The Role of Aquaporin Overexpression in the Modulation of Transcription of Heavy Metal Transporters under Cadmium Treatment in Poplar

Differences in mineral and osmotic balances enhance zinc translocation in an aquaporin overexpressing poplar

Zinc (Zn) is an essential micronutrient for plants, but it is toxic beyond a certain threshold. Populus alba (L.) 'Villafranca' clone is known for its good tolerance to high Zn concentration compared to other poplar species. A line of this species overexpressing the tonoplast intrinsic aquaporin AQUA1 gene has showed an improved tolerance to Zn excess in comparison to the wild-type (wt) line. The aims of this work were to: 1) verify if AQUA1 plants can uptake Zn more efficiently after a longer period of exposure; 2) evaluate if a higher Zn uptake in transgenic lines can have negative effects; 3) assess Zn competing elements (iron and manganese), soluble sugars, osmolytes, and potassium to investigate differences in water and osmotic homeostasis between lines. Under Zn excess, AQUA1 plants showed a twofold Zn translocation factor and a higher xylem sap Zn concentration than the wt plants. Transgenic plants preferentially allocated Zn in aerial biomass and this different behaviour matched with modified manganese and iron balances suggesting that the increased Zn uptake might be related to a decrease in iron transport in the transgenic line. Moreover, a higher instantaneous water use efficiency in control conditions and an increase in bark soluble sugars under Zn excess could allow a higher resistance of AQUA1 plants to the water and osmotic perturbations caused by Zn. Indeed, the Zn excess increased the xylem osmolyte content only in wt plants. Further investigations are required to understand the role of AQUA1 in osmotic regulation.

Cadmium promotes the absorption of ammonium in hyperaccumulator Solanum nigrum L. mediated by ammonium transporters and aquaporins

Cadmium (Cd) is a toxic heavy metal affecting the normal growth of plants. Nitrate (NO₃^-) and ammonium (NH₄⁺) are the primary forms of inorganic nitrogen (N) absorbed by plants. However, the mechanism of N absorption and regulation under Cd stress remains unclear. This study found that: (1) Cd treatment affected the biomass, root length, and Cd²⁺ flux in Solanum nigrum seedling roots. Specifically, 50 μM Cd significantly inhibited NO₃^- influx while increased NH₄⁺ influx compared with 0 and 5 μM Cd treatments measured by non-invasive micro-test technology. (2) qRT-PCR analysis showed that 50 μM Cd inhibited the expressions of nitrate transporter genes, SnNRT2;4 and SnNRT2;4-like, increased the expressions of ammonium transporter genes, SnAMT1;2 and SnAMT1;3, in the roots. (3) Under NH₄⁺ supply, 50 μM Cd significantly induced the expressions of the aquaporin genes, SnPIP1;5, SnPIP2;7, and SnTIP2;1. Our results showed that 50 μM Cd stress promoted NH₄⁺ absorption by up-regulating the gene expressions of NH₄⁺ transporter and aquaporins, suggesting that high Cd stress can affect the preference of N nutrition in S. nigrum.

The Mechanism of Metal Homeostasis in Plants: A New View on the Synergistic Regulation Pathway of Membrane Proteins, Lipids and Metal Ions

Heavy metal stress (HMS) is one of the most destructive abiotic stresses which seriously affects the growth and development of plants. Recent studies have shown significant progress in understanding the molecular mechanisms underlying plant tolerance to HMS. In general, three core signals are involved in plants' responses to HMS; these are mitogen-activated protein kinase (MAPK), calcium, and hormonal (abscisic acid) signals. In addition to these signal components, other regulatory factors, such as microRNAs and membrane proteins, also play an important role in regulating HMS responses in plants. Membrane proteins interact with the highly complex and heterogeneous lipids in the plant cell environment. The function of membrane proteins is affected by the interactions between lipids and lipid-membrane proteins. Our review findings also indicate the possibility of membrane protein-lipid-metal ion interactions in regulating metal homeostasis in plant cells. In this review, we investigated the role of membrane proteins with specific substrate recognition in regulating cell metal homeostasis. The understanding of the possible interaction networks and upstream and downstream pathways is developed. In addition, possible interactions between membrane proteins, metal ions, and lipids are discussed to provide new ideas for studying metal homeostasis in plant cells.

Comparative transcriptomic analysis reveals the coordinated mechanisms of Populus × canadensis 'Neva' leaves in response to cadmium stress

Cadmium (Cd), a heavy metal element has strong toxicity to living organisms. Excessive Cd accumulation directly affects the absorption of mineral elements, inhibits plant tissue development, and even induces mortality. Populus × canadensis 'Neva', the main afforestation variety planted widely in northern China, was a candidate variety for phytoremediation. However, the genes relieving Cd toxicity and increasing Cd tolerance of this species were still unclear. In this study, we employed transcriptome sequencing on two Cd-treated cuttings to identify the key genes involved in Cd stress responses of P. × canadensis 'Neva' induced by 0 (CK), 10 (C10), and 20 (C20) mg/L Cd(NO3)2 4H2O. We discovered a total of 2,656 (1,488 up-regulated and 1,168 down-regulated) and 2,816 DEGs (1,470 up-regulated and 1,346 down-regulated) differentially expressed genes (DEGs) between the CK vs C10 and CK vs C20, respectively. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses in response to the Cd stress indicated that many DEGs identified were involved in the catalytic activity, the oxidoreductase activity, the transferase activity, and the biosynthesis of secondary metabolites. Based on the enrichment results, potential candidate genes were identified related to the calcium ion signal transduction, transcription factors, the antioxidant defense system, and transporters and showed divergent expression patterns under the Cd stress. We also validated the reliability of transcriptome data with the real-time PCR. Our findings deeper the understanding of the molecular responsive mechanisms of P. × canadensis 'Neva' on Cd tolerance and further provide critical resources for phytoremediation applications.