Shall we talk? New details in crosstalk between copper and iron homeostasis uncovered in Arabidopsis thaliana

This article is a Commentary on Cai et al. (2024), 242: 1206–1217.

Loss of OPT3 function decreases phloem copper levels and impairs crosstalk between copper and iron homeostasis and shoot-to-root signaling in Arabidopsis thaliana

Copper (Cu) and iron (Fe) are essential micronutrients that are toxic when accumulating in excess in cells. Thus, their uptake by roots is tightly regulated. While plants sense and respond to local Cu availability, the systemic regulation of Cu uptake has not been documented in contrast to local and systemic control of Fe uptake. Fe abundance in the phloem has been suggested to act systemically, regulating the expression of Fe uptake genes in roots. Consistently, shoot-to-root Fe signaling is disrupted in Arabidopsis thaliana mutants lacking the phloem companion cell-localized Fe transporter, OLIGOPEPTIDE TRANSPORTER 3 (AtOPT3). We report that AtOPT3 also transports Cu in heterologous systems and contributes to its delivery from sources to sinks in planta. The opt3 mutant contained less Cu in the phloem, was sensitive to Cu deficiency and mounted a transcriptional Cu deficiency response in roots and young leaves. Feeding the opt3 mutant and Cu- or Fe-deficient wild-type seedlings with Cu or Fe via the phloem in leaves downregulated the expression of both Cu- and Fe-deficiency marker genes in roots. These data suggest the existence of shoot-to-root Cu signaling, highlight the complexity of Cu/Fe interactions, and the role of AtOPT3 in fine-tuning root transcriptional responses to the plant Cu and Fe needs.

Visualizing Metal Distribution in Plants Using Synchrotron X-Ray Fluorescence Microscopy Techniques

Recent improvements in synchrotron-based X-ray fluorescence (SXRF) microscopy established it as an advanced analytical tool for analyzing 2D- and 3D distribution of mineral elements in plants. Among existing imaging techniques, SXRF microscopy offers several unique capabilities, including in situ metal quantification in plant tissues and high sensitivity, as low as 1 mg kg^-1, at the nanoscale spatial resolution. SXRF is increasingly utilized in different plant science disciplines to provide a fundamental understanding of metal homeostasis, and the function of trace elements in plant metabolism and development. Here, we describe methods for SXRF imaging, including sample preparation, the optimization of conventional SXRF for analyzing trace elements, and the development of confocal SXRF (C-SXRF).

Iron Availability within the Leaf Vasculature Determines the Magnitude of Iron Deficiency Responses in Source and Sink Tissues in Arabidopsis

Iron (Fe) uptake and translocation in plants are fine-tuned by complex mechanisms that are not yet fully understood. In Arabidopsis thaliana, local regulation of Fe homeostasis at the root level has been extensively studied and is better understood than the systemic shoot-to-root regulation. While the root system is solely a sink tissue that depends on photosynthates translocated from source tissues, the shoot system is a more complex tissue, where sink and source tissues occur synchronously. In this study, and to gain better insight into the Fe deficiency responses in leaves, we overexpressed Zinc/Iron-regulated transporter-like Protein (ZIP5), an Fe/Zn transporter, in phloem-loading cells (proSUC2::AtZIP5) and determined the timing of Fe deficiency responses in sink (young leaves and roots) and source tissues (leaves). Transgenic lines overexpressing ZIP5 in companion cells displayed increased sensitivity to Fe deficiency in root growth assays. Moreover, young leaves and roots (sink tissues) displayed either delayed or dampened transcriptional responses to Fe deficiency compared to wild-type (WT) plants. We also took advantage of the Arabidopsis mutant nas4x-1 to explore Fe transcriptional responses in the opposite scenario, where Fe is retained in the vasculature but in an unavailable and precipitated form. In contrast to proSUC2::AtZIP5 plants, nas4x-1 young leaves and roots displayed a robust and constitutive Fe deficiency response, while mature leaves showed a delayed and dampened Fe deficiency response compared to WT plants. Altogether, our data provide evidence suggesting that Fe sensing within leaves can also occur locally in a leaf-specific manner.

Ferroportin 3 is a dual-targeted mitochondrial/chloroplast iron exporter necessary for iron homeostasis in Arabidopsis

Mitochondria and chloroplasts are organelles with high iron demand that are particularly susceptible to iron-induced oxidative stress. Despite the necessity of strict iron regulation in these organelles, much remains unknown about mitochondrial and chloroplast iron transport in plants. Here, we propose that Arabidopsis ferroportin 3 (FPN3) is an iron exporter that is dual-targeted to mitochondria and chloroplasts. FPN3 is expressed in shoots, regardless of iron conditions, but its transcripts accumulate under iron deficiency in roots. fpn3 mutants cannot grow as well as the wild type under iron-deficient conditions and their shoot iron levels are lower compared with the wild type. Analyses of iron homeostasis gene expression in fpn3 mutants and inductively coupled plasma mass spectrometry (ICP-MS) measurements show that iron levels in the mitochondria and chloroplasts are increased relative to the wild type, consistent with the proposed role of FPN3 as a mitochondrial/plastid iron exporter. In iron-deficient fpn3 mutants, abnormal mitochondrial ultrastructure was observed, whereas chloroplast ultrastructure was not affected, implying that FPN3 plays a critical role in the mitochondria. Overall, our study suggests that FPN3 is essential for optimal iron homeostasis.

YSL3-mediated copper distribution is required for fertility, seed size and protein accumulation in Brachypodium

Addressing the looming global food security crisis requires the development of high-yielding crops. In agricultural soils, deficiency in the micronutrient copper significantly decreases grain yield in wheat (Triticum aestivum), a globally important crop. In cereals, grain yield is determined by inflorescence architecture, flower fertility, grain size, and weight. Whether copper is involved in these processes, and how it is delivered to the reproductive organs is not well understood. We show that copper deficiency alters not only the grain set but also flower development in both wheat and its recognized model, Brachypodium distachyon. We then show that the Brachypodium yellow stripe-like 3 (YSL3) transporter localizes to the phloem, transports copper in frog (Xenopus laevis) oocytes, and facilitates copper delivery to reproductive organs and grains. Failure to deliver copper, but not iron, zinc, or manganese to these structures in the ysl3 CRISPR-Cas9 mutant results in delayed flowering, altered inflorescence architecture, reduced floret fertility, grain size, weight, and protein accumulation. These defects are rescued by copper supplementation and are complemented by YSL3 cDNA. This knowledge will help to devise sustainable approaches for improving grain yield in regions where soil quality is a major obstacle for crop production. Copper distribution by a phloem-localized transporter is essential for the transition to flowering, inflorescence architecture, floret fertility, size, weight, and protein accumulation in seeds.

Arabidopsis Pollen Fertility Requires the Transcription Factors CITF1 and SPL7 That Regulate Copper Delivery to Anthers and Jasmonic Acid Synthesis

A deficiency of the micronutrient copper (Cu) leads to infertility and grain/seed yield reduction in plants. How Cu affects fertility, which reproductive structures require Cu, and which transcriptional networks coordinate Cu delivery to reproductive organs is poorly understood. Using RNA-seq analysis, we showed that the expression of a gene encoding a novel transcription factor, CITF1 (Cu-DEFICIENCY INDUCED TRANSCRIPTION FACTOR1), was strongly upregulated in Arabidopsis thaliana flowers subjected to Cu deficiency. We demonstrated that CITF1 regulates Cu uptake into roots and delivery to flowers and is required for normal plant growth under Cu deficiency. CITF1 acts together with a master regulator of copper homeostasis, SPL7 (SQUAMOSA PROMOTER BINDING PROTEIN LIKE7), and the function of both is required for Cu delivery to anthers and pollen fertility. We also found that Cu deficiency upregulates the expression of jasmonic acid (JA) biosynthetic genes in flowers and increases endogenous JA accumulation in leaves. These effects are controlled in part by CITF1 and SPL7. Finally, we show that JA regulates CITF1 expression and that the JA biosynthetic mutant lacking the CITF1- and SPL7-regulated genes, LOX3 and LOX4, is sensitive to Cu deficiency. Together, our data show that CITF1 and SPL7 regulate Cu uptake and delivery to anthers, thereby influencing fertility, and highlight the relationship between Cu homeostasis, CITF1, SPL7, and the JA metabolic pathway.

Iron-Nicotianamine Transporters Are Required for Proper Long Distance Iron Signaling

The mechanisms of root iron uptake and the transcriptional networks that control root-level regulation of iron uptake have been well studied, but the mechanisms by which shoots signal iron status to the roots remain opaque. Here, we characterize an Arabidopsis (Arabidopsis thaliana) double mutant, yellow stripe1-like yellow stripe3-like (ysl1ysl3), which has lost the ability to properly regulate iron deficiency-influenced gene expression in both roots and shoots. In spite of markedly low tissue levels of iron, the double mutant does not up- and down-regulate iron deficiency-induced and -repressed genes. We have used grafting experiments to show that wild-type roots grafted to ysl1ysl3 shoots do not initiate iron deficiency-induced gene expression, indicating that the ysl1ysl3 shoots fail to send an appropriate long-distance signal of shoot iron status to the roots. We present a model to explain how impaired iron localization in leaf veins results in incorrect signals of iron sufficiency being sent to roots and affecting gene expression there. Improved understanding of the mechanism of long-distance iron signaling will allow improved strategies for the engineering of staple crops to accumulate additional bioavailable iron in edible parts, thus improving the iron nutrition of the billions of people worldwide whose inadequate diet causes iron deficiency anemia.

Tentative identification of the second substrate binding site in Arabidopsis phytochelatin synthase

Phytochelatin synthase (PCS) uses the substrates glutathione (GSH, γGlu-Cys-Gly) and a cadmium (Cd)-bound GSH (Cd∙GS2) to produce the shortest phytochelatin product (PC2, (γGlu-Cys)2-Gly) through a ping-pong mechanism. The binding of the 2 substrates to the active site, particularly the second substrate binding site, is not well-understood. In this study, we generated a structural model of the catalytic domain of Arabidopsis AtPCS1 (residues 12-218) by using the crystal structure of the γGlu-Cys acyl-enzyme complex of the PCS of the cyanobacterium Nostoc (NsPCS) as a template. The modeled AtPCS1 revealed a cavity in proximity to the first substrate binding site, consisting of 3 loops containing several conserved amino acids including Arg152, Lys185, and Tyr55. Substitutions of these amino acids (R152K, K185R, or double mutation) resulted in the abrogation of enzyme activity, indicating that the arrangement of these 2 positive charges is crucial for the binding of the second substrate. Recombinant AtPCS1s with mutations at Tyr55 showed lower catalytic activities because of reduced affinity (3-fold for Y55W) for the Cd∙GS2, further suggesting the role of the cation-π interaction in recognition of the second substrate. Our study results indicate the mechanism for second substrate recognition in PCS. The integrated catalytic mechanism of PCS is further discussed.

Phytochelatin synthase is regulated by protein phosphorylation at a threonine residue near its catalytic site

Heavy metals are toxic to most living organisms and cause health problems by contaminating agricultural products. In plants, phytochelatin synthase (PCS, EC 2.3.2.15) uses glutathione (GSH) as its substrate to catalyze the synthesis of heavy metal-binding peptides, known as phytochelatins (PC). PCS has been described as a constitutive enzyme that may be controlled by post-translational modifications. However, the detailed mechanism of its catalytic activity is not clear. In this study, in vitro experiments demonstrate that PCS activity increased following phosphorylation by casein kinase 2 (CK2) and decreased following treatment with alkaline phosphatase. Site-directed mutagenesis experiments at amino acids on AtPCS1 indicate that Thr 49 is the site for phosphorylation. This is further supported by fact that the mutant AtPCS1(T49A) cannot be phosphorylated, and its activity is significantly lower than that of the wild-type enzyme. In the modeled three-dimensional structure of AtPCS1, Arg 183 is within close proximity to Thr 49. The mutant AtPCS1(R183A) can be phosphorylated, but it shows much lower catalytic activity than the wild-type protein. This result suggested that Arg 183 may play an important role in the catalytic mechanism of AtPCS1. The possibility of the presence of a second substrate-binding site as a result of the interaction of these two amino acids is discussed. In addition, the activity of AtPCS1 was also found to be modulated by the C-terminal domain. The N-terminal catalytic domain of AtPCS1 was expressed (AtPCS1-N), and its catalytic activity was found to be even more sensitive to Cd or phosphorylation status than was the full-length enzyme.

Soft tissue chondroma in the finger: a case report and review of the literature

Soft tissue chondromas are rare. A fairly benign condition, it is an uncommon occurrence in the hand. It usually presents as an enlarging mass and local surgery is the treatment of choice. We report a case of a patient in our local population presenting with a soft tissue chondroma in a digit.