Identification Of Low Grain Cadmium Accumulation Maize Genotypes And Its Physiological And Molecular Mechanism

Soil cadmium（Cd）contamination poses adverse impacts on crop yields and quality,and threatens human health via food chain.For soils polluted with slight or medium Cd,one of the cost-effective and practical substitute approaches for safe food production is to breed and cultivate crop genotypes/cultivars with low Cd accumulation in edible parts.Maize（Zea mays L.）is currently the world’s highest produced food crops.Accordingly,the present study was carried out to elucidate the physiological mechanism of low Cd accumulation in maize grains,and to identify relevant genes for Cd-tolerance/low accumulation by comparative transcriptome analysis,based on the selection of genotypes differing in grain Cd accumulation using in 95 maize genotypes.The main results were summarized as follows:1.Identification of maize genotypes with low grain Cd accumulationDifferences in grain Cd concentrations were evaluated among 95 maize genotypes（inbred lines）grown in Changxing Agricultural Experiment Station of Zhejiang University（Changxing,Zhejiang province,China）to select low grain Cd accumulating genotypes.The results showed that there was a significantly genotypic difference in grain Cd concentrations in maize grain samples,with a mean concentration of 0.013 mg kg^-1 DW and the variation of0.001 to 0.06 mg kg^-1 DW.Furthermore,differences between genotypes in the two years were fairly consistent.Genotypes L42 and L15 showed relative lower grain Cd concentrations than L51 and L63.The results showed a potential of selecting suitable genotypes for these areas where Cd concentration in grain exceeds the maximum permissible concentration（MPC）.The genotypic difference was also found in Cr,Fe,Mn,Zn and Cu concentrations.Correlation analysis showed that there were significantly positive correlations between grain Cd and Mn,Cu concentrations.2.Difference in physiological features in response to Cd stress in the two maize genotypes with contrasting grain Cd accumulationHydroponic experiments were conducted to evaluate genotypic difference in response to5μM Cd stress using two maize genotypes with contrasting grain Cd accumulation（L63,low Cd accumulator;L42,high Cd accumulator）.The results showed that L63 demonstrated higher Cd tolerance than L42.Compared with control,5μM Cd stress significantly reduced plant height,root/shoot fresh and dry weight in L42;while,L63 plants almost displayed no significant reduction.Compared with L42,seedlings of L63 recorded higher Cd concentration in roots,but lower in shoots.Cd stress induced significantly reduction in shoot Mn content in both L63 and L42（c.f.22%and 33%,lower than control）,and in shoot Fe content of L63（22%）.In addition,there was highly significantly difference in phytohormone contents in response to 24h Cd stress.Compared with control,significant increases were recorded in ABA content and ethylene（ET）emission rate in L42,and in GA3,JA and ET in L63.Meanwhile,under Cd stress,L63 displayed relatively lower ABA,ET and GA3 than L42.Obvious ultrastructural damage was observed in L42 under Cd stress,especially vacuoles in root tips and mesophyll cells,while L63 was less affected.3.Comparative transcription study in response to Cd stress between the two maize genotypes with contrasting grain Cd accumulationA hydroponic experiment was performed using L42（Cd sensitive and high accumulation）and L63（Cd tolerant and low accumulation）to evaluate genotypic difference in gene expression in response to 5μM Cd by comparative transcriptome analysis.A total of 268 and300 genes were up-and down-regulated in L42,while that was 86 and 26 in L63,respectively.Eighty-three candidate genes related to low-Cd-accumulation or Cd tolerance were identified,whose expression was significantly up-regulated in L63 and non or down-regulated in L42under Cd stress.These include autophagy-related 8d,ATP-sulfurylase 3,ABC transporter,zinc transporter and RING zinc finger domain superfamily protein.These genes categorized in stress response,transport,protein synthesis and degradation,transcription,signal transduction and cell division and growth.Based on these differential expression genes（DEGs）,an integrated schematic diagram was proposed which suggests the key genes associated with low-Cd accumulation or Cd tolerance were mainly involved in cell wall processing,metal transport,vacuolar sequestration,ROS scavenging and ATP production.