Variation in Wheat Root Transcriptome Response to Cadmium

Wheat (Triticum aestivum L.) genotypes vary for the propensity to accumulate cadmium (Cd) in grain.  Cadmium is toxic, and producing low Cd grain is important to a safe, healthful food supply.  In field-grown plants, grain Cd concentration was proportional to above-ground plant Cd concentration at flowering.  In a low-volume hydroponic system, high-Cd wheat genotypes accumulated more Cd in shoots and less Cd in roots than a low-Cd wheat genotype.  Root transcriptome response to 1 μM Cd was evaluated in high and low-Cd wheat genotypes using RNAseq technology.  Downregulated transcripts outnumbered upregulated transcripts in all genotypes.  Gene ontology (GO) singular enrichment analysis identified 22 significantly enriched GO terms in the Cd-upregulated transcripts, including transcripts associated with metal ion binding, and 94 significantly enriched GO terms in the Cd-downregulated transcripts, most notably transcripts associated with nicotianamine biosynthetic processes.  Between the lowest and highest-Cd genotypes, abundance of 134 transcripts responded differently to Cd.  Among these differentially responsive transcripts, 105 had GO annotations, and 28 GO terms were significantly enriched, including transcripts associated with transmembrane transport, heme binding, and nutrient reservoir activity.  The differentially responsive transcript set included transcripts homologous to YS1, an iron-phytosiderophore transporter, which was downregulated in the Cd-accumulating genotype.  Transcripts encoding a group of Mn-binding proteins associated with defense response were differentially upregulated in the high-Cd genotype.  Candidate genes for differential Cd accumulation, including phytochelatin synthase and genes in the glutathione biosynthetic pathway, HMA3, and NRAMP, were not differentially expressed in response to Cd in high and low-Cd genotypes.