Effects Of Polyamines And Ethylene On Amino Acids In Milled Rice And Its Nitrogen Regulation

Polyamines(PAs)and ethylene(ETH)are two important classes of plant growth regulators,which have important regulation effects on the growth and development,yield formation,milling and appearance qualities,and protein composition of rice.However,it is unclear about the regulation effects and mechanisms of PAs and ETH and their interaction on amino acid composition and content in rice grains.To elucidate the regulatory role and mechanism of free PAs and ETH on amino acids in rice grains,six rice varieties with different protein content(two cultivars with low-protein content,two cultivars with medium-protein content,and two cultivars with high-protein content)were used as materials to measure the concentrations of grain free-PAs and 1-aminocylopropane-1-carboxylic acid(ACC,a synthetic precursor of ethylene),ethylene evolution rate(EER),and the activities of key enzymes involved in amino acid biosynthesis,and amino acid contents in milled rice.The effects of exogenous PAs and ETH related chemicals,different nitrogen(N)application rates on endogenous levels of grain free PAs and ETH,activities of key enzymes involved in amino acid synthesis,expression of related coding genes,and amino acid content of rice were also investigated.The main results were shown as follows:1.The role and mechanism of polyamines in regulating amino acid biosynthesis in rice grainsCultivars with higher concentrations of free-Spermidine(Spd)and free-Spermine(Spm)exhibited higher amino acid contents than the others.The concentrations of free-Spd and free-Spm were significantly and positively correlated with the contents of essential amino acids(EAAs),non-essential amino acids(NEAAs)and total amino acids(TAAs)in milled rice,whereas the concentration of free-Putrescine(Put)had no effect on these parameters.The concentrations of free-Spd and free-Spm were linearly associated with the activities of glutamine synthetase(GS),glutamate synthetase(GOGAT),aspartate transaminase(AST),aspartate kinase(AK),and alanine transaminase(ALT),but only GOGAT,AST and ALT were significantly and positively correlated with amino acid content.Tne application of exogenous Spd or Spm to panicles increased the activities of these key enzymes and amino acid contents,whereas the application of MGBG(an inhibitor of Spd or Spm synthesis)had the opposite effect.Applying Put had no significant effect on these traits.These results suggest that free-Spd and free-Spm enhance amino acid biosynthesis during grain filling in rice,primarily by improving the activities of GOGAT,AST,and ALT.2.The role and mechanism of ethylene in regulating amino acid biosynthesis in rice grainsRice cultivars with higher levels of ETH and ACC exhibited lower amino acid contents than the others.The EER and ACC content were highly significantly and negatively correlated with the activities of GOGAT,AST and ALT involved in amino acid biosynthesis,and the contents of EAAs,NEAAs,and TAAs in milled rice.The application of ethephon(an ethylene-releasing agent)or ACC to rice panicles markedly reduced the activities of the above three enzymes and contents of amino acids,whereas the use of aminoethoxyvinylglycine(AVG,an inhibitor of ACC synthase)had the opposite effect.These results suggest that high ETH level negatively affects amino acid biosynthesis mainly by inhibiting the activities of GOGAT,AST,and ALT during the grain filling in rice.3.Effects of different nitrogen application rates on the levels of polyamines and ethylene in rice grains and amino acids in millded riceFree-Spd and free-Spm concentrations,especially ratios of free-PAs to ACC,and TAAs,EAAs,and NEAAs contents were increased with N rate,while EER and ACC concentration were reduced with N increases.There was relatively little difference in the above parameters between MN and HN.Levels of free Spd and Spm,and their ratios to ACC exhibited significant and positive correlations with the activities/gene expressions of key enzymes related to amino acid biosynthesis,and TAAs,NEAAs,and EAAs contents,whereas ACC and EER levels had significantly positive correlations with those parameters.Exogenous Spd,Spm or AVG increased the enzymatic activities and gene expressions,whereas MGBG or ethephon exerted the opposite effect.These results demonstrated that free-PAs and ETH antagonistically interact in mediating the effect of N rate on grain amino acid synthesis of rice.Higher ratios of free PAs to ACC could promote synthesis of amino acids in grains of rice.In summary,free-PAs(mainly free-Spd and free-Spm)and ETH could antagonistically interact in regulating the biosynthesis of amino acids in rice grains,and mediate the role of different nitrogen application rates in regulating amino acid content in milled rice.This study can provide theoretical basis and technical support for genetic improvement and cultivation regulation of rice nutritional quality.