Arabidopsis COPPER TRANSPORTER 1 undergoes degradation in a proteasome-dependent manner

Transcriptome and metabolome analysis reveal key genes and metabolic pathway responses in Leersia hexandra Swartz under Cr and Ni co-stress

Phytoremediation, an eco-friendly approach for mitigating heavy metal contamination, is reliant on hyperaccumulators. This study focused on Leersia hexandra Swart, a known chromium (Cr) hyperaccumulator with demonstrated tolerance to multiple heavy metals. Our objective was to investigate its response to simultaneous Cr and nickel (Ni) stress over 12 days. Results from physiological experiments demonstrated a significant increase in the activities of antioxidant enzymes (APX, SOD, CAT) and glutathione (GSH) content under Cr and Ni stress, indicating enhanced antioxidant mechanisms. Transcriptome analysis revealed that stress resulted in the differential expression of 27 genes associated with antioxidant activity and metal binding, including APX, SOD, CAT, GSH, metallothionein (MT), and nicotinamide (NA). Among them, twenty differentially expressed genes (DEGs) related to GSH metabolic cycle were identified. Notably, GSTU6, GND1, and PGD were the top three related genes, showing upregulation with fold changes of 4.57, 6.07, and 3.76, respectively, indicating their crucial role in metal tolerance. The expression of selected DEGs was validated by quantitative real-time PCR, confirming the reliability of RNA-Seq data. Metabolomic analysis revealed changes in 1121 metabolites, with amino acids, flavonoids, and carbohydrates being the most affected. Furthermore, glucosinolate biosynthesis and amino acid biosynthesis pathways were represented in the KEGG pathway of differentially expressed metabolites (DEMs). This study provides insights into the tolerance mechanisms of L. hexandra under the co-stress of Cr and Ni, offering a new perspective for enhancing its remediation performance.

Boron uptake in rice is regulated post-translationally via a clathrin-independent pathway

Uptake of boron (B) in rice (Oryza sativa) is mediated by the Low silicon rice 1 (OsLsi1) channel, belonging to the NOD26-like intrinsic protein III subgroup, and the efflux transporter B transporter 1 (OsBOR1). However, it is unknown how these transporters cooperate for B uptake and how they are regulated in response to B fluctuations. Here, we examined the response of these two transporters to environmental B changes at the transcriptional and posttranslational level. OsBOR1 showed polar localization at the proximal side of both the exodermis and endodermis of mature root region, forming an efficient uptake system with OsLsi1 polarly localized at the distal side of the same cell layers. Expression of OsBOR1 and OsLsi1 was unaffected by B deficiency and excess. However, although OsLsi1 protein did not respond to high B at the protein level, OsBOR1 was degraded in response to high B within hours, which was accompanied with a significant decrease of total B uptake. The high B-induced degradation of OsBOR1 was inhibited in the presence of MG-132, a proteasome inhibitor, without disturbance of the polar localization. In contrast, neither the high B-induced degradation of OsBOR1 nor its polarity was affected by induced expression of dominant-negative mutated dynamin-related protein 1A (OsDRP1AK47A) or knockout of the mu subunit (AP2M) of adaptor protein-2 complex, suggesting that clathrin-mediated endocytosis is not involved in OsBOR1 degradation and polar localization. These results indicate that, in contrast to Arabidopsis thaliana, rice has a distinct regulatory mechanism for B uptake through clathrin-independent degradation of OsBOR1 in response to high B.

Two ubiquitin-associated ER proteins interact with COPT copper transporters and modulate their accumulation

The endoplasmic reticulum (ER) contains an elaborate protein quality control network that promotes protein folding and prevents accumulation of misfolded proteins. Evolutionarily conserved UBIQUITIN-ASSOCIATED DOMAIN-CONTAINING PROTEIN 2 (UBAC2) is involved in ER-associated protein degradation in metazoans. We have previously reported that two close UBAC2 homologs from Arabidopsis (Arabidopsis thaliana) not only participate in selective autophagy of ER components but also interact with plant-specific PATHOGEN-ASSOCIATED MOLECULAR PATTERN (PAMP)-INDUCED COILED COIL (PICC) protein to increase the accumulation of POWDERY MILDEW-RESISTANT 4 callose synthase. Here, we report that UBAC2s also interacted with COPPER (Cu) TRANSPORTER 1 (COPT1) and plasma membrane-targeted members of the Cu transporter family. The ubac2 mutants were significantly reduced in both the accumulation of COPT proteins and Cu content, and also displayed increased sensitivity to a Cu chelator. Therefore, UBAC2s positively regulate the accumulation of COPT transporters, thereby increasing Cu uptake by plant cells. Unlike with POWDERY MILDEW RESISTANCE 4, however, the positive role of UBAC2s in the accumulation of COPT1 is not dependent on PICC or the UBA domain of UBAC2s. When COPT1 was overexpressed under the CaMV 35S promoter, the increased accumulation of COPT1 was strongly UBAC2-dependent, particularly when a signal peptide was added to the N-terminus of COPT1. Further analysis using inhibitors of protein synthesis and degradation strongly suggested that UBAC2s stabilize newly synthesized COPT proteins against degradation by the proteasome system. These results indicate that plant UBAC2s are multifunctional proteins that regulate the degradation and accumulation of specific ER-synthesized proteins.

Current progress in plant V-ATPase: From biochemical properties to physiological functions

Vacuolar-type adenosine triphosphatase (V-ATPase, VHA) is a highly conserved, ATP-driven multisubunit proton pump that is widely distributed in all eukaryotic cells. V-ATPase consists of two domains formed by at least 13 different subunits, the membrane peripheral V₁ domain responsible for ATP hydrolysis, and the membrane-integral V₀ domain responsible for proton translocation. V-ATPase plays an essential role in energizing secondary active transport and is indispensable to plants. In addition to multiple stress responses, plant V-ATPase is also implicated in physiological processes such as growth, development, and morphogenesis. Based on the identification of distinct V-ATPase mutants and advances in luminal pH measurements in vivo, it has been revealed that this holoenzyme complex plays a pivotal role in pH homeostasis of the plant endomembrane system and endocytic and secretory trafficking. Here, we review recent progress in comprehending the biochemical properties and physiological functions of plant V-ATPase and explore the topics that require further elucidation.

Characterization of a copper transporter 1 from Dermanyssus gallinae as a vaccine antigen

Poultry red mites (Dermanyssus gallinae, PRM) are dangerous ectoparasites that infest chickens and threaten the poultry industry worldwide. PRMs usually develop resistance to chemical acaricides, necessitating the development of more effective preventive agents, and vaccination could be an alternative strategy for controlling PRMs. The suitability of plasma membrane proteins expressed in the midguts as vaccine antigens was evaluated because these molecules are exposed to antibodies in the ingested blood and the binding of antibodies could potentially induce direct damage to midgut tissue and indirect damage via inhibition of the functions of target molecules. Therefore, in the present study, a copper transporter 1-like molecule (Dg-Ctr1) was identified and its efficacy as a vaccine antigen was assessed in vitro. Dg-Ctr1 mRNA was expressed in the midguts and ovaries and in all the life stages, and flow cytometric analysis indicated that Dg-Ctr1 was expressed on the plasma membrane. Importantly, nymphs fed on plasma derived from chickens immunized with the recombinant protein of the extracellular region of Dg-Ctr1 showed a significant reduction in the survival rate. These data indicate that the application of Dg-Ctr1 as a vaccine antigen could reduce the number of nymphs in the farms, contributing to reduction in the economic losses caused by PRMs in the poultry industry. To establish an effective vaccination strategy, the acaricidal effects of the combined use of Dg-Ctr1 with chemical acaricides or other vaccine antigens must be examined.

Metal transporters in organelles and their roles in heavy metal transportation and sequestration mechanisms in plants

Heavy metal toxicity is one of the major concerns for agriculture and health. Accumulation of toxic heavy metals at high concentrations in edible parts of crop plants is the primary cause of disease in humans and cattle. A dramatic increase in industrialization, urbanization, and other high anthropogenic activities has led to the accumulation of heavy metals in agricultural soil, which has consequently disrupted soil conditions and affected crop yield. By now, plants have developed several mechanisms to cope with heavy metal stress. However, not all plants are equally effective in dealing with the toxicity of high heavy metal concentrations. Plants have modified their anatomy, morphophysiology, and molecular networks to survive under changing environmental conditions. Heavy metal sequestration is one of the essential processes evolved by some plants to deal with heavy metals' toxic concentration. Some plants even have the ability to accumulate metals in high quantities in the shoots/organelles without toxic effects. For intercellular and interorganeller metal transport, plants harbor spatially distributed various transporters which mainly help in uptake, translocation, and redistribution of metals. This review discusses different heavy metal transporters in different organelles and their roles in metal sequestration and redistribution to help plants cope with heavy metal stress. A good understanding of the processes at stake helps in developing more tolerant crops without affecting their productivity.

Ubiquitin-independent proteasome system is required for degradation of Arabidopsis COPPER TRANSPORTER 2

Plants have evolved sophisticated mechanisms to adjust to deficiency or excess of nutrients. Membrane transport proteins play a central role in nutrient uptake from soil. In Arabidopsis thaliana, the COPPER TRANSPORTOR (COPT) family encodes high-affinity copper transporters. COPT2 is transcriptionally regulated in response to changing levels of cellular copper. However, little is known about whether COPT2 activity is subject to multiple levels of regulation. Here, we showed that the plasma membrane-/endoplasmic reticulum-resident COPT2 protein is degraded in response to excess copper. Confocal microscopy analysis together with pharmacological treatment with a vesicle trafficking inhibitor or vacuolar ATPase inhibitor indicated that copper-mediated downregulation of COPT2 is unlikely to be controlled by endosomal recycling and vacuolar system. However, COPT2 protein is stabilized by proteasome inhibition. Through site-directed mutagenesis, we found that COPT2 cannot be ubiquitinated, and lysine residues at the C-terminus is dispensable for copper-induced degradation of COPT2 but required for copper acquisition. Altogether, our findings reveal that unlike many metal transporters in Arabidopsis, COPT2 is a substrate of ubiquitin-independent proteasomal degradation but not of vacuolar proteases. These findings highlight the mechanistic diversity and complexity of plasma membrane transporter degradation.