Molybdenum Uptake And Translocation And Its Regulation Mechanism On Nitrogen Metabolism In Strawberry

Molybdenum is an essential element that is involved in nitrogen metabolism.Plants tend to accumulate more nitrate and show lower nitrogen use efficiency?NUE?under molybdenum-deficient conditions.Improving NUE in fruits reduces the negative effect of large applications of chemical fertilizer,but the mechanisms underlying how molybdenum enhances NUE remain unclear.This research included soilless culture and tissue culture experiment.The soilless culture experimrnt was carried out to study the effects of sodium molybdate?Na₂MoO₄?foliar sprays on biomasses,photosynthesis,fruit mineral element contents,fruit qualities?TSS,AsA,TA,and volatile compounds?,molybdenum transporter gene MOT1 expression level,Mo concentration in seedlings,activities on key enzymes of nitrogen metabolism?nitrate reductase,NR;NiR;glutamine synthetase,GS;NADH-glutamate synthase,NADH-GOGAT?,genes expression levels on enzymes of nitrogen metabolism?NRT1.1,NRT2.1,NIA,NIR,GS-1,NADH-GOGAT?,15 N absorption,distribution and utilization.Na₂MoO₄ levels were T1?0.00% Na₂MoO₄?,T2?0.02% Na₂MoO₄?,T3?0.04%Na₂MoO₄?,T4?0.05% Na₂MoO₄?and T5?0.06% Na₂MoO₄?.The tissue culture experiment was carried out to study the effects of plant phenotype,MOT1 expression level,genes expression levels on Moco synthetic pathway?CNX2,CNX3,CNX5,CNX6,CNX7,CNX1?,Mo concentration in seedlings,activities on key enzymes of nitrogen metabolism?NR,NiR,GS,NADH-GOGAT?,15 N utilization,root microstructure and root activity,leaf microstructure and ultramicroscopic structure,and chlorophyll content.The four treatments were CK?-Na₂MoO₄-Na2WO4?,Mo treatment?+Na₂MoO₄?,W treatment?+ Na2WO4?,and Mo+W treatment?+Na₂MoO₄+Na2WO4?.Results were as follows:1.Appropriate Mo foliar sprays can enhance biomasses of seedlings.Mo foliar sprays can improve the contents of N,Mo,Fe,Cu,TSS,TA,sweetness,and some sugars and acids,even some volatile compounds in the fruits.But the concentration of Se and some aroma compounds were not significantly affected by the Mo concentration.Fruits sprayed with0.04% Na₂MoO₄ exhibited the highest TSS and sweetness values,and the highest N and Feconcentration among all the treatments.And Pn value and the content of chlorophyll,fructose,glucose,sorbitol and total sugars in the fruits supplied with 0.04% Na₂MoO₄ were also higher than in other treatments.Fruits sprayed with Mo concentration of 0.02% exhibited significantly higher ascorbic acid?AsA?values than that of control.Ninety-seven volatile compounds were identified from fruits extracted by head-space solid phase microextraction?HS-SPME?coupled with gas chromatography-mass spectrometry?GC-MS?.Fruits sprayed with 0.04% Na₂MoO₄ exhibited the highest contents of six characteristic aroma compounds,methyl butanoate,?-decalactone,ethyl butanoate,methyl hexanoate,?-dodecalactone and ethyl caproate.2.The Mo concentration in every plant tissue analyzed increased gradually as Mo application level rose.The expression level of MOT1 in roots of T3 treatment was higher than the other treatments.When Na₂MoO₄ concentration was increased from 0.05% to 0.06%,MOT1 mRNA level in roots decreased.After 5 days of Na₂MoO₄ treatment,the expression level of MOT1 in leaves increased gradually as Mo application level rose.After 10 and 15 days of Na₂MoO₄ treatment,MOT1 mRNA level in leaves first increased then decreased when Na₂MoO₄ concentration was increased from 0.00% to 0.04%.Mo application affected Fe,Cu,and Se adsorption in roots.Seedlings sprayed with 0.04% Na₂MoO₄ had a higher [15N]shoot:root?S:R?ratio,and 15 NUE.Seedlings sprayed with 0.04% Na₂MoO₄ also had relatively high nitrogen metabolic enzyme activities and up-regulated transcript levels of nitrate uptake genes?NRT1.1;NRT2.1?and nitrate-responsive genes.Furthermore,there was a significantly lower NO3-concentration in the leaves and roots,a higher NH4+ concentration in leaves,and a higher glutamine/glutamate?Gln/Glu?concentration at the concentration of 0.04% Na₂MoO₄.Seedlings sprayed with 0.06% Na₂MoO₄ showed the opposite trend.Our results suggest that0.04% Na₂MoO₄ application was optimal because it enhanced NO3-transport from the roots to the shoots and increased NUE by mediating nitrogen metabolic enzyme activities,nitrate transport,and nitrate assimilation gene activities.3.Strawberry seedlings grew well on the MS medium supplied with Na₂MoO₄ and its leaves were green with normal leaf cell ultrastructure,more chloroplasts and more chlorophyll content.Its root development proceeded normally with higher root activity and arranges of root tip cells were neat and close.Its pitch cells were larger and arranged densely.Strawberryseedlings on the MS medium supplied with Na₂MoO₄ were with irregular epidermis cells,abnormal leaf cell ultrastructure,damaged organelles and endomembrane system,lower chlorophyll content,and yellow leaves.Its roots were longer but grew weaker,with root tip cell deformability and loose arrangement.Strawberry seedlings growing on the MS medium supplied with only Na2WO4 or Na₂MoO₄ and Na2WO4 were smaller and weaker,with damaged organelles,unclear membrane structure,some yellow leaves and decreased chlorophyll content,which was similar to nitrogen deficiency symptoms.Na₂MoO₄ could induce the expression of FaMOT1 in shoots and roots of strawberry seedlings growing on the MS medium,but Na2WO4 showed the opposite trend.The Mo concentration of seedlings growing on the MS medium supplied with Na₂MoO₄ was highest among all the treatments.CNX2,CNX3,CNX7,and CNX1 showed higher expression levels in seedlings growing on the MS medium supplied with Na₂MoO₄ than those growing on the MS medium supplied with Na2WO4.The Mo treatment owned higher activities of NR,NiR,GS and NADH-GOGAT and had lowest NO3--N concentration,highest concentrations of NO2--N and NH4+-N.The isotope15N-Ca?NO3?2 label experiment suggested that seedlings of the Mo treatment absorbed more15 N,then more 15 N was transported from roots to shoots,and 15 N utilization efficiency was highest among the treatments.However,seedlings treated with Na2WO4 inhibitors showed lower 15 N utilization efficiency.