Connecting Proline and γ-Aminobutyric Acid in Stressed Plants through Non-Enzymatic Reactions

The accumulation of proline (Pro) in plants exposed to biotic/abiotic stress is a well-documented and conserved response in most vegetal species. Stress conditions induce the overproduction of reactive oxygen species which can lead to cellular damage. In vitro assays have shown that enzyme inactivation by hydroxyl radicals ((OH)-O-center dot) can be avoided in presence of Pro, suggesting that this amino acid could act as an (OH)-O-center dot scavenger. We applied Density Functional Theory coupled with a polarizable continuum model to elucidate how Pro reacts with (OH)-O-center dot. In this work we suggest that Pro reacts favourably with (OH)-O-center dot by H-abstraction on the amine group. This reaction produces the spontaneous decarboxylation of Pro leading to the formation of pyrrolidin-1-yl. In turn, pyrrolidin-1-yl can easily be converted to Delta(1)-pyrroline, the substrate of the enzyme Delta(1)-pyrroline dehydrogenase, which produces gamma-aminobutyric acid (GABA). GABA and Pro are frequently accumulated in stressed plants and several protective roles have been assigned to these molecules. Thereby we present an alternative non-enzymatic way to synthetize GABA under oxidative stress. Finally this work sheds light on a new beneficial role of Pro accumulation in the maintenance of photosynthetic activity.