Abstract
HY5 (Long Hypocotyles 5) is a key transcription factor in Arabidopsis thaliana that has a pivotal role in seedling development. Soil nitrogen is an essential macronutrient, and its uptake, assimilation and metabolism are influenced by nutrient availability and by lights. To understand the role of HY5 in nitrogen assimilation pathways, we examined the phenotype as well as the expression of selected nitrogen assimilation-related genes in hy5 mutant grown under various nitrogen limiting and nitrogen sufficient conditions, or different light conditions. We report that HY5 positively regulates nitrite reductase gene NIR1 and negatively regulates the ammonium transporter gene AMT1;2 under all nitrogen and light conditions tested, while it affects several other genes in a nitrogen supply-dependent manner. HY5 is not required for light induction of NIR1, AMT1;2 and NIA genes, but it is necessary for high level expression of NIR1 and NIA under optimal nutrient and light conditions. In addition, nitrogen deficiency exacerbates the abnormal root system of hy5. Together, our results suggest that HY5 exhibits the growth-promoting activity only when sufficient nutrients, including lights, are provided, and that HY5 has a complex involvement in nitrogen acquisition and metabolism in Arabidopsis seedlings.
Highlights
HY5 (Long Hypocotyles 5) is a nuclear bZIP type of transcription factor in Arabidopsis thaliana that has been extensively studied for its role in photomorphogenesis [1]
We found that HY5 regulates many nitrogen related genes in a nitrogen concentration dependent manner, and that it constitutively activates NIR1 (Nitrite reductase 1) while suppresses AMT1;2 (Ammonium transporter 1;2), two important genes in nitrogen metabolism
We studied the role of HY5 in nitrogen assimilation pathways by characterizing the hy5 mutant seedlings with respect to its nitrogen-dependent phenotype and the expression of selected genes related to nitrogen assimilation
Summary
HY5 (Long Hypocotyles 5) is a nuclear bZIP type of transcription factor in Arabidopsis thaliana that has been extensively studied for its role in photomorphogenesis [1]. Plants absorb inorganic nitrogen mainly in two forms, nitrate (NO3−) and ammonium (NH4+), and their nitrogen metabolism is dynamically regulated in response to ambient nitrogen sources and levels as well as other environmental factors [10,11]. Higher plants have both high- and low-affinity nitrate uptake systems (HATS and LATS, respectively), which operate under different nitrate concentrations and are thought to be genetically distinct [12]. Light signals play a crucial role in regulating nitrogen uptake, translocation and assimilation into organic compounds [15]. The rates of photosynthesis and respiration are known to vary as a function of tissue nitrogen concentration in various species and growth conditions [16,17]
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