The anion channel SLAH3 interacts with potassium channels to regulate nitrogen-potassium homeostasis and the membrane potential in Arabidopsis.

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.

A chloride efflux transporter, BIG RICE GRAIN 1, is involved in mediating grain size and salt tolerance in rice.

Grain size is determined by the size and number of cells in the grain. The regulation of grain size is crucial for improving crop yield; however, the genes and molecular mechanisms that control grain size remain elusive. Here, we report that a member of the detoxification efflux carrier /Multidrug and Toxic Compound Extrusion (DTX/MATE) family transporters, BIG RICE GRAIN 1 (BIRG1), negatively influences grain size in rice (Oryza sativa L.). BIRG1 is highly expressed in reproductive organs and roots. In birg1 grain, the outer parenchyma layer cells of spikelet hulls are larger than in wild-type (WT) grains, but the cell number is unaltered. When expressed in Xenopus laevis oocytes, BIRG1 exhibits chloride efflux activity. Consistent with this role of BIRG1, the birg1 mutant shows reduced tolerance to salt stress at a toxic chloride level. Moreover, grains from birg1 plants contain a higher level of chloride than those of WT plants when grown under normal paddy field conditions, and the roots of birg1 accumulate more chloride than those of WT under saline conditions. Collectively, the data suggest that BIRG1 in rice functions as a chloride efflux transporter that is involved in mediating grain size and salt tolerance by controlling chloride homeostasis.

A transceptor-channel complex couples nitrate sensing to calcium signaling in Arabidopsis.

Nitrate-induced Ca2+ signaling is crucial for the primary nitrate response in plants. However, the molecular mechanism underlying the generation of the nitrate-specific calcium signature remains unknown. We report here that a cyclic nucleotide-gated channel (CNGC) protein, CNGC15, and the nitrate transceptor (NRT1.1) constitute a molecular switch that controls calcium influx depending on nitrate levels. The expression of CNGC15 is induced by nitrate, and its protein is localized at the plasma membrane after establishment of young seedlings. We found that disruption of CNGC15 results in the loss of the nitrate-induced Ca2+ signature (primary nitrate response) and retards root growth, reminiscent of the phenotype observed in the nrt1.1 mutant. We further showed that CNGC15 is an active Ca2+-permeable channel that physically interacts with the NRT1.1 protein in the plasma membrane. Importantly, we discovered that CNGC15-NRT1.1 interaction silences the channel activity of the heterocomplex, which dissociates upon a rise in nitrate levels, leading to reactivation of the CNGC15 channel. The dynamic interactions between CNGC15 and NRT1.1 therefore control the channel activity and Ca2+ influx in a nitrate-dependent manner. Our study reveals a new nutrient-sensing mechanism that utilizes a nutrient transceptor-channel complex assembly to couple nutrient status to a specific Ca2+ signature.

Interaction Between AtCML9 and AtMLO10 Regulates Pollen Tube Development and Seed Setting.

In higher-plant reproduction, the compatibility of pollen tube germination in the pistil is essential for successful double fertilization. It has been reported that Mildew Locus O (MLO) family gene NTA (MLO7), expressing in synergid cells, can correctly guide pollen tubes. However, the molecular mechanism underlying the interacting partners to MLOs in the fertilization is still unknown. In our study, we identified the direct protein interaction between CML9 and MLO10 within a non-canonical CaMBD. In GUS reporter assays, CML9 expresses in a high level in pollens, whereas MLO10 can be specifically detected in stigma which reaches up to a peaking level before fertilization. Therefore, the spatio-temporal expression patterns of MLO10 and CML9 are required for the time-window of pollination. When we observed the pollen germination in vitro, two cml9 mutant alleles dramatically reduced germination rate by 15% compared to wild-type. Consistently, the elongation rate of pollen tubes in planta was obviously slow while manually pollinating cml9-1 pollens to mlo10-1 stigmas. Additionally, cml9-1 mlo10-1 double mutant alleles had relatively lower rate of seed setting. Taken together, protein interaction between MLO10 and CML9 is supposed to affect pollen tube elongation and further affect seed development.

Two New Members of CsFEXs Couple Proton Gradients to Export Fluoride and Participate in Reducing Fluoride Accumulation in Low-Fluoride Tea Cultivars.

The accumulation of fluoride in tea leaves from various cultivars exhibits significant differences. However, the molecular basis and mechanism remain largely unknown. Here, we reported that two genes of CsFEX (fluoride export genes in Camellia sinensis), CsFEX1 and CsFEX2, transport fluoride out of cells, alleviate the cellular fluoride toxin, and rescue the yeast mutant (FEX1ΔFEX2Δ) and Arabidopsis mutant (fex), as their efflux activities are coupled with proton gradients. Further analysis found that CsFEX1 and CsFEX2 localize to the plasma membrane both in yeast and Arabidopsis cells. CsFEX2 is more effective to reduce fluoride toxicity in yeast and Arabidopsis compared with CsFEX1 even at low pH. CsFEX2 induced by fluoride treatment is around tenfold higher in a low-fluoride cultivar (Yunkang 10) than that in a high-fluoride cultivar (Pingyang Tezaocha), suggesting that CsFEX2 possibly plays a critical role in reducing fluoride accumulation in tea leaves.