Abstract
BackgroundMetal tolerance is often an integrative result of metal uptake and distribution, which are fine-tuned by a network of signaling cascades and metal transporters. Thus, with the goal of advancing the molecular understanding of such metal homeostatic mechanisms, comparative RNAseq-based transcriptome analysis was conducted to dissect differentially expressed genes (DEGs) in maize roots exposed to cadmium (Cd) stress.ResultsTo unveil conserved Cd-responsive genes in cereal plants, the obtained 5166 maize DEGs were compared with 2567 Cd-regulated orthologs in rice roots, and this comparison generated 880 universal Cd-responsive orthologs groups composed of 1074 maize DEGs and 981 rice counterparts. More importantly, most of the orthologous DEGs showed coordinated expression pattern between Cd-treated maize and rice, and these include one large orthologs group of pleiotropic drug resistance (PDR)-type ABC transporters, two clusters of amino acid transporters, and 3 blocks of multidrug and toxic compound extrusion (MATE) efflux family transporters, and 3 clusters of heavy metal-associated domain (HMAD) isoprenylated plant proteins (HIPPs), as well as all 4 groups of zinc/iron regulated transporter protein (ZIPs). Additionally, several blocks of tandem maize paralogs, such as germin-like proteins (GLPs), phenylalanine ammonia-lyases (PALs) and several enzymes involved in JA biosynthesis, displayed consistent co-expression pattern under Cd stress.Out of the 1074 maize DEGs, approximately 30 maize Cd-responsive genes such as ZmHIPP27, stress-responsive NAC transcription factor (ZmSNAC1) and 9-cis-epoxycarotenoid dioxygenase (NCED, vp14) were also common stress-responsive genes reported to be uniformly regulated by multiple abiotic stresses. Moreover, the aforementioned three promising Cd-upregulated genes with rice counterparts were identified to be novel Cd-responsive genes in maize.Meanwhile, one maize glutamate decarboxylase (ZmGAD1) with Cd co-modulated rice ortholog was selected for further analysis of Cd tolerance via heterologous expression, and the results suggest that ZmGAD1 can confer Cd tolerance in yeast and tobacco leaves.ConclusionsThese novel findings revealed the conserved function of Cd-responsive orthologs and paralogs, which would be valuable for elucidating the genetic basis of the plant response to Cd stress and unraveling Cd tolerance genes.
Highlights
Metal tolerance is often an integrative result of metal uptake and distribution, which are fine-tuned by a network of signaling cascades and metal transporters
Cd uptake and allocation is associated with a number of metal-regulated transporters including heavy metal ATPase (HMA), ATP-binding cassette transporters (ABC) subfamilies ABCC and ABCG, natural resistance-associated macrophage protein (Nramp), and zinc/iron regulated transporter protein (ZIP), which are indispensable for the homeostasis of essential metals [2,3,4,5,6,7,8]
Functional characterization of early cd-responsive differentially expressed genes (DEGs) in maize roots To investigate transcripts that were regulated in short-term Cd stressed maize roots, RNAseq data from replicated samples were processed through TopHat-Cufflinks pipeline to perform pair-wise comparisons between 1 h Cd-treated (Cd1h) and untreated maize seedlings (Additional file 1: Table S1)
Summary
Metal tolerance is often an integrative result of metal uptake and distribution, which are fine-tuned by a network of signaling cascades and metal transporters. With the goal of advancing the molecular understanding of such metal homeostatic mechanisms, comparative RNAseq-based transcriptome analysis was conducted to dissect differentially expressed genes (DEGs) in maize roots exposed to cadmium (Cd) stress. Cadmium (Cd) is one of the most hazardous heavy metals, it can be absorbed by the roots from the soil and transported to the aboveground parts, indirectly induces oxidative stress and negatively affects nutrient uptake and homeostasis, eventually causes stunted growth and reductions in productivity of crop plants [1]. It is critically important to understand the mechanisms underlying Cd uptake, translocation and accumulation in plants. Despite the identification of those genes, the underlying knowledge of molecular mechanisms for plant Cd tolerance is still fragmental. RNAseq studies allowed the identification of long non-coding RNAs and cis-natural antisense transcription in response to Cd stress in rice [15, 16]
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