image: When plants are subjected to Zn deficiency, the transcriptional levels of Zn-absorbing transporters (such as ZIP4, ZIP9, and ZIP12) are significantly upregulated due to the plant's stress response, facilitating the uptake of Zn from the soil. To prevent the disorder that might arise from excessive expression of ZIP transporters or to avoid unnecessary high expression levels, it is essential to appropriately reduce the protein levels. The CBL-CIPK complex perceives specific Ca2+ signals and mediates the partial degradation of ZIP12 through the ubiquitin-proteasome pathway via phosphorylation. This negative feedback mechanism not only prevents the excessive accumulation of Zn transporters caused by the plant's emergency response but also dynamically adapts to fluctuations in environmental Zn concentrations. Consequently, it maintains the physiological balance of Zn absorption by precisely regulating the homeostasis of ZIP12 protein.
Credit: ©Science China Press
Zn, often referred to as "the flower of life" and "the source of wisdom," is deficient in nearly 30% of global agricultural soils and 50% of paddy soils. Soil Zn deficiency can lead to Zn deficiency in humans, contributing to hidden hunger, a prevalent global food security issue.
Ca2+ serves as a fundamental regulator of plant growth, development, and stress responses, representing a significant area of interest within botanical research. Calcineurin B-like proteins (CBLs), as distinctive Ca2+ sensors in plants, establish a complex regulatory network with CBL-interacting protein kinases (CIPKs). This network decodes and transmits Ca2+ signals by phosphorylating downstream substrates, thereby regulating the absorption and utilization of various mineral elements. Despite this understanding, the role of Ca2+ signaling and the CBL-CIPK functional module in the regulation of Zn absorption and transport, along with its specific molecular mechanisms, remains to be elucidated.
CBL1/4/5/8/9-CIPK3/9/23/26 responds to Zn deficiency stress and interacts with Zn transporter ZIP12.
Through genetic screening, the research team identified that high-order mutants of cbl1/4/5/8/9 and cipk3/9/23/26 exhibited a pronounced tolerance to Zn deficiency, characterized by increased primary root length, plant fresh weight, and Zn content. Furthermore, using bimolecular fluorescence complementation assays, the study detected protein interactions between CIPK26 and 11 Zn transporters located in the plasma membrane, revealing a particularly strong interaction between CIPK26 and ZIP12. Bimolecular fluorescence complementation detection utilizing the plasma membrane marker CBL1-OFP demonstrated that CIPK3/9/23/26 interacts with the ZIP12 protein at the plasma membrane. The interaction between CIPK3/9/23/26 and the ZIP12 protein was substantiated through firefly luciferase complementation assay and in vivo co-immunoprecipitation assay.
CIPK3/9/23/26 phosphorylates ZIP12 at Ser185 and affects its protein stability
In this investigation, both in vitro and in vivo phosphorylation assays indicated that CIPK3/9/23/26 predominantly phosphorylates the Ser185 residue of ZIP12. To elucidate the impact of this phosphorylation site on ZIP12 function, the study included protein stability analysis, complementary phenotype assessments of transgenic lines, and in vivo measurements of Ca2+ signaling alterations. The findings revealed that Zn deficiency stress induces a pronounced Ca2+ signal in Arabidopsis roots. The Ca2+ sensors CBL1/4/5/8/9 recruit their interacting protein kinase CIPK3/9/23/26, which phosphorylates the Ser185 site of the Zn transporter ZIP12, thereby modulating the protein stability of ZIP12.
In conclusion, under conditions of Zn deficiency, plants exhibit a pronounced stress response characterized by a substantial upregulation of the transcription levels of Zn uptake transporters, such as ZIP4, ZIP9, and ZIP12, thereby enhancing Zn absorption from the soil. Given that the increase in transcription levels can reach several hundred to a thousand-fold, plants may seek to prevent excessive expression of ZIP transporters, which could lead to cellular disruptions, or they may find such elevated expression levels unnecessary and thus aim to modulate protein levels accordingly. The CBL-CIPK complex plays a crucial role in this regulatory process by sensing specific Ca2+ signals, phosphorylating ZIP12, and initiating its partial degradation of the ZIP12 protein. This negative feedback mechanism effectively fine-tunes the plant's response to Zn deficiency environments, facilitating efficient resource utilization and maintaining Zn homeostasis under such conditions.
See the article:
Plasma membrane-associated calcium signaling modulates zinc homeostasis in Arabidopsis
https://www.sciencedirect.com/science/article/pii/S2095927325001690
Journal
Science Bulletin