Abstract
Nitrate is an important source of nitrogen and also acts as a signaling molecule to trigger numerous physiological, growth, and developmental processes throughout the life of the plant. Many nitrate transporters, transcription factors, and protein kinases participate in the regulation of nitrate signaling. Here, we identified a gene encoding the chrysanthemum calcineurin B-like interacting protein kinase CmCIPK23, which participates in nitrate signaling pathways. In Arabidopsis, overexpression of CmCIPK23 significantly decreased lateral root number and length and primary root length compared to the WT when grown on modified Murashige and Skoog medium with KNO3 as the sole nitrogen source (modified MS). The expression of nitrate-responsive genes differed significantly between CmCIPK23-overexpressing Arabidopsis (CmCIPK23-OE) and the WT after nitrate treatment. Nitrate content was significantly lower in CmCIPK23-OE roots, which may have resulted from reduced nitrate uptake at high external nitrate concentrations (≥ 1 mM). Nitrate reductase activity and the expression of nitrate reductase and glutamine synthase genes were lower in CmCIPK23-OE roots. We also found that CmCIPK23 interacted with the transcription factor CmTGA1, whose Arabidopsis homolog regulates the nitrate response. We inferred that CmCIPK23 overexpression influences root development on modified MS medium, as well as root nitrate uptake and assimilation at high external nitrate supply. These findings offer new perspectives on the mechanisms by which the chrysanthemum CBL interacting protein kinase CmCIPK23 influences nitrate signaling.
Highlights
Nitrogen (N) is one of the most important macronutrients for plant growth and d evelopment[1]; it is integral to the structure of key cellular macromolecules and participates in many plant physiological processes[2]
Previous studies have shown that Arabidopsis CIPK23 is a central component of nitrate signaling pathways[31,62] and inhibits ammonium transport[33,64]
We found that the nitrate-induced transcript levels of the primary nitrate response (PNR) genes AtNIA1, AtNIR, AtHHO1, and AtHRS1 differed significantly between the CmCIPK23-OE lines and the wild type (WT) (Fig. 3)
Summary
Nitrogen (N) is one of the most important macronutrients for plant growth and d evelopment[1]; it is integral to the structure of key cellular macromolecules and participates in many plant physiological processes[2]. CIPK8 positively regulates the nitrate-induced expression of primary nitrate response (PNR) genes, including nitrate transporter genes and genes involved in nitrate assimilation. The primary root of Arabidopsis cipk[8] mutants was longer when nitrate was either the sole nitrogen source or was applied together with ammonium (as N H4NO3)[28]. A previous study demonstrated that the Arabidopsis cipk[23] mutant had greater PR and LR lengths and more LRs than the wild type (WT) after 10 days (d) of growth on Hoagland’s medium containing 2 mM KNO333. Ho et al.[31] demonstrated that AtCIPK23 plays a negative role in the PNR: following nitrate exposure, upregulation of the nitrate responsive gene AtNRT2.1 was higher in cipk[23] mutants than in the wild type. A few important nitrate regulatory genes that act in the PNR have been identified in Arabidopsis. Chen et al.[43] found that CmCLCa plays an important role in NO3− storage in leaf vacuoles and is a candidate gene for the improvement of chrysanthemum N-starvation tolerance
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