Two Calcium Sensor-Activated Kinases Function in Root Hair Growth.

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 alleviation of ammonium toxicity in plants.

Nitrogen (N) is an essential macronutrient for plants and profoundly affects crop yields and qualities. Ammonium (NH4 + ) and nitrate (NO3 - ) are major inorganic N forms absorbed by plants from the surrounding environments. Intriguingly, NH4 + is usually toxic to plants when it serves as the sole or dominant N source. It is thus important for plants to coordinate the utilization of NH4 + and the alleviation of NH4 + toxicity. To fully decipher the molecular mechanisms underlying how plants minimize NH4 + toxicity may broadly benefit agricultural practice. In the current minireview, we attempt to discuss recent discoveries in the strategies for mitigating NH4 + toxicity in plants, which may provide potential solutions for improving the nitrogen use efficiency (NUE) and stress adaptions in crops.

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.

Nitrate transporter NRT1.1 and anion channel SLAH3 form a functional unit to regulate nitrate-dependent alleviation of ammonium toxicity.

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.

Kinase SnRK1.1 regulates nitrate channel SLAH3 engaged in nitrate-dependent alleviation of ammonium toxicity.

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.

Synergistic removal of aniline by carbon nanotubes and the enzymes of Delftia sp. XYJ6.

Synergistic removal of aniline by carbon nanotubes and the enzymes of Delftia sp. XYJ6, a newly isolated bacterial strain for biodegrading aniline, was investigated. It showed that biodegradation rate of aniline was increased with the augment of protein concentration in cell-free extract of Delftia sp. XYJ6. The adsorption amount of aniline by multi-walled carbon nanotubes (MWCNTs) was slightly higher than that by single-walled carbon nanotubes (SWCNTs), however the adsorption amount of protein of Delftia sp. XYJ6 by MWCNTs was lower than that by SWCNTs. Much more amount of aniline could be removed by CE of Delftia sp. XYJ6 in the presence of SWCNTs than MWCNTs, which indicated that an efficient reaction between aniline and enzymes of Delftia sp. XYJ6 on the surface of SWCNTs played a key role in the rapid enzymatic biodegradation of aniline. This study is not previously reported and may be useful in basic research and the removal of aniline from wastewater.

Effects of nicotinamide and riboflavin on the biodesulfurization activity of dibenzothiophene by Rhodococcus erythropolis USTB-03.

Rhodococcus erythropolis USTB-03 is a promising bacterial strain for the biodesulfurization of dibenzothiophene (DBT) via a sulfur-specific pathway in which DBT is converted to 2-hydroxybiphenyl (2HBP) as an end product. The effects of nicotinamide and riboflavin on the sulfur specific activity (SA) of DBT biodesulfurization by R. erythropolis USTB-03 were investigated. Both nicotinamide and riboflavin were found to enhance the expression of SA, which was not previously reported. When R. erythropolis USTB-03 was grown on a medium containing nicotinamide of 10.0 mmol or riboflavin of 50.0 micromol, SA was raised from 68.0 or so to more than 130 mmol 2HBP/(kg dry cells. x ). When R. erythropolis USTB-03 was grown in the presence of both nicotinamide of 5.0 mmol and riboflavin of 25.0 Cmicrool, SA was further increased to 159.0 mmol 2HBP/(kg dry cells. x ). It is suggested that the biological synthesis of reduced form of flavin mononucleotide (FMNH2), an essential coenzyme for the activities of biodesulfurization enzyme Dsz C and A, might be enhanced by nicotinamide and riboflavin, which was responsible for the increased SA of R. erythropolis USTB-03.