Spatiotemporal auxin distribution in Arabidopsis tissues is regulated by anabolic and catabolic reactions under long-term ammonium stress

Background The plant hormone auxin is a major coordinator of plant growth and development in response to diverse environmental signals, including nutritional conditions. Sole ammonium (NH4+) nutrition is one of the unique growth-suppressing conditions for plants. Therefore, the quest to understand NH4+-mediated developmental defects led us to analyze auxin metabolism. Results Indole-3-acetic acid (IAA), the most predominant natural auxin, accumulates in the leaves and roots of mature Arabidopsis thaliana plants grown on NH4+, but not in the root tips. We found changes at the expressional level in reactions leading to IAA biosynthesis and deactivation in different tissues. Finally, NH4+ nutrition would facilitate the formation of inactive oxidized IAA as the final product. Conclusions NH4+-mediated accelerated auxin turnover rates implicate transient and local IAA peaks. A noticeable auxin pattern in tissues correlates with the developmental adaptations of the short and highly branched root system of NH4+-grown plants. Therefore, the spatiotemporal distribution of auxin might be a root-shaping signal specific to adjust to NH4+-stress conditions.

Thymol ameliorates ammonium toxicity via repressing polyamine oxidase-derived hydrogen peroxide and modulating ammonium transporters in rice root

Background Ammonium is an indispensable nutrient for crop growth, but anoxic conditions or inappropriate fertilizer usage result in the increase in ammonium content in soil. Excessive ammonium causes phytotoxicity. Thymol is a kind of natural phenolic compound with anti-microbial properties. However, little is known about the role of thymol in modulating plant physiology. Here we find the novel role of thymol in protecting rice from ammonium toxicity. Results Thymol remarkably rescued rice seedlings growth from ammonium stress, which may resulted from the attenuation of reactive oxygen species (ROS) accumulation, oxidative injury, and cell death in both shoots and roots. Polyamine oxidase (PAO) metabolizes polyamines to produce ROS in plants in response to stress conditions. Thymol blocked ammonium-induced upregulation of a set of rice PAOs, which contributed to the decrease in ROS content. In rice seedlings upon ammonium stress, thymol downregulate the expression of ammonium transporters (AMT1;1 and AMT1;2); thymol upregulated the expression of calcineurin B-like interacting protein kinase 23 (CIPK23) and calcineurin B-like binding protein 1 (CBL1), two negative regulators of AMTs. This may help rice avoid ammonium overload in excessive ammonium environment. Correlation analysis indicated that PAOs, AMTs, and CBL1 were the targets of thymol in the detoxification of excessive ammonium. Conclusion Thymol facilitates rice tolerance against ammonium toxicity by decreasing PAO-derived ROS and modulating ammonium transporters. Such findings may be applicable in the modulation of nutrient acquisition during crop production. Graphical abstract

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 homologue 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.