Effects And Mechanisms Of 2'-deoxymugineic Acid (DMA) On Cadmium Absorption In Rice

Cadmium,as a toxic heavy metal,inhibits plants growth seriously.Excess Cd accumulation pose a great threat to human health via food chain.Rice is one of primary food crops in the world,especially in Asia area.However,increasing number of environmental pollution lead to a great deal of Cd accumulation in soil.Due to stronger mobilization,Cd rapidly translocate to aerial parts after absorbed by rice roots,depositing in grains eventually.Therefore,approach for reducing Cd contamination in rice has been an urgent target in the agricultural field.Root exudates from rice,such as organic acids,have been considered and proved to play a positive role in Cd resistance.Organic acids can reduce Cd accumulation by inhibiting Cd uptake by roots,among which 2'-deoxymugineic acid?DMA?is a specific product of graminaceous plants,such as rice.DMA serves as Fe carrier,helping plants to absord and translocate Fe.Prior reports indicates that DMA can reduce Cd accumulation by inhibiting Cd uptake by roots as well,whereas effects and mechanisms of DMA on Cd uptake in plants,especially in rice,is still unclear,which needs further investigation.Hydroponics for rice in this study.Nipponbare?Oryza sativa L.cv.Nipponbare?and naat1mutant isolated from EMS mutagenized Nipponbare were selected as materials,investigating effects of Cd stress on growth of wild type and DMA deficient mutant naat1.We revealed effects and mechanisms of DMA on Cd adsorption in rice by measuring physiological index,flux of Cd²⁺from roots;analyzing expressions of genes involved in Fe?Cd transport,DMA biosynthesis and secretion;conducting exogenous DMA addition experiment.The results were as follows:1.naat1 mutant exhibited more Cd toxicity than wild type.Inhibitary growth of naat1 mutant was significantly greater than wild type under Cd stress.chlorophyll content,root activity and dry weight content in naat1 mutant were lower than wild type,but Cd content in naat1 mutant was higher than wild type.2.Influx of Cd in naat1 mutant roots was obviously greater than wild type by non-invasive micro-test technique,indicating that speed of Cd uptake by naat1 mutant roots was faster than wild type.In addition,Cd stress highly up-regulated expressions of genes involved in Cd transport,promoting Cd uptake consequently.3.Cd stress induced DMA biosynthesis and secretion of wild type roots.In the presence of Cd,enhanced DMA release was observed from wild type roots.Meanwhile,genes expressions involved in DMA biosynthesis and secretion were significantly up-regulated in wild type under Cd stress.4.Exogenous DMA application could inhibit Cd uptake by promoting both Fe²⁺and Fe³⁺uptake.However,higher Fe level and reduced Cd accumulation was not directly attributed to DMA-mediated Fe/Cd transporter expressions.5.Cd stress resulted in higher Fe accumulation in roots and reduced Fe in shoots.Cd-induced Fe uptake was not primary reason for higher Fe accumulation in roots but limited Fe translocation from roots to aerial parts.Over-accumulated Fe was identified in root apoplast,where emerged cell walls.Pectin in cell wall was the key adsorption site for Fe.Rice could utilized over-accumulated Fe in cell wall to form homologous Fe plague on root surface,inhibiting Cd entrance into roots and enhancing Cd tolerance.