Nitrate (⁠ $NO3−$⁠) and ammonium (⁠ $NH4+$⁠) are major inorganic nitrogen (N) supplies for plants, but $NH4+$ as the sole or dominant N source causes growth inhibition in many plants, known as ammonium toxicity. Small amounts of $NO3−$ can significantly mitigate ammonium toxicity, and the anion channel SLAC1 homolog 3 (SLAH3) is involved in this process, but the mechanistic detail of how SLAH3 regulates nitrate-dependent alleviation of ammonium toxicity is still largely unknown. In this study, we identified SnRK1.1, a central regulator involved in energy homeostasis, and various stress responses, as a SLAH3 interactor in Arabidopsis (Arabidopsis thaliana). Our results suggest that SNF1-related protein kinase 1 (SnRK1.1) functions as a negative regulator of SLAH3. Kinase assays indicate SnRK1.1 strongly phosphorylates the C-terminal of SLAH3 at the site S601. Under high- $NH4+$/low-pH condition, phospho-mimetic and phospho-dead mutations in SLAH3 S601 result in barely rescued phenotypes and fully complemented phenotypes in slah3. Furthermore, SnRK1.1 migrates from cytoplasm to nucleus under high- $NH4+$/low-pH conditions. The translocation of SnRK1.1 from cytosol to nucleus under high-ammonium stress releases the inhibition on SLAH3, which allows SLAH3-mediated $NO3−$ efflux leading to alleviation of high- $NH4+$/low-pH stress. Our study reveals that the C-terminal phosphorylation also plays important role in SLAH3 regulation and provides additional insights into nitrate-dependent alleviation of ammonium toxicity in plants.

Ammonium (NH₄⁺) and nitrate (NO₃⁻) are major inorganic nitrogen (N) sources for plants. When serving as the sole or dominant N supply, NH₄⁺ often causes root inhibition and shoot chlorosis in plants, known as ammonium toxicity. NO₃⁻ usually causes no toxicity and can mitigate ammonium toxicity even at low concentrations, referred to as nitrate‐dependent alleviation of ammonium toxicity. Our previous studies indicated a NO₃⁻ efflux channel SLAH3 is involved in this process. However, whether additional components contribute to NO₃⁻‐mediated NH₄⁺ detoxification is unknown. Previously, mutations in NO₃⁻ transporter NRT1.1 were shown to cause enhanced resistance to high concentrations of NH₄⁺. Whereas, in this study, we found when the high‐NH₄⁺ medium was supplemented with low concentrations of NO₃⁻, nrt1.1 mutant plants showed hyper‐sensitive phenotype instead. Furthermore, mutation in NRT1.1 caused enhanced medium acidification under high‐NH₄⁺/low‐NO₃⁻ condition, suggesting NRT1.1 regulates ammonium toxicity by facilitating H⁺ uptake. Moreover, NRT1.1 was shown to interact with SLAH3 to form a transporter‐channel complex. Interestingly, SLAH3 appeared to affect NO₃⁻ influx while NRT1.1 influenced NO₃⁻ efflux, suggesting NRT1.1 and SLAH3 regulate each other at protein and/or gene expression levels. Our study thus revealed NRT1.1 and SLAH3 form a functional unit to regulate nitrate‐dependent alleviation of ammonium toxicity through regulating NO₃⁻ transport and balancing rhizosphere acidification.

Plants encounter a variety of mechanical stimuli during their growth and development. It is currently believed that mechanosensitive ion channels play an essential role in the initial perception of mechanical force in plants. Over the past decade, the study of Piezo, a mechanosensitive ion channel in animals, has made significant progress. It has been proved that the perception of mechanical force in various physiological processes of animals is indispensable. However, little is still known about the function of its homologs in plants. In this study, by investigating the function of the AtPiezo gene in the model plant Arabidopsis thaliana, we found that AtPiezo plays a role in the perception of mechanical force in plant root cap and the flow of Ca²⁺ is involved in this process. These findings allow us to understand the function of AtPiezo from the perspective of plants and provide new insights into the mechanism of plant root cap in response to mechanical stimuli.

Nitrogen (N) and potassium (K) are essential macronutrients for plants. Sufficient N and K uptake from the environment is required for successful growth and development. However, how N and K influence each other at the molecular level in plants is largely unknown. In this study, we found loss-of-function mutation in SLAH3 (SLAC1 HOMOLOGUE 3), encoding a NO₃⁻ efflux channel in Arabidopsis thaliana, enhanced tolerance to high KNO₃ concentrations. Surprisingly, slah3 mutants were less sensitive to high K⁺ but not NO₃⁻. Addition of NO₃⁻ led to reduced phenotypic difference between wild-type and slah3 plants, suggesting SLAH3 orchestrates NO₃⁻-K⁺ balance. Non-invasive Micro-test Technology analysis revealed reduced NO₃⁻ efflux and enhanced K⁺ efflux in slah3 mutants, demonstrating that SLAH3-mediated NO₃⁻ transport and SLAH3-affected K⁺ flux are critical in response to high K ⁺. Further investigation showed that two K⁺ efflux channels, GORK (GATED OUTWARDLY-RECTIFYING K⁺ CHANNEL) and SKOR (STELAR K⁺ OUTWARD RECTIFIER), interacted with SLAH3 and played key roles in high K⁺ response. The gork and skor mutants were slightly more sensitive to high K⁺ conditions. Less depolarization occurred in slah3 mutants and enhanced depolarization was observed in gork and skor mutants upon K⁺ treatment, suggesting NO₃⁻/K⁺ efflux-mediated membrane potential regulation is involved in high K⁺ response. Electrophysiological results showed that SLAH3 partially inhibited the activities of GORK and SKOR in Xenopus laevis oocytes. This study revealed that the anion channel SLAH3 interacts with the potassium channels GORK and SKOR to modulate membrane potential by coordinating N–K balance.

Plant pollen tubes and root hairs typical polarized tip growth. It is well established that calcium ions (Ca²⁺) play essential roles in maintaining cell polarity and guiding cell growth orientation. Ca²⁺ signals are encoded by Ca²⁺ channels and transporters and are decoded by a variety of Ca²⁺-binding proteins often called Ca²⁺ sensors, in which calcineurin B-like protein (CBL) proteins function by interacting with and activating a group of kinases, activate CBL-interacting protein kinases (CIPKs). Some CBL-CIPK complexes, such as CBL2/3-CIPK12/19, act as crucial regulators of pollen tube growth. Whether these calcium decoding components regulate the growth of root hairs, another type of plant cell featuring Ca²⁺-regulated polarized growth, remains unknown. In this study, we identified CIPK13 and CIPK18 as genes specifically expressed in Arabidopsis (Arabidopsis thaliana) root hairs. The cipk13 cipk18 double mutants showed reduced root hair length and lower growth rates. The calcium oscillations at the root hair tip were attenuated in the cipk13 cipk18 mutants as compared to the wild-type plants. Through yeast two-hybrid screens, CBL2 and CBL3 were identified as interacting with CIPK13 and CIPK18. cbl2 cbl3 displayed a shortened root hair phenotype similar to cipk13 cipk18. This genetic analysis, together with biochemical assays showing activation of CIPK13/18 by CBL2/3, supported the conclusion that CBL2/3 and CIPK13/18 may work as Ca²⁺-decoding modules in controlling root hair growth. Thus, the findings that CIPK12/19 and CIPK13/18 function in pollen tube and root hair growth, respectively, illustrate a molecular mechanism in which the same CBLs recruit distinct CIPKs in regulating polarized tip growth in different types of plant cells.

The plant-environment interactions are finely tuned by plant endogenous signals and environmental cues. Mechanical forces serve as important exogenous stimuli regulating plant growth and development and shaping plant structures. Studies have shown that mechanosensitive ion channels play essential roles in the responses to mechanical signals in plants. The biological functions of animal Piezos, a group of mechanosensitive ion channels, have been extensively studied and revealed to be required for normal physiological processes. However, little is known about the functions of the homologous genes of animal Piezo genes in plants. We have recently pinpointed that AtPiezo plays an important role in the root cap in response to mechanical forces in Arabidopsis thaliana. Here, we further show that AtPiezo responds to mechanical stimuli at the transcriptional level. The results provide additional evidence for the involvement of Piezo in mechanical responses in plants.