Role of NO in modulating UV-B stress responses in the cyanobacterium Synechococcus PCC 7335

FERNÁNDEZ, MARÍA BELÉN; DELUCA, IARA; LATORRE, LUCAS; CORREA-ARAGUNDE, NATALIA; CASSIA, RAÚL

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Conferencia; PNO9; 2024; 2024

National Institue of Plant Genome Research

Nitric oxide (NO) is a highly diffusible antioxidant and signal molecule that modulates physiological responses in living organisms. NO can be produced by enzymatic (nitric oxide synthase, NOS, and nitrate reductase, NR) or non- enzymatic pathways. NO effects have been extensively studied in plants during growth, development, and stress responses including ultraviolet radiation (UV-B). UV-B is the part of the solar spectrum that ranges from 280 to 315nm. Although most of it is absorbed by the ozone layer, about 5% reaches Earth´s surface and induces photomorphogenesis or damage responses in terrestrial and aquatic organisms. In plants, NO is mainly produced by NR and triggers signaling responses, including tolerance to UV-B exposure. In the cyanobacteria Nostoc muscurum and Anabaena sp. pharmacological studies show that NOS- produced NO protects cell cultures from UV-B exposure. Nevertheless, this represents the sole report connecting NO and UV-B in these organisms. Synechococcus PCC 7335 is a marine free-living cyanobacterium with several adaptations to light fluctuations, such as complementary chromatic adaptation and far-red light photoacclimation. It also encodes a photolyase operon, suggesting high resistance to UV-B through coordinated regulation of enzymes involved in the repair of UV-B-damaged DNA. S. PCC 7335 present a non- canonical NOS, which produces both NO and nitrate using arginine as substrate. The aim of this work was to analyze the effect of UV-B on S. PCC 7335 cultures and the role of NO in this response. To this end, physiological and molecular studies were performed. Flow cytometry analysis showed that exposition to 4 hours of UV-B (3.4 W.m-2) for 3 days did not cause cell death of the culture indicating that S. PCC 7335 is highly resistant to this stress. NO quantification demonstrated a continuously increase during 4 hours of UV-B exposure in S. PCC 7335. Treatment with tungstate, a non- specific inhibitor of NR, did not affect the raise of NO levels upon UV-B irradiation, suggesting that NR is no implicated in this process. Also, the rate of NO production with the addition of arginine was the same regardless of UV-B treatment. Moreover, RT-qPCR analysis showed a decrease in transcript levels of NOS and NR during UV-B compared to control condition. In addition, cyclobutane pyrimidine dimers in DNA generated by UV-B irradiation were not repaired after one hour of culture recovery in white light, despite an increase in transcript levels of photolyases operon. The upregulation of this operon by UV-B was not regulated by endogenous NO. Pigment quantification revealed a decrease in phycobiliproteins upon UV-B stress, which was not ameliorated by the addition of arginine before the irradiation treatment. Furthermore, UV-B promoted the dispersal of S. PCC 7335 biofilm, which was reversed by the scavenging of endogenous NO. Consistently, NO donors (such as GSNO and SNP) also facilitated biofilm dispersal. In summary, the results indicate that UV-B- induced NO is produced by non- enzymatic pathways, do not improve pigment degradation, nor participate in photolyase upregulation, however, is involved in biofilm dispersal as a strategy to avoid stressful environmental conditions.