Nitric oxide synthase, Arginine and NO: are they involved in responses to nitrogen deficiency in Ostreococcus tauri?

CALÓ GONZALO; MARTINEZ-NOEL GISELLE; CORREA ARAGUNDE NATALIA; FORESI NOELIA; NEJAMKIN ANDRES; LAMATTINA LORENZO; DEL CASTELLO FIORELLA; SALERNO GRACIELA

Szeged

Simposio; 8 th plant nitric oxide international meeting; 2021

Nitric oxide (NO) is a key signaling molecule in plant cell physiology. However, its production and function in photosynthetic organisms remains partially unsolved. The best characterized NO production pathway involves the reduction of nitrite to NO through different enzymatic or non-enzymatic mechanisms. Besides this reductive pathway, several reports acknowledge the possible existence of an arginine-dependent oxidative pathway, similar to that described in animals and relying in NO synthase (NOS) activity. However, neither proteins, cDNA nor homologous gene sequences to NOS have been found in the embryophytes so far. Our research group characterized a NOS enzyme in the photosynthetic microorganism, Ostreococcus tauri (OtNOS). This green marine unicellular alga is a good model for the study of physiological and molecular aspects of the NOS expression in photosynthetic microorganisms. In this study, we analyze the effect of nitrogen (N) deficiency on O. tauri growth and the utilization of the NOS substrate Arginine (L- Arg) as source of N. We have grown Ostreococcus cells in different culture media: i) N-complete medium (0.95 mM), ii) N-depleted medium (0 mM) and iii) N-depleted medium supplemented with 0.1 mM L-Arg. Results showed that N deficiency triggers a growth fast inhibition and a decrease of chlorophyll content decrease in the first days of treatment. On the other hand, L-Arg supplementation allows this culture growth at similar rates to those of N-completed medium. Regarding chlorophyll levels in O. tauri growing in L-Arg as the unique N source, it remains constant up to 10 days at the same levels of N-complete medium while then decreases. Interestingly, the NOS inhibitor L-NAME blocks the effect of L-Arg showing a cell growth inhibition similar to the N-deficient condition. These data strongly suggest that NOS activity could be involved in cell N metabolism, at least under poor N availability. Besides, results indicate that NO level increases under both N-depleted and L-Arg supplemented mediums. All together, these results provide evidences that NO has a role in N-deficiency response in O. tauri and that L-Arg may overcome the lack of this nutrient. The pool of enzymes involved in Arg degradation and the putative action of NOS-derived NO within cell N metabolism will be discussed.