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

FORESI N; CALO G; DEL CASTELLO F; NEJAMKIN A; MARTÍNEZ NOËL G; SALERNO G; LAMATTINA L; CORREA-ARAGUNDE N

Congreso; 8th Plant Nitric Oxide International Meeting (NO82020); 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, nor transcripts, nor gene sequences homologous 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 in the first days of treatment. On the other hand, L-Arg supplementation allows this culture grow at similar rates to those of N-complete medium. Regarding the chlorophyll level, it remains constant up to 10 days in O. tauri cells growing in L-Arg as the unique N source (similar to cells in N-complete medium), and 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 media. Altogether, these results provide evidence that NO has a role in N-deficiency response in O. tauri and that L-Arg may overcome the lack of this nutrient. The role of enzymes involved in Arg degradation and the putative action of NOS-derived NO within cell N metabolism will be discussed.