Heavy metal contamination of agricultural fields has become a global concern as it causes a direct impact on human health. Rice is the major food crop for almost half of the world population and is grown under diverse environmental conditions, including heavy metal‐contaminated soil. In recent years, the impact of heavy metal contamination on rice yield and grain quality has been shown through multiple approaches. In this review article, different aspects of heavy metal stress, that is uptake, transport, signaling and tolerance mechanisms, are comprehensively discussed with special emphasis on rice. For uptake, some of the transporters have specificity to one or two metal ions, whereas many other transporters are able to transport many different ions. After uptake, the intercellular signaling is mediated through different signaling pathways involving the regulation of various hormones, alteration of calcium levels, and the activation of mitogen‐activated protein kinases. Heavy metal stress signals from various intermediate molecules activate various transcription factors, which triggers the expression of various antioxidant enzymes. Activated antioxidant enzymes then scavenge various reactive oxygen species, which eventually leads to stress tolerance in plants. Non‐enzymatic antioxidants, such as ascorbate, metalloids, and even metal‐binding peptides (metallothionein and phytochelatin) can also help to reduce metal toxicity in plants. Genetic engineering has been successfully used in rice and many other crops to increase metal tolerance and reduce heavy metals accumulation. A comprehensive understanding of uptake, transport, signaling, and tolerance mechanisms will help to grow rice plants in agricultural fields with less heavy metal accumulation in grains.
Iron is not only important for plant physiology, but also a very important micronutrient in human diets. The vacuole is the main site for accumulation of excess amounts of various nutrients and toxic substances in plant cells. During the past decade, many Vacuolar Iron Transporter (VIT) and VIT‐Like (VTL) genes have been identified and shown to play important roles in iron homeostasis in different plants. Furthermore, recent reports identified novel roles of these transporter genes in symbiotic nitrogen fixation (SNF) in legume crops as well as in the blue coloration of petals in flowers. The literature indicates their universal role in Fe transport across different tissues (grains, nodules, flowers) to different biological processes (cellular iron homeostasis, SNF, petal coloration) in different plants. Here, we have systematically reviewed different aspects, such as structure, molecular evolution, expression, and function of VIT/VTL proteins. This will help future studies aimed at functional analysis of VIT/VTL genes in other plant species, vacuolar transportation mechanisms, and iron biofortification at large.
Rice, a staple food worldwide, contains varying amount of nutrients in different grain tissues. The underlying molecular mechanism of such distinct nutrient partitioning remains poorly-investigated. Here, an optimized rapid Laser Capture Microdissection (LCM) approach was used to individually collect pericarp, aleurone, embryo and endosperm from 10 Days After Fertilization (DAF) old grains. Subsequent RNA-Seq analysis in these tissues have identified 7760 differentially expressed genes (DEGs). Analysis of promoter sequences of tissue specific genes identified many known and novel cis-elements important for grain filling and seed development. Using identified DEGs, comprehensive spatial gene expression pathways were built for spatial accumulation of starch, proteins, lipids and iron. The extensive transcriptomic analysis has provided many novel insights about nutrient partitioning mechanisms, for instance, it reveals a gradient in Seed Storage Protein accumulation across the analysed four tissue-types. It further reveals that partitioning of various minerals, such as iron, is most likely regulated through transcriptional control of their transporters. In addition, the extensive analysis of this study is presented as an interactive online tool (https://biogeek.shinyapps.io/DEGs/) that provides a much-needed resource for future functional genomics studies aimed to improve grain quality and seed development.
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