Nitrate Signaling and Its Role in Regulating Flowering Time in Arabidopsis thaliana

Abstract

Plant growth is coordinated with the availability of nutrients that ensure its development. Nitrate is a major source of nitrogen (N), an essential macronutrient for plant growth. It also acts as a signaling molecule to modulate gene expression, metabolism, and a variety of physiological processes. Recently, it has become evident that the calcium signal appears to be part of the nitrate signaling pathway. New key players have been discovered and described in Arabidopsis thaliana (Arabidopsis). In addition, knowledge of the molecular mechanisms of how N signaling affects growth and development, such as the nitrate control of the flowering process, is increasing rapidly. Here, we review recent advances in the identification of new components involved in nitrate signal transduction, summarize newly identified mechanisms of nitrate signaling-modulated flowering time in Arabidopsis, and suggest emerging concepts and existing open questions that will hopefully be informative for further discoveries.

Keywords: Arabidopsis; NLP7; calcium signaling; flowering time; nitrate signaling.

Figure 1

The nitrate signaling pathway in Arabidopsis thaliana. At elevated nitrate levels, it leads to the activation of calcium−channel activity of the NRT1.1−CNGC15 complex, resulting in a rapid nitrate−induced influx of Ca²⁺ into the cytoplasm. As cytosolic Ca²⁺ levels increase, the calcium−dependent kinases CPK10, CPK30, and CPK32 promote the phosphorylation of NLP7/NLP6. This triggers the nuclear translocation of NLPs. Meanwhile, NLP7 senses nitrate directly in the cytoplasm. Once nitrate binds to NLP7, it causes NLP7 to undergo a conformational change and to be transcriptionally derepressed. This occurs simultaneously and synergistically with the phosphorylation process. In the nucleus, NLP6 and NLP7 can bind directly to nitrate−responsive cis−elements (NRE) in the promoter regions of PNR genes and activate nitrate−responsive transcription. Abbreviations: NO₃⁻, nitrate; Ca²⁺, calcium; P, phosphorylation; NRT1.1, NITRATE TRANSPORTER 1.1; CNGC15, CYCLIC NUCLEOTIDE-GATED CHANNEL 15; CPK10, CALCIUM-DEPENDENT PROTEIN KINASE 1; CPK30, CALCIUM-DEPENDENT PROTEIN KINASE 30; CPK32, CALCIUM-DEPENDENT PROTEIN KINASE 32; NLP7, NIN LIKE PROTEIN 7; NLP6, NIN LIKE PROTEIN 6; NRE, nitrate-responsive cis-elements; PNR, primary nitrate response.

Figure 2

Nitrate signaling regulates flowering time in Arabidopsis thaliana. N signaling as an input to the central circadian clock, by modulating the cellular CRY1 protein abundance, affects the expression of key components of the central oscillator. Subsequently, as a circadian output, the expression of some flowering genes in the photoperiod pathway, such as CO, is altered, and this affects flowering time. Additionally, the phosphorylation state of the FBH4 protein can be modulated by N conditions. Under low N conditions, FBH4 phosphorylation levels decrease, promoting FBH4 nuclear localization and transcriptional activation of the direct target CO and downstream florigen FT to accelerate flowering. In another pathway, nitrate availability controls bolting and flowering time by modulating SMZ and SNZ gene expression via the GA pathway. In the third pathway, nitrate signaling directly integrates into the age pathway in a tissue-specific manner. NLP7 and NLP6 directly regulate SPL3 and SPL5 at the transcriptional level in the SAM. SOC1 is considered the main flowering integrator involved in this pathway. Abbreviations: P, phosphorylation; FNR1, FERREDOXIN-NADP (+) OXIDOREDUCTASE 1; CRY1, cryptochrome blue-light receptor 1; CCA1, CIRCADIAN CLOCK ASSOCIATED 1; LHY, LATE ELONGATED HYPOCOTYL 1; TOC1, TIMING OF CAB EXPRESSION 1; CO, CONSTANS; FT, FLOWERING LOCUS T; FBH4, FLOWERING BHLH 4; GNC, GATA TRANSCRIPTION FACTOR 21; GNL/CGA1, GATA TRANSCRIPTION FACTOR 22; SMZ, SCHLAFMUTZE; SNZ, SCHNARCHZAPFEN; NLP6, NIN LIKE PROTEIN 6; NLP7, NIN LIKE PROTEIN 7; SPL3, SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 3; SLP5, SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 5.