Wavelength-dependent induction of a mycosporine-like amino acid in a rice-field cyanobacterium, Nostoc commune: role of inhibitors and salt stress

Abstract

Wavelength-dependent induction of a mycosporine-like amino acid (MAA) was studied in a nitrogen-fixing rice-field cyanobacterium, Nostoc commune. HPLC studies showed the presence of shinorine, a bisubstituted MAA containing both glycine and serine groups and having an absorption maximum at 334 nm. Exposure of cultures to simulated solar radiation in combination with various cut-off filters (WG 280, 295, 305, 320, 335, 345, GG 400, 420, 455, 475, OG 515, 530, 570, RG 645, 665; all Schott filter series) clearly indicated that MAAs were induced by UV-B radiation, while UV-A and PAR had very little effect on MAA induction in this organism. The ratio of the absorption at 334 nm (shinorine) to 665 nm (chlorophyll a) and the derived action spectrum also revealed the induction of MAAs to be UV-B dependent with a prominent peak at 290 nm and a second small peak at 310 nm. Various concentrations (50–300 mM) of NaCl were used to test whether another common stress factor, such as osmotic stress, also induces MAAs, as has been reported for other cyanobacterial species. The results indicate that cells grown at high concentration of NaCl but without UV-B did not show any MAA induction. In order to elucidate the possible photoreceptors, the effects of various inhibitors/quenchers on the induction of MAAs were studied. There was a marked reduction in the amount of MAA when the cells were irradiated with UV-B in the presence of inhibitors of the shikimate pathway (glyphosate, 1 mM), photosynthesis [3-(3,4-dichlorophenyl)-1,1-dimethylurea, 20 µM], protein synthesis (chloramphenicol, 25 µg ml⁻¹), pterin synthesis (N-acetylserotonin, 5 mM, and 2,4-diamino-6-hydroxypyrimidine, 5 mM) and a quencher of the excited state of flavins and pterins (phenylacetic acid, 1 mM).