Characterization of Nucleobase Cation Symporter 1 (NCS1) Localized to the Plastids of Nicotiana tabacum.

Jasmonate-mediated defense responses redirect plant resources from growth to defense, imposing significant demands on the nitrogen economy to produce nitrogenous compounds, such as alkaloids. However, the mechanisms by which plants maintain essential central metabolic processes, such as nucleotide synthesis, under nitrogen resource competition remain unclear. Therefore, in this study, we characterized the nucleobase cation symporter NtNCS1 in Nicotiana tabacum. Notably, NtNCS1 mRNA exhibited opposing organ-specific regulation by jasmonic acid, with induction in roots (the site of nicotine synthesis) but repression in leaves. NtNCS1 was localized to the plastid envelope in metabolically active cells, including mesophyll, phloem-associated, and, notably, guard cells. This dual regulation suggests that NtNCS1 plays a key role in reallocating nitrogen resources. In roots, its expression possibly contributes to maintaining the essential nucleotide pools for basal metabolism when nitrogen is diverted to alkaloid production. Conversely, its repression in leaves possibly conserves nitrogen by downregulating growth, thereby reallocating resources to defense. Overall, this study highlights the potential roles of NCS1 transporters in safeguarding central metabolism and mediating the plant growth-defense trade-off.