High Light-Induced Nitric Oxide Production Induces Autophagy and Cell Death inChlamydomonas reinhardtii

Autophagy plays a role in regulating important cellular functions in response to stress conditions. The role of nitric oxide (NO) in the regulation of autophagy inChlamydomonas reinhardtiihas been not studied. Illumination ofC. reinhardtiicells under a high light (HL, 1,600 mu mol m(-2)s(-1)) condition induced a NO burst through NO synthase- and nitrate reductase-independent routes, and cell death. The abundance of CrATG8 protein, an autophagy marker ofC. reinhardtii, increased after HL illumination along with a linear increase in the transcript abundance of autophagy-associated genes (CrVPS34, CrATG1, CrATG3, CrATG4, CrATG6, CrATG7, CrATG8, and CrATG12), which were suppressed in the presence of an NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO). The cells were treated with NO donors,S-nitroso-N-acetyl-penicillamine, andS-nitrosoglutathione, under a normal light (50 mu mol m(-2)s(-1)) condition to elucidate the role of NO in autophagy activation and cell death. Treatment with 0.05 mM or 0.1 mM NO donors increased the abundance of ATG8 protein and CrATG transcripts, which were suppressed in the presence of cPTIO. Moreover, treatment with 0.05 mM NO donors did not affect cell viability, while 0.1 mM NO donors elicited a transient decrease in cell growth and death that recovered after 12 h. The transient effect could be prevented by the presence of cPTIO. However, treatment with 1 mM H(2)O(2)and 0.1 mM NO donors enhanced autophagy induction and resulted in cell death after 24 h. The interaction of H(2)O(2)and NO can be prevented by cPTIO treatment. This implies that NO is critical for the interaction of H(2)O(2)and NO that induces cell death and autophagy. Furthermore, exposure to 0.1 mM NO donors under a non-lethal HL condition (750 mu mol m(-2)s(-1)) evoked autophagy and cell death. In conclusion, the present findings demonstrated that the NO-mediated autophagy pathway is activated inC. reinhardtiiunder lethal high intensity illumination and may interact with H(2)O(2)for HL-induced cell death. The relationships between autophagy and cell death are discussed.