AtCIPK8, a CBL‐interacting protein kinase, regulates the low‐affinity phase of the primary nitrate response

Abstract

SummaryNitrate, the major nitrogen source for most plants, is not only a nutrient but also a signaling molecule. For almost two decades, it has been known that nitrate can rapidly induce transcriptional expression of several nitrate-related genes, a process that is referred to as the primary nitrate response. However, little is known about how plants actually sense nitrate and how the signal is transmitted in this pathway. In this study, a calcineurin B-like (CBL) -interacting protein kinase (CIPK) gene, CIPK8, was found to be involved in early nitrate signaling. CIPK8 expression was rapidly induced by nitrate. Analysis of two independent knockout mutants and a complemented line showed that CIPK8 positively regulates the nitrate-induced expression of primary nitrate response genes, including nitrate transporter genes and genes required for assimilation. Kinetic analysis of nitrate induction levels of these genes in wild-type plants indicated that there are two response phases: a high-affinity phase with a K m of approximately 30 lM and a low-affinity phase with a K m of approximately 0.9 mM. As cipk8 mutants were defective mainly in the low-affinity response, the high-affinity and low-affinity nitrate signaling systems are proposed to be genetically distinct, with CIPK8 involved in the low-affinity system. In addition, CIPK8 was found to be involved in long-term nitrate-modulated primary root growth and nitrate-modulated expression of a vacuolar malate transporter. Taken together, our results indicate that CBL-CIPK networks are responsible not only for stress responses and potassium shortage, but also for nitrate sensing.

Nuclear retention of the transcription factor NLP7 orchestrates the early response to nitrate in plants

“…RNA was extracted with Trizol method (Invitrogen). First-strand cDNAs were synthesized using Moloney murine leukaemia virus reverse transcriptase and oligo(dT) 15 primers (Promega) 38,39 . PCR was performed as for ChIP-PCR.…”

Reciprocal regulation between AtNRT2.1 and AtAMT1.1 expression and the kinetics of NH4+ and NO3− influxes

“…Recently, we have advanced in molecular knowledge of N sensing in plants, such as the dual function of the nitrate transport CHL1 (Muños et al, 2004;Ho et al, 2009;Wang et al, 2009), the roles of different transcription factors (Rubin et al, 2009;Wang et al, 2009) or of the protein kinases (Ho et al, 2009;Hu et al, 2009). Proof of the complexity of the process can be obtained when observing interactions among different genes.…”

Arabidopsis NIN-like transcription factors have a central role in nitrate signalling

“…Another Arabidopsis gene, GNC, has also been identified as a nitrateinducible gene encoding a putative transcription factor of the GATA family 22 . In addition to these putative transcription factors, two Arabidopsis proteins, NRT1.1 dual-affinity nitrate transporter and CIPK8 protein kinase, have also been suggested to have roles in nitrate response as a nitrate sensor 23 and a regulator in the presence of high concentrations of nitrate 24 , respectively. Although an increasing number of factors involved in nitrate signalling and response has been identified, the key factor in the mechanism for nitrate response, namely the transcription factor that primarily responds to nitrate signalling, has not yet been identified.…”