Exploring new insights into the role of nitric oxide synthase in nitrogen metabolism among photosynthetic organisms

CORREA-ARAGUNDE N; DEL CASTELLO F; NEJAMKIN A; ECIOLAZA MAGDALENA; DE MARCO A; CALO G; MARTÍNEZ NOËL G; FORESI N

Congreso; NO meeting 2024; 2024

Nitric oxide synthase (NOS) plays a crucial role in catalyzing the conversion of L-arginine into L-citrulline and nitric oxide (NO). Existing evidence suggests the absence of NOS in higher plants; however, they have been identified in numerous photosynthetic microorganisms. In our laboratory, we are investigating the functionality of the NOS in the cyanobacterium Synechococcus PCC 7335 (syNOS) and the algae Ostreococcus tauri (otNOS). The objective of our research is to assess whether NOS is involved in primary nitrogen metabolism, given that arginine serves as a major organic nitrogen source. Our findings indicate that O. tauri can sustain a consistent growth rate when L-arginine is the sole nitrogen source, despite lacking conventional pathways for metabolizing this amino acid into assimilable nitrogen. The transcript level of genes involved in nitrogen uptake and metabolism were increased in nitrogen-starved condition while the addition of L- Arg reduced their induction. Notably, levels of NO increased in O. tauri cells growing in L-arginine, suggesting the potential involvement of the otNOS enzyme in L-arginine metabolism. Conversely, syNOS exhibits an additional domain, distinct from the classical oxygenase and reductase, encoding for a globin. Consequently, syNOS can facilitate the oxidation of Arg to citrulline and NO, followed by the further oxidation of NO to nitrate. To decipher the function of syNOS, we expressed the syNOS protein in Synechococcus PCC 7942, a strain lacking NOS. Preliminary results indicate that syNOS expression enhances bacterial growth under nitrogen scarcity. Our working hypothesis posits that syNOS expression contribute to degrade internal arginine pools dealing with nitrogen starvation. An early indicator of nitrogen deficiency is the rapid degradation of phycobiliproteins in cyanobacteria. The expression of syNOS in S. PCC 7942 accelerates this process, suggesting that this strain may perceive nitrogen deficiency more rapidly. Our current results strongly imply the involvement of NOSs in the growth and primary nitrogen metabolism of photosynthetic microorganisms. These findings encourage us to evaluate the effect of NOS expression in higher plants. Transgenic Arabidopsis and potato lines expressing syNOS display higher yield respect to wild type plants in growing either higher or low nutrient conditions. The metabolic profile of nitrogen organic compounds, including proline and serine, experienced a notable decrease in wild-type plants under nitrogen deficiency. In contrast, these compounds showed relatively little change in potato transgenic lines under the same conditions. In summary, this research expands our understanding of nitrogen recycling and metabolism in alga and cyanobacteria and holds potential applications for enhancing nitrogen use efficiency in higher plants. Supported by the Agencia Nacional de Promoción Científica y Tecnológica (AGENCIA), Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Universidad de Mar del Plata (UNMdP), Argentina.