Abstract
In Arabidopsis (Arabidopsis thaliana), the NRT2.1 gene codes for the main component of the root nitrate (NO(3)(-)) high-affinity transport system (HATS). Due to the strong correlation generally found between high-affinity root NO(3)(-) influx and NRT2.1 mRNA level, it has been postulated that transcriptional regulation of NRT2.1 is a key mechanism for modulation of the HATS activity. However, this hypothesis has never been demonstrated, and is challenged by studies suggesting the occurrence of posttranscriptional regulation at the NRT2.1 protein level. To unambiguously clarify the respective roles of transcriptional and posttranscriptional regulations of NRT2.1, we generated transgenic lines expressing a functional 35S::NRT2.1 transgene in an atnrt2.1 mutant background. Despite a high and constitutive NRT2.1 transcript accumulation in the roots, the HATS activity was still down-regulated in the 35S::NRT2.1 transformants in response to repressive nitrogen or dark treatments that strongly reduce NRT2.1 transcription and NO(3)(-) HATS activity in the wild type. In some treatments, this was associated with a decline of NRT2.1 protein abundance, indicating posttranscriptional regulation of NRT2.1. However, in other instances, NRT2.1 protein level remained constant. Changes in abundance of NAR2.1, a partner protein of NRT2.1, closely followed those of NRT2.1, and thus could not explain the close-to-normal regulation of the HATS in the 35S::NRT2.1 transformants. Even if in certain conditions the transcriptional regulation of NRT2.1 contributes to a limited extent to the control of the HATS, we conclude from this study that posttranscriptional regulation of NRT2.1 and/or NAR2.1 plays a predominant role in the control of the NO(3)(-) HATS in Arabidopsis.
Highlights
MRNA level, it has been postulated that transcriptional regulation of NRT2.1 is a key mechanism for modulation of the HATS activity
Even if in certain conditions the transcriptional regulation of NRT2.1 contributes to a limited extent to the control of the HATS, we conclude from this study that posttranscriptional regulation of NRT2.1 and/or NAR2.1 plays a predominant role in the control of the NO32 HATS in Arabidopsis
These data support the previous proposal that posttranscriptional regulatory mechanisms participate in the control of the HATS, because expression of a 35S::NpNRT2.1 or RolD:: NpNRT2.1 transgene in N. plumbaginifolia did not prevent down-regulation of the HATS activity by high NH4NO3 supply (Fraisier et al, 2000)
Summary
MRNA level, it has been postulated that transcriptional regulation of NRT2.1 is a key mechanism for modulation of the HATS activity. Many studies have shown that regulation of the HATS is always highly correlated with changes in NRT2.1 transcript accumulation in the roots In more detail, both NO32 HATS activity and NRT2.1 expression are induced by NO32 (Filleur and Daniel-Vedele, 1999; Lejay et al, 1999; Zhuo et al, 1999), repressed by high N supply to the plant (Lejay et al, 1999; Zhuo et al, 1999; Cerezo et al, 2001; Gansel et al, 2001), repressed by darkness (Lejay et al, 1999), and stimulated by sugar supply to the roots (Lejay et al, 2003, 2008). Many studies have focused on the identification of regulatory proteins governing NRT2.1 gene expression (Munos et al, 2004; Krouk et al, 2006; Castaings et al, 2009; Ho et al, 2009; Wang et al, 2009; Girin et al, 2010; Widiez et al, 2011)
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