Effects Of Glutamate Receptor Channels And Its Modulators On Cadmium Uptake And Transport In Rice

Roots and vegetative organs of rice can concentrate Cd from soil,and then transport Cd to grains through phloem.Glutamate receptor channels（GLRs）are widely distributed in the cell membranes as a kind of cation channels,which are involved in many metabolic processes such as ions transport,resistance response and signal transduction.Therefore,it is of great significance to study the regulatory mechanism of selectively conducting essential ions and Cd²⁺by GLRs in rice cells for the development of agronomic measures to reduce Cd in grains.This paper studied the effects of soil Cd levels,malate and amino acid metabolism on GLRs in rice.The main results were as follows:Soil Cd levels significantly inhibited the expression levels of Os GLRs and decreased the ability of ion channels to recognize and intercept Cd.The content of glutamate in grains and vegetative organs decreased significantly,while the content of Cd and Mn and the fault-tolerant rate of channels for Cd increased significantly with the increase of soil Cd levels.Low-Cd-accumulating mutant fc8 displayed significantly higher content of Ca,Fe and Zn,but lower Mn content than its wild type Nipponbare.Ion channels in fc8 displayed stronger ability to recognize and intercept Cd,and lower fault-tolerant rate for Cd than that in Nipponbare.When soil Cd increased from 0.1 to 2.4 mg·kg^-1,the fault-tolerant rate of ion channels for Cd in fc8 and NPB grains increased from 0.003%to 0.07%and from 0.01%to 0.12%,respectively.Spraying MA made the expression levels of Os GLRs and amino acid metabolism were significantly increased while Cd content and the Cd:Ca ratio were significantly decreased in grains.After foliar spraying MA at rice anthesis stage,the expression levels of Os GLR3.1-3.5 were significantly increased,Mn and Cd transporter genes Os NRAMP1,2,3,5 were significantly decreased,Ca and glutamate content in grains increased by 12.5～29.8%,content of Cd and other toxic elements in grains reduced by 27.9–41.0%.Meanwhile,the expression levels of Os CESA1,4,7,8,9 for the cellulose synthesis were down-regulated,the content of cellulose and soluble sugars was decreased.Adding ion channel modulators effectively inhibited the absorption and transport of Cd in rice.Content of Cd in roots and shoots of rice seedlings was significantly decreased by adding glutamate（Glu）,glycine（Gly）,serine（Ser）and methionine（Met）in rhizosphere solution.After foliar spraying 5～10mmol·L^-1 Ser at rice anthesis stage,Ca content in grains increased by 5.45～9.45%,Cd content decreased by 33.39～48.03%and the Cd:Ca ratio decreased from 0.31%to 0.15%.Increasing glutamate concentration in rhizosphere solution significantly increased the expression levels of Os GLRs and inhibited the absorption and transport of Cd in rice seedings.Adding 0.05～3.00mmol·L^-1 glutamate resulted in reductions in Cd content by 14.53～68.07%and 13.92～61.62%in roots and shoots of X24,by 35.74～73.61%and 16.65～66.73%in roots and shoots of Z35,respectively.Meanwhile,the expression levels of Os GLR3.1-3.4,glutamate and Ca content in rice seedings were significantly increased,while the Cd:Ca ratio were significantly decreased.In conclusion,GLRs played an important role in the transmembrane transport of Cd in rice.Moderate and heavy Cd pollution in soil led to selectivity damaged of rice GLRs.The expression levels of Os GLRs and Glu content of rice were significantly increased by adding ion channel modulators or breeding low-Cd-accumulating lines.These regulatory mechanisms enhanced the capable of ion channels to recognized and intercepted Cd and inhibited the uptake and accumulation of Cd in rice.