Abstract
Although many members encoding different ammonium- and nitrate-transporters (AMTs, NRTs) were identified and functionally characterized from several plant species, little is known about molecular components for - and acquisition/transport in tobacco, which is often used as a plant model for biological studies besides its agricultural and industrial interest. We reported here the first molecular identification in tobacco (Nicotiana tabacum) of nine AMTs and four NRTs, which are respectively divided into four (AMT1/2/3/4) and two (NRT1/2) clusters and whose functionalities were preliminarily evidenced by heterologous functional-complementation in yeast or Arabidopsis. Tissue-specific transcriptional profiling by qPCR revealed that NtAMT1.1/NRT1.1 mRNA occurred widely in leaves, flower organs and roots; only NtAMT1.1/1.3/2.1NRT1.2/2.2 were strongly transcribed in the aged leaves, implying their dominant roles in N-remobilization from source/senescent tissues. N-dependent expression analysis showed a marked upregulation of NtAMT1.1 in the roots by N-starvation and resupply with N including , suggesting a predominant action of NtAMT1.1 in uptake/transport whenever required. The obvious leaf-expression of other NtAMTs e.g., AMT1.2 responsive to N indicates a major place, where they may play transport roles associated with plant N-status and (-)N movement within aerial-parts. The preferentially root-specific transcription of NtNRT1.1/1.2/2.1 responsive to N argues their importance for root uptake and even sensing in root systems. Moreover, of all NtAMTs/NRTs, only NtAMT1.1/NRT1.1/1.2 showed their root-expression alteration in a typical diurnal-oscillation pattern, reflecting likely their significant roles in root N-acquisition regulated by internal N-demand influenced by diurnal-dependent assimilation and translocation of carbohydrates from shoots. This suggestion could be supported at least in part by sucrose- and MSX-affected transcriptional-regulation of NtNRT1.1/1.2. Thus, present data provide valuable molecular bases for the existence of AMTs/NRTs in tobacco, promoting a deeper understanding of their biological functions.
Highlights
Ammonium (NH+4 ) and nitrate (NO−3 ) are principal soil nitrogen (N) sources available to plants
To explore molecular bases of NH+4 and NO−3 movement in tobacco, sequences of Arabidopsis, rice and poplar ammoniumand nitrate-transporters (e.g., AtAMT1.1, AtAMT2.1, OsAMT3.1, PtrAMT4.1, AtNRT1.1, and AtNRT2.1) were used as references to search for homologous sequences in tobacco database at Sol Genomics Network using an E-value cutoff of 1e-5
Regarding tobacco NRTs, NtNRT1 and 2 possess predicted transmembrane domains (TMD), especially for NtNRT1.1 and 1.2 with a relatively large hydrophilic-loop located between TMD6 and TMD7 (Figure S1), similar to their orthologs from Arabidopsis (Tsay et al, 2007)
Summary
Ammonium (NH+4 ) and nitrate (NO−3 ) are principal soil nitrogen (N) sources available to plants. As for the movement of N into and within the plant, the activity of different transport systems for NO−3 and NH+4 was extensively investigated (Nacry et al, 2013). NO−3 /NH+4 uptake/transport and assimilation were proved to be tightly controlled by the concentration of their substrates and/or whole-plant signal(s) of N status (Nacry et al, 2013). The acquisition of NO−3 /NH+4 by the plant can be impacted by the photosynthesis, and displays a diurnal-rhythm pattern that is attributed to the regulation by shoot-to-root transport of carbohydrates (O’Brien et al, 2016). A recent study demonstrated that a transcription factor HY5 many serve as a shoot-to-root signal to induce AtNRT2.1 function in roots in response to light irradiation, enabling homeostatic maintenance of carbon (C)-N balance in varied light environments (Chen et al, 2016)
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