The maintenance of cellular phosphate (Pi) homeostasis is of great importance in living organisms. The SPX domain-containing protein 1 (SPX1) proteins from both Arabidopsis and rice have been proposed to act as sensors of Pi status. The molecular signal indicating the cellular Pi status and regulating Pi homeostasis in plants, however, remains to be identified, as Pi itself does not bind to the SPX domain. Here, we report the identification of the inositol pyrophosphate InsP 8 as a signaling molecule that regulates Pi homeostasis in Arabidopsis. Polyacrylamide gel electrophoresis profiling of InsPs revealed that InsP 8 level positively correlates with cellular Pi concentration. We demonstrated that the homologs of diphosphoinositol pentakisphosphate kinase (PPIP5K), VIH1 and VIH2, function redundantly to synthesize InsP 8 , and that the vih1 vih2 double mutant overaccumulates Pi. SPX1 directly interacts with PHR1, the central regulator of Pi starvation responses, to inhibit its function under Pi-replete conditions. However, this interaction is compromised in the vih1 vih2 double mutant, resulting in the constitutive induction of Pi starvation-induced genes, indicating that plant cells cannot sense cellular Pi status without InsP 8 . Furthermore, we showed that InsP 8 could directly bind to the SPX domain of SPX1 and is essential for the interaction between SPX1 and PHR1. Collectively, our study suggests that InsP 8 is the intracellular Pi signaling molecule serving as the ligand of SPX1 for controlling Pi homeostasis in plants.

Continuous and real‐time sensoring has received much attention in biomarker monitoring, toxicity assessment, and therapeutic agent tracking. However, its on‐site application is seriously limited by several stubborn defects including liability to fouling, signal drifting, short service life, poor repeatability, etc. Additionally, most current methods require extra sample pretreatment, delaying timely acquisition of testing results. To address these issues, MXene‐Ti3C2Tx based screen‐printed electrode incorporated with a dialysis microfluidic chip is constructed for a direct and continuous multicomponent analysis of whole blood. Dual‐function of MXene is developed and allows for simultaneous quantification of different target compounds through one device. Importantly, ratiometric sensing tactic is easily implemented in the system, which greatly alleviates signal drifting. As a proof of concept, this novel sensor is applied in hemodialysis, and continuous assay of urea, uric acid, and creatinine levels in human blood is realized. This work paves a new path for 2D MXene in biomedical and sensing applications.

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