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Phosphatase promotes nitrate uptake
Plants require nitrogen, either from the soil mostly in the form of nitrate (NO3−) or from a symbiotic relationship with nitrogen-fixing bacteria or fungi. Léran et al. identified ABI2, a phosphatase that is inhibited by the stress hormone abscisic acid (ABA), as a key positive regulator of the nitrate transporter NPF6.3. ABI2 dephosphorylated components of a calcium-sensing and kinase complex that phosphorylated and inhibited NPF6.3-dependent NO3− uptake, sensing, and signaling in roots. Because when dephosphorylated by ABI2 a related calcium-sensing kinase complex also stimulates K+ influx, the identification of this ABA-regulated phosphatase suggests a mechanism for integrating regulation of ionic balance and energy-consuming nitrate reduction during periods of plant stress.
Abstract
Living organisms sense and respond to changes in nutrient availability to cope with diverse environmental conditions. Nitrate (NO3−) is the main source of nitrogen for plants and is a major component in fertilizer. Unraveling the molecular basis of nitrate sensing and regulation of nitrate uptake should enable the development of strategies to increase the efficiency of nitrogen use and maximize nitrate uptake by plants, which would aid in reducing nitrate pollution. NPF6.3 (also known as NRT1.1), which functions as a nitrate sensor and transporter; the kinase CIPK23; and the calcium sensor CBL9 form a complex that is crucial for nitrate sensing in Arabidopsis thaliana. We identified two additional components that regulate nitrate transport, sensing, and signaling: the calcium sensor CBL1 and protein phosphatase 2C family member ABI2, which is inhibited by the stress-response hormone abscisic acid. Bimolecular fluorescence complementation assays and in vitro kinase assays revealed that ABI2 interacted with and dephosphorylated CIPK23 and CBL1. Coexpression studies in Xenopus oocytes and analysis of plants deficient in ABI2 indicated that ABI2 enhanced NPF6.3-dependent nitrate transport, nitrate sensing, and nitrate signaling. These findings suggest that ABI2 may functionally link stress-regulated control of growth and nitrate uptake and utilization, which are energy-expensive processes.
