Cadmium (Cd) pollution in soil has become a major environmental issue worldwide. However, the underlying molecular mechanism of low grain-Cd accumulation (GCA) in maize is still largely unknown. Herein, we report the mechanistic basis for low GCA in maize by a multiomics approach. The low GCA genotype L63 showed normal vacuolar formation and a lower capacity of xylem loading of Cd than the high-accumulator L42 under Cd stress. Transcriptomic sequencing identified 84 low-GCA-associated genes which are mainly involved in the S-adenosylmethionine (SAM) cycle, metal transport, and vacuolar sequestration. A metabolome analysis revealed that L63 plants had a more active SAM cycle and a greater capacity for terpenoid synthesis and phenylalanine metabolism than L42. Combining the analysis of transcriptome and metabolome characterized several genes as key genes involved in the determination of Cd accumulation. Our study identifies a mechanistic basis for low Cd accumulation in maize grains and provides candidate genes for genetic improvement of crops.
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Excessive cadmium (Cd) accumulation in grains of rice (Oryza sativa L.) is a risk to food security. The transporters in the nodes of rice are involved in the distribution of mineral elements including toxic elements to different tissues such as grains. However, the mechanism of Cd accumulation in grains is largely unknown. Here, we report a node-expressed transporter gene, OsCCX2, a putative cation/calcium exchanger, mediating Cd accumulation in the grains of rice. Knockout of OsCCX2 caused a remarkable reduction of Cd content in the grains. Further study showed that disruption of this gene led to a reduced root-to-shoot translocation ratio of Cd. Moreover, Cd distribution was also disturbed in different level of internode and leaf. OsCCX2 is localized to plasma membrane, and OsCCX2 is mainly expressed in xylem region of vascular tissues at the nodes. OsCCX2 might function as an efflux transporter, responsible for Cd loading into xylem vessels. Therefore, our finding revealed a novel Cd transporter involved in grain Cd accumulation, possibly via a calcium (Ca) transport pathway in the nodes of rice.
Soil cadmium (Cd) contamination is a global environmental and food safety production issue. microRNAs (miRNAs) are proven to be involved in plant growth and development, and abiotic/biotic stress response, but their role in Cd tolerance is largely unknown in maize. To understand the genetic basis of Cd tolerance, two maize genotypes differing in Cd tolerance (L42, a sensitive genotype and L63, a tolerant genotype) were selected, and miRNA sequencing was carried out at nine-day-old seedlings exposed to 24 h Cd stress (5 μM CdCl2). A total of 151 differentially expressed miRNAs were identified, including 20 known miRNAs and 131 novel miRNAs. The results revealed that 90 and 22 miRNAs were up-regulated and down-regulated by Cd in Cd-tolerant genotype L63, and there were 23 and 43 miRNAs in Cd-sensitive genotype L42, respectively. Twenty-six miRNAs were up-regulated in L42 and unchanged or down-regulated in L63, or unchanged in L42 and down-regulated in L63. There were 108 miRNAs that were up-regulated in L63 and unchanged or down-regulated in L42, or unchanged in L63 and down-regulated in L42. Their target genes were enriched mainly in peroxisomes, glutathione (GSH) metabolism, ABC transporter, and ubiquitin-protease system. Among them, target genes involved in the peroxisome pathway and GSH metabolism might play key roles in Cd tolerance in L63. Besides, several ABC transporters which might involve in Cd uptake and transport were identified. The differentially expressed miRNAs or target genes could be used for breeding low grain Cd accumulation and high Cd tolerance cultivars in maize.
Although cultivation of hybrid rice varieties has been increasing, there are risks that high levels of cadmium (Cd) will accumulate in grain when such rice is grown in Cd-polluted environments.To produce Cd-safe hybrid rice, one practical approach is the generation of low Cd-accumulating parental lines.In two-line hybrid breeding, thermosensitive genic male sterile (TGMS) lines function as female parents to yield hybrid seeds.Recently, Cd accumulation-related genes have been identified; however, the effect of these genes on Cd accumulation in the grains of TGMS lines has yet to be reported.Here, 174 TGMS lines were selected for Cd accumulation phenotyping, and 30 TGMS lines, including 15 stable low-Cd and 15 high-Cd lines, were selected for single-nucleotide polymorphism (SNP) genotyping and association analysis.Association studies were conducted to identify the relationship between Cd accumulation and variable sites within seven candidate Cd-associated genes using logistic models.Nine sequence variant sites in four of the candidate genes were found to be significantly associated with Cd accumulation, two of which in OsNRAMP1 and OsNRAMP5 are low-Cd favorable variants, explaining 46.4% and 22.6% of the phenotypic variation, respectively.These loci could be developed as new molecular markers for identification of Cd accumulation characteristics and low-Cd marker-assisted breeding.
Summary The considerable drought tolerance of wild cereal crop progenitors has diminished during domestication in the pursuit of higher productivity. Regaining this trait in cereal crops is essential for global food security but requires novel genetic insight. Here, we assessed the molecular evidence for natural variation of drought tolerance in wild barley ( Hordeum spontaneum ), wild emmer wheat ( Triticum dicoccoides ), and Brachypodium species collected from dry and moist habitats at Evolution Canyon, Israel (ECI). We report that prevailing moist vs dry conditions have differentially shaped the stomatal and photosynthetic traits of these wild cereals in their respective habitats. We present the genomic and transcriptomic evidence accounting for differences, including co‐expression gene modules, correlated with physiological traits, and selective sweeps, driven by the xeric site conditions on the African Slope (AS) at ECI. Co‐expression gene module ‘circadian rhythm’ was linked to significant drought‐induced delay in flowering time in Brachypodium stacei genotypes. African Slope‐specific differentially expressed genes are important in barley drought tolerance, verified by silencing Disease‐Related Nonspecific Lipid Transfer 1 ( DRN1 ), Nonphotochemical Quenching 4 ( NPQ4 ), and Brassinosteroid‐Responsive Ring‐H1 ( BRH1 ). Our results provide new genetic information for the breeding of resilient wheat and barley in a changing global climate with increasingly frequent drought events.
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