Nitrate sensing and uptake in Arabidopsis are enhanced by ABI2, a phosphatase inactivated by the stress hormone abscisic acid

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.

Nitrate transporters: an overview in legumes

“…3). This hypothesis is supported by recent results showing that, in A. thaliana, ABI2 (ABAinsensitive 2; a protein phosphatase 2C) enhanced AtNPF6.3-dependent nitrate transport, nitrate sensing and nitrate signaling (Léran et al 2015). (Pellizzaro et al 2014).…”

Environmental Nitrate Stimulates Abscisic Acid Accumulation in Arabidopsis Root Tips by Releasing It from Inactive Stores

“…The root-expressed NPF4.6/AIT1/NRT1.2 protein transports both ABA and nitrate in Arabidopsis, but nitrate has no effect on ABA transport, and the affinity of this transporter for ABA is several orders of magnitude higher than for nitrate, providing no obvious role for nitrate transport in this system (Huang et al, 1999;Kanno et al, 2012Kanno et al, , 2013. Recently, the nitrate/ auxin transporter NPF6.3 (NRT1.1) has been shown to interact with the ABA coreceptor, ABI2, such that ABA represses nitrate uptake by NRT1.1/AtNPF6.3 by inactivating ABI2, thus further strengthening the link between nitrate sensing and ABA signaling (Ma et al, 2009;Park et al, 2009;Léran et al, 2015).…”

Environmental nitrate signals through abscisic acid in the root tip

“…35,36 AtABI2 is a PP2C that functions both as a coreceptor for ABA, 35,36 and additionally serves to inactivate the CIPK23 kinase complex that phosphorylates the nitrate/auxin transporter AtNPF6.3, inhibiting its ability to transport nitrate. 30,37 Interestingly, when a PP2C binds the ABA/receptor complex, its phosphatase ability is inhibited. 35,36 As ABA levels rise within the cell, more ABI2 molecules become bound to, and inactivated by, the ABA/receptor complex, thereby relieving repression of the CIPK23/CBL1 kinase complex, which is now free to phosphorylate AtNPF6.3, inhibiting nitrate transport into the cell (Fig.…”

“…1). 37 This gradual accumulation of intracellular ABA in response to increased environmental nitrate thus activates a slow-acting negative feedback loop, mediated by AtBG1 and ABA, that reduces further nitrate import and stimulates nitrate assimilation.…”

“…13,63 Once ABI2 is inactivated, the CIPK23/CBL1 complex is free to act, phosphorylating AtNPF6.3 and inhibiting its ability to transport. 37 Thus, nitrate transport into the cell activates a slow-acting negative feedback loop, mediated by BG1 and ABA, that reduces further nitrate import and stimulates nitrate assimilation.…”