Micronutrients: the Borderline Between their Beneficial Role and Toxicity in Plants

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Review
16 November 2021

Cobalt is a transition metal located in the fourth row of the periodic table and is a neighbor of iron and nickel. It has been considered an essential element for prokaryotes, human beings, and other mammals, but its essentiality for plants remains obscure. In this article, we proposed that cobalt (Co) is a potentially essential micronutrient of plants. Co is essential for the growth of many lower plants, such as marine algal species including diatoms, chrysophytes, and dinoflagellates, as well as for higher plants in the family Fabaceae or Leguminosae. The essentiality to leguminous plants is attributed to its role in nitrogen (N) fixation by symbiotic microbes, primarily rhizobia. Co is an integral component of cobalamin or vitamin B12, which is required by several enzymes involved in N2 fixation. In addition to symbiosis, a group of N2 fixing bacteria known as diazotrophs is able to situate in plant tissue as endophytes or closely associated with roots of plants including economically important crops, such as barley, corn, rice, sugarcane, and wheat. Their action in N2 fixation provides crops with the macronutrient of N. Co is a component of several enzymes and proteins, participating in plant metabolism. Plants may exhibit Co deficiency if there is a severe limitation in Co supply. Conversely, Co is toxic to plants at higher concentrations. High levels of Co result in pale-colored leaves, discolored veins, and the loss of leaves and can also cause iron deficiency in plants. It is anticipated that with the advance of omics, Co as a constitute of enzymes and proteins and its specific role in plant metabolism will be exclusively revealed. The confirmation of Co as an essential micronutrient will enrich our understanding of plant mineral nutrition and improve our practice in crop production.

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Original Research
17 February 2021
Evidences for a Nutritional Role of Iodine in Plants
Claudia Kiferle
8 more and 
Pierdomenico Perata
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Little is known about the role of iodine in plant physiology. We evaluated the impact of low concentrations of iodine on the phenotype, transcriptome and proteome of Arabidopsis thaliana. Our experiments showed that removal of iodine from the nutrition solution compromises plant growth, and restoring it in micromolar concentrations is beneficial for biomass accumulation and leads to early flowering. In addition, iodine treatments specifically regulate the expression of several genes, mostly involved in the plant defence response, suggesting that iodine may protect against both biotic and abiotic stress. Finally, we demonstrated iodine organification in proteins. Our bioinformatic analysis of proteomic data revealed that iodinated proteins identified in the shoots are mainly associated with the chloroplast and are functionally involved in photosynthetic processes, whereas those in the roots mostly belong and/or are related to the action of various peroxidases. These results suggest the functional involvement of iodine in plant nutrition.

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Boron mediating crucial developmental processes in roots and shoots. (A) The asymmetric cell divisions at early embryos exhibit a remarkable pattern related to BOR1, whereas NIP5;1 are the better marker for these divisions on late embryogenesis, suggesting the B role in orientating embryo development. Curiously, the B deficiency induction on mature embryos alters not only BOR1 and NIP5;1 gene expression patterns but also disrupts auxin transport among the different cell-types impairing root apical meristem (RAM) formation, which result in a rootless A. thaliana plants. Furthermore, B deficiency on mature plants reduce length and identity of quiescent center (QC) at RAM, which altogether suggest that B display influence on overall root developmental stages. (B) Shoot apical meristem (SAM) exhibits the B transport differently regulated according with developmental phase, the TRUNCATED LEAF SYNDROME (TLS) transporter remobilize low B levels into SAM at juvenile phase. Apparently, the reduced levels of B enables the activity of Brahma (BRM) chromatin remodeler allowing the expression of miR156A, which arrest the flowering transition in juvenile tissues. The adult phase transition to flowering would be mediated by the increase on B levels at SAM, which would promote blocking of BRM activity, which represses miR156A expression releasing flowering induction. Dotted arrows indicate mechanisms not completely elucidated. Figures were created using the software biorender (app.biorender.com).
Review
14 January 2021
Boron: More Than an Essential Element for Land Plants?
Greice Leal Pereira
4 more and 
Wagner L. Araújo

Although boron (B) is an element that has long been assumed to be an essential plant micronutrient, this assumption has been recently questioned. Cumulative evidence has demonstrated that the players associated with B uptake and translocation by plant roots include a sophisticated set of proteins used to cope with B levels in the soil solution. Here, we summarize compelling evidence supporting the essential role of B in mediating plant developmental programs. Overall, most plant species studied to date have exhibited specific B transporters with tight genetic coordination in response to B levels in the soil. These transporters can uptake B from the soil, which is a highly uncommon occurrence for toxic elements. Moreover, the current tools available to determine B levels cannot precisely determine B translocation dynamics. We posit that B plays a key role in plant metabolic activities. Its importance in the regulation of development of the root and shoot meristem is associated with plant developmental phase transitions, which are crucial processes in the completion of their life cycle. We provide further evidence that plants need to acquire sufficient amounts of B while protecting themselves from its toxic effects. Thus, the development of in vitro and in vivo approaches is required to accurately determine B levels, and subsequently, to define unambiguously the function of B in terrestrial plants.

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