Regulatory role of gamma-aminobutyric acid in cyanobacteria challenged with UV-B: The implication of nitric oxide

The present study examined the alleviatory role of gamma-aminobutyric acid (GABA) in UV-B challenged nitrogen fixing cyanobacteria and the significant role of nitric oxide (NO) as a downstream signal in GABA-mediated responses. UV-B (0.48 W m⁻²) stress reduced the growth by 37% and 33% in Anabaena sp. PCC 7120 and Nostoc muscorum ATCC 27893, respectively. The diminished growth under UV-B stress was due to the damaging effects on photosynthetic pigment contents [chlorophyll a (Chl a), carotenoids, phycocyanin, allophycocyanin, and phycoerythrin], photosynthetic oxygen evolution, and photosystem (PS) II photochemistry, with a decline in Chl a fluorescence parameters [maximum photosynthetic quantum yield (Fv/Fm or Phi_Po), quantum yield of electron transport flux (Phi_Eo), yield of electron transport per trapped exciton (Psi_o), size and number of active photosynthetic reaction centers (Fv/Fo) and performance index (PIABS)]. However, a rise in energy flux parameters [absorption flux per reaction center (ABS/RC), trapping flux per reaction center (TRo/RC), dissipation flux per reaction center (DIo/RC), and electron transport flux per reaction center (ETo/RC)] was observed in both cyanobacteria. Additionally, UV-B stress induced a significant rise in levels of oxidative markers [superoxide radicals (O₂.⁻), hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) equivalents], and respiratory uptake of oxygen in the tested cyanobacteria. Contrarily, exogenous supplementation of GABA (50 nM) and NO [as sodium nitroprusside (SNP); 10 µM] resulted in a significant recovery of growth and photosystem (PS) II activity and reduction in oxidative damage. Furthermore, application of NO scavenger [2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO; 20 µM)] and NO synthase inhibitor [NG-nitro-L-arginine methyl ester (L-NAME; 100 µM)] significantly diminished the exogenously supplemented GABA-mediated responses under UV-B stress exposure. These results suggest that NO is the final requirement for the GABA-mediated tolerance against UV-B stress in the tested cyanobacteria.