Abstract
The effects of 1,4-naphthoquinone (NQ) and 2-hydroxy-1,4-naphthoquinone (NQ-2-OH) on indole-3-acetic acid (IAA)-induced growth, medium pH changes and membrane potential (Em) in maize (Zea mays L.) coleoptile cells were determined. In addition, the redox cycling properties of both naphthoquinones were also compared. The dose-response curves constructed for the effects of NQ and NQ-2-OH on endogenous and IAA-induced growth differ in shape. It was found that NQ was by 10–50% more effective in inhibiting IAA-induced growth in maize coleoptile segments than NQ-2-OH. Simultaneous measurements of growth and external medium pH indicated that NQ and NQ-2-OH reduced or eliminated proton extrusion at all of the concentrations used, excluding NQ at 1 µM. It was found that both naphthoquinones at concentrations higher than 10 µM caused the depolarisation of the membrane potential (Em). Additionally, compared to the controls, NQ- and NQ-2-OH-exposure of coleoptile segments, at concentrations higher than 10 µM, caused an elevation of the hydrogen peroxide (H2O2) production and plasma membrane redox activity. The highest catalase activity was observed at 10 µM NQ and it was ca. 18-fold greater (at 4 h) than in the control medium. Moreover, it was also found that NQ and NQ-2-OH, at all concentrations studied, increased the malondialdehyde content of coleoptile segments at 4 h of the experiment. The data presented here are discussed taking into account the “acid growth hypothesis” of auxin action and the mechanisms by which naphthoquinones interact with biological systems.
Highlights
Naphthoquinones are a group of organic compounds that are widely distributed in nature
NQ-2-OH, which was added to the incubation medium at the same time protocol and concentrations as NQ, inhibited the elongation growth of maize coleoptile segments incubated in the presence of indole-3-acetic acid (IAA), similar to NQ (Fig. 2)
NQ2-OH, which was added to the incubation medium at the same time protocol and concentrations as NQ, stimulated ferricyanide reduction by a mean 50% (1.5-fold) at moderate concentrations (0.1, 1 and 10 μM), while NQ-2-OH at 100 and 1000 μM induced a 2.4-fold and 4.6-fold increase in hexacyanoferrate III (HCF III) reduction within two hours compared to the control (Fig. 7), respectively
Summary
Naphthoquinones are a group of organic compounds that are widely distributed in nature. The biological activity of naphthoquinones is primarily based on two primary mechanisms—one is the covalent modification of biological molecules at their nucleophilic sites, such as the thiols in proteins and glutathione (GSH), in which quinones act as electrophiles, while the other mechanism consists of redox cycling, in which reactive oxygen species (ROS) are generated (reviewed in El-Najjar et al 2011; Klotz et al 2014; Kumagai et al 2012) Occurring naphthoquinones such as juglone (5-hydroxy1,4-naphthoquinone), lawsone (2-hydroxy-1,4-naphthoquinone), plumbagin (2-methyl-5-hydroxy-1,4-naphthoquinone), naphthazarin (5,8-dihydroxy-1,4-naphthoquinone) and others along with their synthetic derivatives have been studied intensively for many years in biology, medicine, agriculture and industry. Lawsone (2-hydroxy-1,4-naphthoquinone) is one of the naturally available 1,4-naphthoquinone derivatives
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