Study of Zn accumulation and tolerance of HMA4 TILLING mutants of Brassica rapa grown under Zn deficiency and Zn toxicity

Toxicity effects of zinc supply on growth revealed by physiological and transcriptomic evidences in sweet potato (Ipomoea batatas (L.) Lam)

Zinc toxicity affects crop productivity and threatens food security and human health worldwide. Unfortunately, the accumulation patterns of zinc and the harmful effects of excessive zinc on sweet potato have not been well explored. In the present research, two genotypes of sweet potato varieties with different accumulation patterns of zinc were selected to analyze the effects of excessive zinc on sweet potato via hydroponic and field cultivation experiments. The results indicated that the transfer coefficient was closely related to the zinc concentration in the storage roots of sweet potato. Excessive zinc inhibited the growth of sweet potato plants by causing imbalanced mineral concentrations, destroying the cellular structure and reducing photosynthesis. Furthermore, a total of 17,945 differentially expressed genes were identified in the two genotypes under zinc stress by transcriptomic analysis. Differentially expressed genes involved in the absorption and transport of zinc, defense networks and transcription factors played important roles in the response to zinc stress. In conclusion, this study provides a reference for the selection of sweet potato varieties in zinc contaminated soil and lays a foundation for investigating the tolerance of sweet potato to excessive zinc, which is meaningful for environmental safety and human health.

Is it the end of TILLING era in plant science?

Since its introduction in 2000, the TILLING strategy has been widely used in plant research to create novel genetic diversity. TILLING is based on chemical or physical mutagenesis followed by the rapid identification of mutations within genes of interest. TILLING mutants may be used for functional analysis of genes and being nontransgenic, they may be directly used in pre-breeding programs. Nevertheless, classical mutagenesis is a random process, giving rise to mutations all over the genome. Therefore TILLING mutants carry background mutations, some of which may affect the phenotype and should be eliminated, which is often time-consuming. Recently, new strategies of targeted genome editing, including CRISPR/Cas9-based methods, have been developed and optimized for many plant species. These methods precisely target only genes of interest and produce very few off-targets. Thus, the question arises: is it the end of TILLING era in plant studies? In this review, we recap the basics of the TILLING strategy, summarize the current status of plant TILLING research and present recent TILLING achievements. Based on these reports, we conclude that TILLING still plays an important role in plant research as a valuable tool for generating genetic variation for genomics and breeding projects.

Appraising growth, daily intake, health risk index, and pollution load of Zn in wheat (Triticum aestivum L.) grown in soil differentially spiked with zinc

Zinc (Zn) is a vital nutrient element required for plants normal growth and development. It performs imperative functions in numerous metabolic pathways in the plants. However, potentially noxious levels of Zn in terrestrial environment can lead to inhibited photosynthesis, growth, respiratory rate and imbalanced mineral nutrition. In micronutrient malnutrition, Zn deficiency is a global human health problem owing to the human dependence on cereals grains especially wheat-based diet. Therefore, this study investigated the Zn uptake efficacy in Triticum aestivum that is grown under two different doses (100 g/kg or 200 g/kg) of various soil amendments in both pot and field experimentation. Results of this study revealed that mean Zn concentration in different wheat varieties and treatments were varied from 1.53 to 6.03 mg/kg, 11.27 to 40.65 mg/kg, 11.28 to 39.93 mg/kg, and 11.32 to 37.70 mg/kg in amended soil, root, shoot, and grains, respectively. All observed Zn values in soil and wheat parts were lower than the FAO/WHO standards. Zinc values observed for pollution load index (0.034-0.134 mg/kg), daily intake (0.00492-0.01533 mg/kg), and health risk (0.0164-0.0570 mg/kg) index were lower than 1 except bio-concentration factor. Bio-concentration factor (5.076-10.165 mg/kg) revealed that DHARABI-11 variety showed maximum Zn uptake efficacy in farmyard manure treatment. The daily intake and health risk index values also showed that Zn level in grains is safe for inhabitants consumption. Overall, study recommended that these organic amendments are a good source of fertilizers, essentially required for the sustainable management of soil and increases the Zn accumulation in wheat grains which can ultimately reduce the Zn malnutrition in human food chain.

A comparison study of physiological response and TaZIPs expression in seedlings of two wheat (Triticum aestivum L.) cultivars with contrasting grain zinc accumulation

Screening and breeding of high-Zn-accumulating wheat cultivars have received increasing attention in recent years. However, the exact mechanism of Zn uptake and accumulation in wheat is not fully understood. Here, we investigated the physiological responses and TaZIPs gene expression in a low (Zhengmai0856, ZM0856) and a high (Aikang58, AK58) grain-Zn-accumulating wheat cultivars under hydroponic conditions with different levels of Zn supply. Results showed that AK58 was a Zn sensitive cultivar with better growth advantage, while ZM0856 was a Zn tolerant cultivar with higher capacity of Zn uptake. In addition, gene expression analysis showed that, the expression levels of the TaZIP3, TaZIP5, and TaZIP7 in roots were increased in both cultivars under Zn deficiency. In shoots, TaZIP3 and TaZIP6 transcript accumulation was lower in AK58 than ZM0856, whereas TaZIP7 showed the opposite effect. Moreover, multivariate statistical analysis (Pearson's correlation and PCA) showed that the mechanisms involved in Zn uptake and translocation was closely related to subcellular biosynthesis and ZIP gene expression regulation, whereas adequate Zn supply improved the Zn uptake and root-to-shoot translocation. These novel findings might be helpful for the molecular-assisted selecting and breeding of Zn-rich wheat cultivars.

Zinc toxicity in plants: a review

This review highlights the most recent updated information available about Zn phytotoxicity at physiological, biochemical and molecular levels, uptake mechanisms as well as excess Zn homeostasis in plants. Zinc (Zn) is a natural component of soil in terrestrial environments and is a vital element for plant growth, as it performs imperative functions in numerous metabolic pathways. However, potentially noxious levels of Zn in soils can result in various alterations in plants like reduced growth, photosynthetic and respiratory rate, imbalanced mineral nutrition and enhanced generation of reactive oxygen species. Zn enters into soils through various sources, such as weathering of rocks, forest fires, volcanoes, mining and smelting activities, manure, sewage sludge and phosphatic fertilizers. The rising alarm in environmental facet, as well as, the narrow gap between Zn essentiality and toxicity in plants has drawn the attention of the scientific community to its effects on plants and crucial role in agricultural sustainability. Hence, this review focuses on the most recent updates about various physiological and biochemical functions perturbed by high levels of Zn, its mechanisms of uptake and transport as well as molecular aspects of surplus Zn homeostasis in plants. Moreover, this review attempts to understand the mechanisms of Zn toxicity in plants and to present novel perspectives intended to drive future investigations on the topic. The findings will further throw light on various mechanisms adopted by plants to cope with Zn stress which will be of great significance to breeders for enhancing tolerance to Zn contamination.

Effects of zinc acquired through the plant-aphid-ladybug food chain on the growth, development and fertility of Harmonia axyridis

Heavy metal pollution is an increasingly serious problem in agricultural ecosystems. Zinc accumulation in the food chain may harm the physiological functions of organisms, including herbivorous and predatory insects. Its effects on development and reproduction in Harmonia axyridis are largely unknown. In this study, five Zn solutions (25, 50, 100, and 150 mg/kg) plus control (0 mg/kg) were used to treat broad beans and to water the resulting seedlings. Aphids fed on these seedlings were eaten by H. axyridis ladybugs. Zn accumulation was found at all three trophic levels. Compared with the control group, ladybugs in the 25, 50, and 100 mg/kg groups had significantly reduced weight gain from the 4^th instar to adulthood. Pupae and larvae (instars 1-4) in the 150 mg/kg group had the lowest survival of any group; pupal mortality in the 100 mg/kg group was significantly higher than that in the control group. Under Zn stress, female adults had inhibited expression of Vg1, Vg2 and VgR, reducing egg production and hatchability. Zn thus negatively affected their fertility. These results provide a theoretical basis for future exploration of soil heavy metal pollution impacts in ecosystems.

Biofortification of soybean (Glycine max L.) with Se and Zn, and enhancing its physiological functions by spiking these elements to soil during flowering phase

Soybean is recognized as one of the most important prospective protein sources for human nutrition under conditions of climate change and population growth. Occurrence of Se and Zn deficiency in vast areas over the globe inhabited by up to 2 billion people, induced search for a comprehensive solution to these problems through the efficient Se/Zn biofortification of soybean seeds (beans). To assess the Se/Zn accumulation efficiency and the physiological status of soybean plants, a pot experiment on Se and Zn enrichment in beans was conducted. It consisted of applying 15 different Se-deficient soil treatments with these elements during the flowering phase, alone or in dose combinations. Application of Se alone, besides Se accumulation in soybean, reduced Zn uptake from soil, but caused alterations in Zn translocation, and its multiple enrichment in beans. Addition of Zn alone promoted both Zn and Se enrichment in beans. Joint Se/Zn application in increasing doses appeared to have a strong synergistic effect on accumulation of these elements in beans and enhanced the physiological functions of the soybean. This manifested itself in the growth of photosynthetic production and soybean biomass, and in the improvement of lipid peroxidation status (REC, MDA and proline content indices). Toxicity symptoms indicated the maximum Se/Zn doses. Several-fold higher contents of Se and Zn in soybean straw compared to spiked soil suggest its possible use as Se/Zn-rich soil amendment in accordance with the circular economy goals. These novel findings may significantly contribute to human health improvement in Se and Zn deficient regions.