The Effects Of Sugars And Amino Acids On The IHATS In Rice (Oryza Sativa L.)

The growth of plants depends on their taking carbon from the air and nitrogen from the soil in a ratio of approximately 30 C:1 N.Thus the assimilatory mechanisms of plants are dominated by photosynthesis and nitrate uptake.If photosynthesis is perturbed(by changes in light intensity or CO₂ concentration,for instance) then nitrate uptake must change to maintain the N supply at the necessary rate,and if nitrate uptake is perturbed it must recover its former rate in order to continue matching the rate of photosynthesis(or photosynthesis must respond).Such adjustments have been observed.In previous studies, the results of feeding sugars(sucrose,glucose and fructose,but not sugar derivatives) to the roots suggest that new photosynthate,moving to the root in the phloem,acts as a signal.This would be positive feed-forward control.Alternatively,if photosynthesis and growth speed up,the N arriving from the root will be used up faster,the concentration of free amino acids and other N compounds would fall,and the flow of reduced N to the root system would also fall and act as a signal to increase the nitrate uptake and transport rates. This would be negative feedback control.We have invented a system for measuring nitrate uptake with very high time resolution in Oryza sativa L.The system has enabled us to measure nitrate uptake every 10 minutes for many hours on the same plant and on successive days.More frequent measurements are also possible.In the parent studies,by using this system,we studied the effects of sugars and amino acids on the iHATS(induced high affinity nitrate transport system) in rice.The main results are as fellowes:1,Nine different sugars were chosen,among which,sucrose,glucose and fructose are substrates transported in the phloem,and galactose,raffinose,mannitol,lactose,arabinose and mannose are components of the cell wall.Lactose is a disaccharide only present in some special structure.The duration of nitrate uptake was accelerated by sucrose,and this accelarated could last for 1.5 h,before reaching a maximum,and the extent was 298%.By contrast,the acceleration of nitrate uptake by glucose,galactose,fructose and raffinose lasted for 40 min,and the increase of nitrate uptake rates were 223%,200%,192%and 140%respectively.In addition,there were obvious platform periods after the glucose, galactose,fructose and raffinose treatment.In addition,mannose showed inhibition of the nitrate uptake.Therefore sucrose appears to be a specific signal molecule to nitrate uptake.2,The effects of ten different amino acids on nitrate uptake in rice roots were measured. A short-term acceleration of 10-20 min was recorded after the Glu and Asp treatment,the extent of acceleration were 377%and 186%respectively.Gln,Asp,Arg and Lys showed obvious inhibition on the nitrate uptake rate,while Pro,Phe,val,and ser had no effects on the nitrate uptake in rice foot.No single amino acid was found to inhibit the net nitrate uptake immediately.3,The gene expression analysis showed that sucrose,glucose,and galactose enhanced the OsNrt2.1(coding iHAT transporter) gene expression,among which the enhanced expression was the most induced by sucrose.In addition,the gene expression could be obviously enhanced 30 min after sucrose treatment;the enhancement was continuously increased until it reached to a maximum after 4 h of sucrose treatment.4,The effects of six different amino acids(Asp,Asn,Glu,Gln,Arg,and Lys)on nitrate uptake in rice roots were measured.The six amino acids all could inhibit the after the 2 h treatment.No single amino was founded to inhibit the OsNrt2.1 gene expression mostly.The results of experiment showed that sucrose might be the major signal molecule regulating the iHATS,and the amino acids had no direct effect on the iHATS.