms behind it from these two aspects.And then,we researched the effects of P.liquidambari on primary metabolism and mineral elements accumulation in rice under stagnant condition,which are associated with aerenchyma formation.First of all,we applied bacterial suspension of P.liquidambari to rice roots to observed its colonization in roots.After that,we conducted pot experiments to investigate the differences of aerenchyma area,root porosity(POR),radial oxygen loss(ROL)and oxidation-reduction potential(ORP)of rice with or without P.liquidambari treatment during exposure three different N levels at four growth stages.Our results showed that P.liquidambari could successfully colonize in epidermal cell layer of rice root as an endophyte.The larger aerenchyma area in rice roots were observed following P.liquidambari treatment,and caused substantially elevated POR,leading to the enhancement of ROL and ORP,compared with the control.Furthermore,we detected plant hormones,including ethylene,indole acetic acid(IAA),abscisic acid(ABA)and cytokinin(CTK),which associated with aerenchyma formation,and found that ethylene and auxin signaling responded obviously to P.liquidambari treatment.The obtained results offer novel evidence concerning that ethylene and auxin signalling may participate in P.liquidambari-induced aerenchyma production.To explore the role of ethylene and auxin in P.liquidambari-promoted aerenchyma formation of rice roots,differences in aerenchyma area were assessed by exposing plants to various concentrations of ethylene precursor ACC and the inhibitor AOA,IAA and the inhibitor PCIB,respectively.The results showed that aerenchyma area was significantly increased by exogenous application of ACC or IAA,while it was severely impaired by their inhibitors AOA or PCIB.However,the P.liquidambari treatment partly mitigated the adverse effects caused by inhibitors.These results indicated that ethylene and IAA played an essential role in P.liquidambari-promoted aerenchyma formation of rice roots.In addition,we found that the P.liquidambari significantly increases IAA and ethylene levels in rice by stimulating the expression of genes involved in auxin and ethylene biosyntheses,leading to high expression of genes related to aerenchyma formation.Furthermore,auxin,partly acting upstream of ethylene signaling,played an essential role in P.liquidambari-mediated promotion of aerenchyma formation.Stagnant condition was established to investigate the effects of P.liquidambari on primary metabolism and mineral elements absorption in rice.The contents of chlorophyll and soluble sugar,activities of key enzymes of aerobic and anaerobic respiration,and contents of ATP and mineral elements in rice were measured under stagnant condition.The results showed that P.liquidambari enhanced soluble sugar accumulation caused by the improvement of photosynthesis,which activated anaerobic and aerobic respiration enzymes in roots.Compared to the uninfected plants,more ATP contents produced by respiration were found in rice roots upon endophyte infection under anoxic condition,which contributed to the synergy absorption of multiple mineral elements.Therefore,we deemed that P.liquidambari could improve primary metabolism level and mineral elements accumulation in waterlogging rice,which were possibly associated with aerenchyma formation induced by endophyte.Taken together,the obtained results verified the direct contribution of P.liquidambari in promoting aerenchyma formation via the accumulation of IAA to enhance ethylene synthesis in rice roots.Furthermore,aerenchyma formation induced by endophyte may make an important contribution for the improvement of metabolism level and mineral elements accumulation.