Endogenous NO Is Involved in Dissimilar Responses to Rehydration and Pb(NO3)2 in Ramalina farinacea Thalli and Its Isolated Phycobionts.

Lichens undergo desiccation/rehydration cycles and are permeable to heavy metals, which induce free radicals. Nitrogen monoxide (NO) regulates important cellular functions, but the research on lichen NO is still very scarce. In Ramalina farinacea thalli, NO seems to be involved in the peroxidative damage caused by air pollution, antioxidant defence and regulation of lipid peroxidation and photosynthesis. Our hypothesis is that NO also has a critical role during the rehydration and in the responses to lead of its isolated phycobionts (Trebouxia sp. TR9 and Trebouxia jamesii). Therefore, we studied the intracellular reactive oxygen species (ROS) production, lipid peroxidation and chlorophyll autofluorescence during rehydration of thalli and isolated microalgae in the presence of a NO scavenger and Pb(NO3)2. During rehydration, NO scavenging modulates free radical release and chlorophyll autofluorescence but not lipid peroxidation in both thalli and phycobionts. Pb(NO3)2 reduced free radical release (hormetic effect) both in the whole thallus and in microalgae. However, only in TR9, the ROS production, chlorophyll autofluorescence and lipid peroxidation were dependent on NO. In conclusion, Pb hormetic effect seems to depend on NO solely in TR9, while is doubtful for T. jamesii and the whole thalli.

Inhibition of NO Biosynthetic Activities during Rehydration of Ramalina farinacea Lichen Thalli Provokes Increases in Lipid Peroxidation.

Lichens are poikilohydrous symbiotic associations between a fungus, photosynthetic partners, and bacteria. They are tolerant to repeated desiccation/rehydration cycles and adapted to anhydrobiosis. Nitric oxide (NO) is a keystone for stress tolerance of lichens; during lichen rehydration, NO limits free radicals and lipid peroxidation but no data on the mechanisms of its synthesis exist. The aim of this work is to characterize the synthesis of NO in the lichen Ramalina farinacea using inhibitors of nitrate reductase (NR) and nitric oxide synthase (NOS), tungstate, and NG-nitro-L-arginine methyl ester (L-NAME), respectively. Tungstate suppressed the NO level in the lichen and caused an increase in malondialdehyde during rehydration in the hyphae of cortex and in phycobionts, suggesting that a plant-like NR is involved in the NO production. Specific activity of NR in R. farinacea was 91 µU/mg protein, a level comparable to those in the bryophyte Physcomitrella patens and Arabidopsis thaliana. L-NAME treatment did not suppress the NO level in the lichens. On the other hand, NADPH-diaphorase activity cytochemistry showed a possible presence of a NOS-like activity in the microalgae where it is associated with cytoplasmatic vesicles. These data provide initial evidence that NO synthesis in R. farinacea involves NR.