Effects of salt stress on the expression of key genes related to nitrogen assimilation and transport in the roots of the cultivated tomato and its wild salt-tolerant relative

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Inorganic nitrogen is a key element for plant growth under salt stress. A comparative study including physiological responses, ion content, transcript regulation of ammonium/nitrate transporters (AMTs/NRTs) as well as key enzymes for nitrogen assimilation was undertaken in wild salt-tolerant tomatoes (Solanum pennellii) and cultivated tomatoes (Solanum lycopersicon) exposed to 100mM NaCl for 1 and 7days. In comparison to S. lycopersicon, S. pennellii was more salt tolerant as evidenced by its higher survival rate, lower biomass reduction, and less salt injury (reduced electrolyte leakage and proline accumulation). In root tissues of both species, salt exposure (7days) reduced the mRNA expression levels of low affinity nitrate transporters (NRT1.1 and NRT1.2). This was associated with a decline in both nitrate content and expression level of the nitrate reductase gene (NR). Salt-stressed root tissues of S. pennellii showed relatively higher mRNA expression of the high affinity ammonium transporters (AMT1.1 and AMT1.2) compared to S. lycopersicon. The root ammonium content was increased only in S. lycopersicon going hand in hand with a reduction in mRNA level of cytosolic glutamine synthetase (GS1) after 7days of salt stress, whereas the expression level of GS1 was unchanged in S. pennellii, suggesting a lower salt-induced inhibition in ammonium assimilation in this species. Our comparative study demonstrated that the salt-tolerant and salt-sensitive tomato species show differential contribution of the nitrogen transporters and key genes associated with nitrogen assimilation under salt stress. While the reduction in the expression of key components of NO3− uptake (NRT1.1, NRT1.2) and assimilation (NR gene) in both species, likely contributed to the reduction in plant growth under salt stress, the observed salt tolerance for S. pennellii was associated with relative higher mRNA expression of ammonium uptake and assimilation genes. These results provide crucial knowledge for tomato breeding employing salt-tolerant wild species in salt-induced nitrogen- deficient environments.