CHL1 Functions as a Nitrate Sensor in Plants

Abstract

Ions serve as essential nutrients in higher plants and can also act as signaling molecules. Little is known about how plants sense changes in soil nutrient concentrations. Previous studies showed that T101-phosphorylated CHL1 is a high-affinity nitrate transporter, whereas T101-dephosphorylated CHL1 is a low-affinity transporter. In this study, analysis of an uptake- and sensing-decoupled mutant showed that the nitrate transporter CHL1 functions as a nitrate sensor. Primary nitrate responses in CHL1T101D and CHLT101A transgenic plants showed that phosphorylated and dephosphorylated CHL1 lead to a low- and high-level response, respectively. In vitro and in vivo studies showed that, in response to low nitrate concentrations, protein kinase CIPK23 can phosphorylate T101 of CHL1 to maintain a low-level primary response. Thus, CHL1 uses dual-affinity binding and a phosphorylation switch to sense a wide range of nitrate concentrations in the soil, thereby functioning as an ion sensor in higher plants. For a video summary of this article, see the PaperFlick file with the Supplemental Data available online.

Nitrate transporters: an overview in legumes

“…Since MtNPF6.8 does not influence the nitrate uptake at 250 lM of nitrate supply nor the global nitrogen status of the plant, it is possible that the role of MtNPF6.8 in response to nitrate is independent of its nitrate transport function. So, MtNPF6.8 could be a nitrate transceptor, having a nitratetransporter and a nitrate-sensor function (Pellizzaro et al 2014), as found for AtNPF6.3 (Ho et al 2009;Ho and Tsay 2010). In addition, Pellizzaro et al (2014) noticed that the induction of NR1 and NR2 was reduced in npf6.8 mutants by 60%, suggesting the existence of other nitrate sensors in M. truncatula (Fig.…”

“…So, to date, both studied nitrate transporters from M. truncatula, MtNPF6.8 and MtNPF1.7, can transport nitrate at low concentrations when expressed in heterologous transport system, without having a HATS activity in planta (Morère-Le Paven et al 2011; Bagchi et al 2012;Pellizzaro et al 2014). Interestingly, AtNPF6.3 of A. thaliana, known to be a dual-affinity nitrate transporter in oocytes, has been shown to be involved in HATS in planta (Wang et al 1998;Liu et al 1999;Ho et al 2009), although this is a matter of debate (Glass and Kotur 2013).…”

Environmental nitrate signals through abscisic acid in the root tip

“…26,27 Two of these transporters, AtNPF6.3 and AtNRT2.1, have also been proposed to function as receptors, thus earning the name of "transceptor". [28][29][30][31] The coupling of transport to sensing allows the cell to have an accurate perception of the amount of nitrate it is acquiring; once nitrate enters the cytoplasm, it can either be moved to the vacuole, exported from the cell or rapidly converted to other forms of inorganic nitrogen and subsequently assimilated, making accurate assessment of nitrate pools in the cytoplasm problematic. 20,32 The strongest case can be made for AtNPF6.3 being a transceptor, with a nitrate sensing role that can be uncoupled from transport.…”

“…20,32 The strongest case can be made for AtNPF6.3 being a transceptor, with a nitrate sensing role that can be uncoupled from transport. 30 Nitrate activation of ABA signaling by release of bioactive ABA from inactive stores Once nitrate is increased in the environment of the root, it triggers the gradual accumulation of ABA in the root tip, stimulating ABA signaling, and ultimately regulating first nitrate metabolism, then uptake ( Fig. 1).…”

“…As levels of nitrate (NO 3 ) rise in the environment, nitrate is actively transported into the root cell via the dual-affinity AtNPF6.3 (AtNRT1.1) transporter, which also functions as a nitrate sensor. 30 Transport of nitrate through AtNPF6.3 competitively inhibits transport of its other ligand, auxin (IAA), into the cell. 62 Once inside the cell, nitrate is converted into ammonium (NH 4 C ), by the sequential action of the nitrate reductase and nitrite reductase enzymes, encoded by NIA1 and NIA2, and NIR1, respectively.…”