Nicotianamine synthase gene family as central components in heavy metal and phytohormone response in maize

Persistent legacy effects on soil metagenomes facilitate plant adaptive responses to drought

Both chronic and acute drought alter the composition and physiology of soil microbiomes, with implications for globally important processes including carbon cycling and plant productivity. When water is scarce, selection favors microbes with thicker peptidoglycan cell walls, sporulation ability, and constitutive osmolyte production (Schimel, Balser, and Wallenstein 2007)-but also the ability to degrade complex plant-derived polysaccharides, suggesting that the success of plants and microbes during drought are inextricably linked. However, communities vary enormously in their drought responses and subsequent interactions with plants. Hypothesized causes of this variation in drought resilience include soil texture, soil chemistry, and historical precipitation patterns that shaped the starting communities and their constituent species (Evans, Allison, and Hawkes 2022). Currently, the physiological and molecular mechanisms of microbial drought responses and microbe-dependent plant drought responses in diverse natural soils are largely unknown (de Vries et al. 2023). Here, we identify numerous microbial taxa, genes, and functions that characterize soil microbiomes with legacies of chronic water limitation. Soil microbiota from historically dry climates buffered plants from the negative effects of subsequent acute drought, but only for a wild grass species native to the same region, and not for domesticated maize. In particular, microbiota with a legacy of chronic water limitation altered the expression of a small subset of host genes in crown roots, which mediated the effect of acute drought on transpiration and intrinsic water use efficiency. Our results reveal how long-term exposure to water stress alters soil microbial communities at the metagenomic level, and demonstrate the resulting "legacy effects" on neighboring plants in unprecedented molecular and physiological detail.

Nicotianamine: A Key Player in Metal Homeostasis and Hyperaccumulation in Plants

Nicotianamine (NA) is a low-molecular-weight N-containing metal-binding ligand, whose accumulation in plant organs changes under metal deficiency or excess. Although NA biosynthesis can be induced in vivo by various metals, this non-proteinogenic amino acid is mainly involved in the detoxification and transport of iron, zinc, nickel, copper and manganese. This review summarizes the current knowledge on NA biosynthesis and its regulation, considers the mechanisms of NA secretion by plant roots, as well as the mechanisms of intracellular transport of NA and its complexes with metals, and its role in radial and long-distance metal transport. Its role in metal tolerance is also discussed. The NA contents in excluders, storing metals primarily in roots, and in hyperaccumulators, accumulating metals mainly in shoots, are compared. The available data suggest that NA plays an important role in maintaining metal homeostasis and hyperaccumulation mechanisms. The study of metal-binding compounds is of interdisciplinary significance, not only regarding their effects on metal toxicity in plants, but also in connection with the development of biofortification approaches to increase the metal contents, primarily of iron and zinc, in agricultural plants, since the deficiency of these elements in food crops seriously affects human health.

The role of sulfur nutrition in plant response to metal(loid) stress: Facilitating biofortification and phytoremediation

Metal(loid)s contamination poses a serious threat to ecosystem biosafety and human health. Phytoremediation is a cost-effective and eco-friendly technology with good public acceptance, although the process does require a significant amount of time for success. To enhance the phytoremediation efficiency, numerous approaches have been explored, including soil amendments application with chelators to facilitate remediation. Sulfur (S), a macronutrient for plant growth, plays vital roles in several metabolic pathways that can actively affect metal(loid)s phytoextraction, as well as attenuate metal(loid) toxicity. In this review, different forms of S-amendments (fertilizers) on uptake and translocation in plants upon exposure to various metal(loid) are evaluated. Possible mechanisms for S application alleviating metal(loid) toxicity are documented at the physiological, biochemical and molecular levels. Furthermore, this review highlights the crosstalk between S-assimilation and other biomolecules, such as phytohormones, polyamines and nitric oxide, which are also important for metal(loid) stress tolerance. Given the effectiveness and potential of S amendments on phytoremediation, future studies should focus on optimizing phytoremediation efficiency in long-term field studies and on investigating the appropriate S dose to maximize the food safety and ecosystem health.

Comparative Transcriptome Analysis of Iron and Zinc Deficiency in Maize (Zea mays L.)

Globally, one-third of the population is affected by iron (Fe) and zinc (Zn) deficiency, which is severe in developing and underdeveloped countries where cereal-based diets predominate. The genetic biofortification approach is the most sustainable and one of the cost-effective ways to address Fe and Zn malnutrition. Maize is a major source of nutrition in sub-Saharan Africa, South Asia and Latin America. Understanding systems’ biology and the identification of genes involved in Fe and Zn homeostasis facilitate the development of Fe- and Zn-enriched maize. We conducted a genome-wide transcriptome assay in maize inbred SKV616, under –Zn, –Fe and –Fe–Zn stresses. The results revealed the differential expression of several genes related to the mugineic acid pathway, metal transporters, photosynthesis, phytohormone and carbohydrate metabolism. We report here Fe and Zn deficiency-mediated changes in the transcriptome, root length, stomatal conductance, transpiration rate and reduced rate of photosynthesis. Furthermore, the presence of multiple regulatory elements and/or the co-factor nature of Fe and Zn in enzymes indicate their association with the differential expression and opposite regulation of several key gene(s). The differentially expressed candidate genes in the present investigation would help in breeding for Fe and Zn efficient and kernel Fe- and Zn-rich maize cultivars through gene editing, transgenics and molecular breeding.

Sulfur application reduces cadmium uptake in edible parts of pakchoi (Brassica chinensis L.) by cadmium chelation and vacuolar sequestration

Sulfur (S) application in pakchoi (Brassica chinensis L.) cultivation is vital for reducing cadmium (Cd) accumulation in the plants. However, the mechanism of S application on Cd uptake and translocation in pakchoi is unclear. In this study, a hydroponic experiment was performed to investigate the effects of S application on Cd accumulation in pakchoi at one Cd concentration (50 μM, in comparison to the control condition, 0 μM) and three S levels (0, 2, 4 mM). The results showed that excessive S application (4 mM) reduced Cd accumulation and alleviated pakchoi growth inhibition caused by Cd stress in shoots and roots. With increased S application, the proportion of Cd in the vacuolar fraction and the proportion of NaCl-extractable Cd increased in roots. Additionally, S application increased the content of glutathione (GSH) and phytochelatins (PCs). The reduced Cd uptake and accumulation in pakchoi shoots could have been due to increased Cd chelation and vacuolar sequestration in roots. In addition, sufficient S application (2 mM) increased the expression of γ-glutamylcysteine synthetase (GSH1) and nicotinamide synthase (NAS) in roots, and excessive S application upregulated the expression of ATP sulfurylase (ATPS) and phytochelatin synthase (PCs). This study provides evidence for the mechanism of mitigating Cd toxicity in pakchoi and will be helpful for developing strategies to reduce Cd accumulation in the edible parts of pakchoi through S fertilizer application.

Identification and molecular characterization of the nicotianamine synthase gene family in bread wheat

Nicotianamine (NA) is a non-protein amino acid involved in fundamental aspects of metal uptake, transport and homeostasis in all plants and constitutes the biosynthetic precursor of mugineic acid family phytosiderophores (MAs) in graminaceous plant species. Nicotianamine synthase (NAS) genes, which encode enzymes that synthesize NA from S-adenosyl-L-methionine (SAM), are differentially regulated by iron (Fe) status in most plant species and plant genomes have been found to contain anywhere from 1 to 9 NAS genes. This study describes the identification of 21 NAS genes in the hexaploid bread wheat (Triticum aestivum L.) genome and their phylogenetic classification into two distinct clades. The TaNAS genes are highly expressed during germination, seedling growth and reproductive development. Fourteen of the clade I NAS genes were up-regulated in root tissues under conditions of Fe deficiency. Protein sequence analyses revealed the presence of endocytosis motifs in all of the wheat NAS proteins as well as chloroplast, mitochondrial and secretory transit peptide signals in four proteins. These results greatly expand our knowledge of NAS gene families in graminaceous plant species as well as the genetics underlying Fe nutrition in bread wheat.

Arbuscular mycorrhizal symbiosis alters the expression patterns of three key iron homeostasis genes, ZmNAS1, ZmNAS3, and ZmYS1, in S deprived maize plants

Nicotianamine is an essential molecule for Fe homeostasis in plants, its primary precursor is the S-containing compound methionine, and it is biosynthesized by the enzyme family of nicotianamine synthases (NASs). In maize, a graminaceous plant that follows Strategy II for Fe uptake, ZmNAS genes can be subgrouped into two classes, according to their roles and tissue specific expression profiles. In roots, the genes of class I provide NA for the production of deoxymugineic acid (DMA), which is secreted to the rhizosphere and chelates Fe(III). The Fe(III)-DMA complex is then inserted to the root via a ZmYS1 transporter. The genes of class II provide NA for local translocation and detoxification of Fe in the leaves. Due to the connection between S and Fe homeostasis, S deficiency causes Fe deprivation responses to graminaceous plants and when S is supplied, these responses are inverted. In this study, maize plants were grown in pots with sterile river sand containing FePO4 and were inoculated with the mycorrhizal fungus Rhizophagus irregularis. The plants were grown under S deficient conditions until day 60 from sowing and on that day sulfate was provided to the plants. In order to assess the impact of AM symbiosis on Fe homeostasis, the expression patterns of ZmNAS1, ZmNAS3 (representatives of ZmNAS class I and class II), and ZmYS1 were monitored before and after S supply by means of real time RT-PCR and they were used as indicators of the plant Fe status. In addition, total shoot Fe concentration was determined before and after S supply. AM symbiosis prevented Fe deprivation responses in the S deprived maize plants and iron was possibly provided directly to the mycorrhizal plants through the fungal network. Furthermore, sulfate possibly regulated the expression of all three genes revealing its potential role as signal molecule for Fe homeostasis.

The development dynamics of the maize root transcriptome responsive to heavy metal Pb pollution

Lead (Pb), as a heavy metal element, has become the most important metal pollutant of the environment. With allocating a relatively higher proportion of its biomass in roots, maize could be a potential important model to study the phytoremediation of Pb-contaminated soil. Here we analyzed the maize root transcriptome of inbred lines 9782 under heavy metal lead (Pb) pollution, which was identified as a non-hyperaccumulator for Pb in roots. In the present study, more than 98 millions reads were mapped to define gene structure and detect polymorphism, thereby to qualify transcript abundance along roots development under Pb treatment. A total of 17,707, 17,440, 16,998 and 16,586 genes were identified in maize roots at four developmental stages (0, 12 h, 24 h and 48 h) respectively and 2,825, 2,626, 2161 and 2260 stage-specifically expressed genes were also identified respectively. In addition, based on our RNA-Seq data, transcriptomic changes during maize root development responsive to Pb were investigated. A total of 384 differentially expressed genes (DEGs) (log2Ratio ≥ 1, FDR ≤ 0.001) were identified, of which, 36 genes with significant alteration in expression were detected in four developmental stages; 12 DEGs were randomly selected and successful validated by qRT-PCR. Additionally, many transcription factor families might act as the important regulators at different developmental stages, such as bZIP, ERF and GARP et al. These results will expand our understanding of the complex molecular and cellular events in maize root development and provide a foundation for future study on root development in maize under heavy metal pollution and other cereal crops.

Comparative transcriptional profiling of two wheat genotypes, with contrasting levels of minerals in grains, shows expression differences during grain filling

Wheat is one of the most important cereal crops in the world. To identify the candidate genes for mineral accumulation, it is important to examine differential transcriptome between wheat genotypes, with contrasting levels of minerals in grains. A transcriptional comparison of developing grains was carried out between two wheat genotypes- Triticum aestivum Cv. WL711 (low grain mineral), and T. aestivum L. IITR26 (high grain mineral), using Affymetrix GeneChip Wheat Genome Array. The study identified a total of 580 probe sets as differentially expressed (with log2 fold change of ≥2 at p≤0.01) between the two genotypes, during grain filling. Transcripts with significant differences in induction or repression between the two genotypes included genes related to metal homeostasis, metal tolerance, lignin and flavonoid biosynthesis, amino acid and protein transport, vacuolar-sorting receptor, aquaporins, and stress responses. Meta-analysis revealed spatial and temporal signatures of a majority of the differentially regulated transcripts.