Abstract
Ammonium (NH4+) is one of the principal nitrogen (N) sources in soils, but is typically toxic already at intermediate concentrations. The phytohormone abscisic acid (ABA) plays a pivotal role in responses to environmental stresses. However, the role of ABA under high-NH4+ stress in rice (Oryza sativa L.) is only marginally understood. Here, we report that elevated NH4+ can significantly accelerate tissue ABA accumulation. Mutants with high (Osaba8ox) and low levels of ABA (Osphs3-1) exhibit elevated tolerance or sensitivity to high-NH4+ stress, respectively. Furthermore, ABA can decrease NH4+-induced oxidative damage and tissue NH4+ accumulation by enhancing antioxidant and glutamine synthetase (GS)/glutamate synthetasae (GOGAT) enzyme activities. Using RNA sequencing and quantitative real-time PCR approaches, we ascertain that two genes, OsSAPK9 and OsbZIP20, are induced both by high NH4+ and by ABA. Our data indicate that OsSAPK9 interacts with OsbZIP20, and can phosphorylate OsbZIP20 and activate its function. When OsSAPK9 or OsbZIP20 are knocked out in rice, ABA-mediated antioxidant and GS/GOGAT activity enhancement under high-NH4+ stress disappear, and the two mutants are more sensitive to high-NH4+ stress compared with their wild types. Taken together, our results suggest that ABA plays a positive role in regulating the OsSAPK9-OsbZIP20 pathway in rice to increase tolerance to high-NH4+ stress.
Highlights
Nitrate (NO3−) and ammonium (NH4+) are the two major inorganic nitrogen (N) forms accessed by plant roots
Our data show that high NH4+ induces free NH4+ accumulation in wild-type roots, and that it increases in phs3 and decreases in aba8ox3 (Fig. 3A)
The data indicate that abscisic acid (ABA) can accelerate NH4+ assimilation by activating the enzyme activities of glutamine synthetase (GS), GOGAT, and glutamate dehydrogenase (GDH) under high-NH4+ stress and that root growth can be maintained under high-NH4+ stress
Summary
Nitrate (NO3−) and ammonium (NH4+) are the two major inorganic nitrogen (N) forms accessed by plant roots. NH4+ is frequently absorbed by roots more readily, and its assimilation requires less energy than that of NO3−, excessive NH4+ is toxic to plants (Britto et al, 2001; Kronzucker et al, 2001; Britto and Kronzucker, 2002; Li et al, 2014). NH4+ toxicity takes place when plants accumulate high tissue levels of free NH4+, resulting from both excessive NH4+ exposure and disturbance of NH4+ assimilation in plant cells (Barker and Corey, 1991; Bittsánszky et al, 2015). A recent study in Arabidopsis thaliana revealed that AtNRT1.1 negatively regulates NH4+ tolerance by inhibiting the activities of GS, GOGAT, and GDH through a nitrate-independent pathway, and knockout of AtNRT1.1 enhances NH4+ assimilation and reduces free NH4+ accumulation in nrt1.1 mutants (Jian et al, 2018). The regulation of NH4+ assimilation under high NH4+ warrants further study
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