The Allelochemical L-DOPA Increases Melanin Production and Reduces Reactive Oxygen Species in Soybean Roots

Abstract

The non-protein amino acid, L-3,4-dihydroxyphenylalanine (L-DOPA), is the main allelochemical released from the roots of velvetbean and affects seed germination and root growth of several plant species. In the work presented here, we evaluated, in soybean roots, the effects of L-DOPA on the following: polyphenol oxidase (PPO), superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities; superoxide anion (O·-2), hydrogen peroxide (H(2)O(2)), and melanin contents; and lipid peroxidation. To this end, 3-day-old seedlings were cultivated in half-strength Hoagland's solution (pH 6.0), with or without 0.1 to 1.0 mM L-DOPA in a growth chamber (at 25°C, with a light/dark photoperiod of 12/12 hr and a photon flux density of 280 μmol m(-2) s(-1)) for 24 hr. The results showed that L-DOPA increased the PPO activity and, further, the melanin content. The activities of SOD and POD increased, but CAT activity decreased after the chemical exposure. The contents of reactive oxygen species (ROS), such as O·-2 and H(2)O(2), and the levels of lipid peroxidation significantly decreased under all concentrations of L-DOPA tested. These results suggest that L-DOPA was absorbed by the soybean roots and metabolized to melanin. It was concluded that the reduction in the O·-2 and H(2)O(2) contents and lipid peroxidation in soybean roots was due to the enhanced SOD and POD activities and thus a possible antioxidant role of L-DOPA.

The role of L-DOPA in plants

“…Increases in PPO activity and decreases in lipid peroxidation were observed in cucumber plants exposed to L-DOPA, 36 results also noted by Soares et al 37 in soybean roots. The reduction in lipid peroxidation may be related to the activity of antioxidant enzymes and/or suppression of ROS generation by quinoprotein formation.…”

“…Furthermore, the application of an exogenous antioxidant agent (ascorbic acid) decreased PPO activity and lipid peroxidation, and reversed the growth-inhibition caused by L-DOPA in lettuce. 45 In this context, Soares et al 37 reported that L-DOPA (0.1-1.0 mM) increased PPO activity and melanin synthesis (roots become black) in soybean. The results showed that the increase in the PPO activity was associated with the browning root, suggesting that melanin synthesis came from the oxidation of L-DOPA.…”

“…This has been attributed to increased superoxide dismutase (SOD) and POD activities, enzymes considered as ROS-scavengers, and also due to a possible direct antioxidant activity of L-DOPA. 37 In addition, the consumption of H 2 O 2 during lignification might have contributed to reducing the levels of this ROS, since L-DOPA increases lignification in soybean. 34 The decreases in ROS contents were corroborated by reductions in lipid peroxidation; thus, Soares et al 37 suggests that the phytotoxicity of L-DOPA in soybean is not due to ROS production during its detoxification to melanin.…”

“…37 In addition, the consumption of H 2 O 2 during lignification might have contributed to reducing the levels of this ROS, since L-DOPA increases lignification in soybean. 34 The decreases in ROS contents were corroborated by reductions in lipid peroxidation; thus, Soares et al 37 suggests that the phytotoxicity of L-DOPA in soybean is not due to ROS production during its detoxification to melanin. To the contrary, at least under the conditions and concentrations tested, L-DOPA seems to have an antioxidant role in soybean roots, reducing the levels of ROS.…”

Uso potencial de aminoácidos não-proteinogênicos como aleloquímicos

“…In lettuce, L-DOPA (0.1 and 1 mM) doubled the PPO activity (Mushtaq et al, 2013a). Plants of Cucumis sativus and Glycine max presented inhibition of root growth and increased PPO activity after addition of L-DOPA (Soares et al, 2011;Mushtaq et al, 2013b). In lettuce, L-DOPA (1 mM) increased about 4 times the production of O -2 and H2O2 compared to untreated control.…”