C cortex, M medulla, PL photobiont layer, Pho photobiont, Hy fungal hyphae Air oxidation of NO in an aqueous environment results in the near exclusive generation of NO2 -, which is further oxidized to NO3 = [23]. NO end-products (NOx) were quantified by the classical method of Griess. NOx levels see more increased over 2 h to reach a maximum (Figure 4C). By 4 h, NOx levels had decreased to slightly below the initial levels, reaching a minimum, after which the levels remained constant for up to 24 h.

Effect of NO scavenging during lichen rehydration on ROS production, chlorophyll autofluorescence and lipid peroxidation To study the role of NO during rehydration, R. farinacea thalli were rehydrated with 200 μM of the membrane-permeable compound 3-Methyladenine supplier c-PTIO, which specifically reacts with NO to inhibit its biological actions. NO scavenging with c-PTIO completely suppressed DAN fluorescence emission (image not shown). It also produced a remarkable increase in ROS production

in both the cortex and the medulla (Figure 2F). The confocal laser beam produced an oxidative burst in the photobionts, leading to chlorophyll photo-oxidation and DCF fluorescence onset within seconds (Figure 2F). The kinetics study (Figure 3B, solid triangles) confirmed that NO inhibition during rehydration multiplies the levels of intracellular free radicals at 0 min (52.1 ± 2.85 versus 18.4 ± 1.67 a.u.). Moreover, check details inhibition of NO eliminates the initial exponential phase of free radical production seen during physiological rehydration of thalli (Figure 3B, solid squares). Chlorophyll autofluorescence was simultaneously measured and no evident differences between physiological and NO-inhibited rehydration could be observed (Figure 3C, solid triangles). However, NO inhibition in 24h-hydrated Myosin thalli resulted in an important decrease in chlorophyll autofluorescence that tends to recover normal values after 1 h (Figure 3D, solid triangles). Lipid peroxidation during NO-specific inhibition with c-PTIO was measured quantitatively; the results are presented in Figure 4B. MDA levels reach a maximum at 2 h and

a minimum at 4 h. The MDA levels measured following rehydration with cPTIO were the opposite of those obtained under physiological conditions. Figure 4D shows that, overall, NO end-products decreased in amount when c-PTIO was used. Microscopy studies of isolated algae Confocal studies clearly showed that NO deprivation caused photo-oxidative damage in the photobiont (Figure 2F). NO is known to reduce photo-oxidative stress in some species of green algae. A specific role for NO in the prevention of photo-oxidation in Trebouxia algae was confirmed in the following studies. A suspension of axenically cultured Trebouxia sp., the photobiont isolated from R. farinacea, was treated with 200 μM c-PTIO in the presence of both DCFH2-DA and DAN. The images of control cells are presented in Figure 6A.