miRNA transcriptome reveals key miRNAs and their targets contributing to the difference in Cd tolerance of two contrasting maize genotypes.

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.

Integration of Transcriptome and Metabolome Analyses Reveals the Mechanistic Basis for Cadmium Accumulation in Maize.

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.

Identification of low grain cadmium accumulation genotypes and its physiological mechanism in maize (Zea mays L.).

Soil cadmium (Cd) contamination poses adverse impacts on crop yield and quality. Maize is a widely cultivated cereal throughout the world. In this study, field and hydroponic experiments were conducted to investigate the genotypic difference in Cd accumulation and tolerance in maize. There were significant genotypic differences in grain Cd concentrations among 95 genotypes. From these 95 genotypes, L42 which showed a higher grain Cd concentration and L63 which showed a lower grain Cd concentration was selected for further study. Under Cd stress, L63 showed much less reduction in plant growth than L42 compared with the control. Seedlings of L63 recorded higher Cd concentration in roots, but lower in shoots L42, indicating that the low grain Cd concentration in L63 is mainly due to the low rate of transportation of Cd from roots to shoots. Most Cd accumulated in epidermis and xylem vessels of L63, while the green fluorescent was found across almost the entire cross-section of root in L42. Obvious ultrastructural damage was observed in L42 under Cd stress, especially in mesophyll cells, while L63 was less affected. These findings could contribute to developing low Cd accumulation and high tolerance maize cultivars.

Metabolome Analysis Revealed the Mechanism of Exogenous Glutathione to Alleviate Cadmium Stress in Maize (Zea mays L.) Seedlings.

Cadmium (Cd) is one of the major heavy metal pollutants in the environment and imposes severe limitations on crop growth and production. Glutathione (GSH) plays an important role in plant Cd tolerance which is able to scavenge stresses-induced reactive oxygen species (ROS) and is involved in the biosynthesis of phytochelatins (PCs). Our previous study revealed that Cd stress affects maize growth, and the GSH treatment could relieve Cd stress in maize seedlings. In this study, we attempted to characterize the metabolomics changes in maize leaves and roots under Cd stress and exogenous GSH conditions. We identified 145 and 133 metabolites in the leaves and roots, respectively. Cd stress decreased the tricarboxylic acid cycle (TCA cycle) metabolism and increased the amino acid contents in the leaves, while it decreased the amino acid contents, increased the TCA cycle metabolism, the sugar contents, and shikimic acid metabolism in the roots. On the other hand, exogenous GSH increased the GSH content, changed the production of metabolites related to antioxidant systems (such as ascorbic acid-related metabolites and flavonoid-related metabolites), and alleviated lipid peroxidation, thereby alleviating the toxic effect of Cd stress on maize. These findings support the idea that GSH alleviates Cd-induced stress in maize and may help to elucidate the mechanism governing Cd-induced stress and the GSH-driven alleviation effect.