Abstract
Naphthazarin (5,8-dihydroxy-1,4-naphthoquinone, DHNQ) is a naturally occurring 1,4-naphthoquinone derivative. In this study, we focused on elucidating the toxic effect of this secondary metabolite on the growth of plant cells. The dose–response curves that were obtained for the effects of DHNQ on endogenous and IAA-induced growth in maize coleoptile segments differ in shape; in the first case, it is linear, while in the presence of auxin it is bell-shaped with the maximum at 1 μM. It was found that DHNQ at almost all concentrations studied, when added to the incubation medium inhibited endogenous growth (excluding naphthazarin at 0.001 μM) as well as growth in the presence of IAA. Simultaneous measurements of the growth and external medium pH of coleoptile segments indicated that DHNQ diminished or eliminated proton extrusion at all of the concentrations that were used. Interestingly, the oxidative stress in maize coleoptile cells, which was measured as hydrogen peroxide (H2O2) production, catalase activity, redox activity and malondialdehyde (MDA) content, increased at the lower concentrations of DHNQ (<1 μM), thus suggesting a specific character of its action. It was also found that naphthazarin at concentration higher than 0.1 μM caused the depolarization of the membrane potential (Em). An analysis of the organization and anisotropy of the cortical microtubules showed that naphthazarin at all of the concentrations that were studied changed the IAA-induced transverse microtubule reorientation to an oblique reorientation. Our results indicate that naphthazarin diminished the growth of maize coleoptile cells by a broad spectrum of its toxic effects, thereby suggesting that naphthazarin might be a hypothetical component of new bioherbicides and biopesticides.
Highlights
Naphthoquinones are the products of the bacterial and fungal metabolism as well as the secondary metabolism in higher plants, where they are produced and used as natural defense chemicals (Babula et al, 2009; War et al, 2012)
The dose–response curves that were constructed for the effect of DHNQ on the endogenous and its ability to regulate auxin (IAA)-induced elongation growth of the maize coleoptile segments differed in their shapes (Figure 3A)
This high level of biological activity of naphthoquinones is based on two primary mechanisms: the first is the covalent modification of biological molecules at their nucleophilic sites in which quinones act as electrophiles, while the second consists in redox cycling in which reactive oxygen species (ROS) are generated
Summary
Naphthoquinones are the products of the bacterial and fungal metabolism as well as the secondary metabolism in higher plants, where they are produced and used as natural defense chemicals (Babula et al, 2009; War et al, 2012). It has been found that quinones, via the alkylation of proteins or interactions with other organic molecules, reduce the nutritional value of plant components for insects, thereby leading to a reduction in their growth and development Secondary metabolites, such as quinones and their derivatives, may have a direct toxic effect on insect-attacking plants as a result of the initiation of the redox cycles and the production of reactive oxygen species (ROS). It was found that quinone derivatives are primarily synthesized during the stress that is associated with a herbivorous attack (Duffey and Stout, 1996; Eilenberg and Zilberstein, 2008; War et al, 2012)
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