Modes of action of phytotoxins from plants

Abstract

Relatively little is known of the mode of action of natural phytotoxins, when compared to our understanding of the mode of action of synthetic herbicides (Devine et al. 1993; Fedtke and Duke, 2005). Furthermore, most of the scientific literature on mode of action of natural phytotoxins focuses on compounds from microbial origins. Relatively little effort has been made to study the mode of action of plant-produced phytotoxins, including allelochemicals involved in plant/plant interactions. This review will cover phytotoxins produced by higher plants, as well as algae and lichens, even though some of the phytotoxins from lichens may be produced by the fungal component. We will not discuss compounds from microbes nor non-lichen fungi. Where possible, we will discuss molecular target sites, however, much of the literature on mode of action of these compounds does not provide definitive proof of the molecular target site. Those with known molecular target sites will be discussed separately from those for which some mode of action information is available, but the precise target site is unknown. Any strong phytotoxin will eventually affect most physiological processes at sufficient dosage. Indeed, this fact has been the basis for many inconclusive publications. We will try to include mostly publications that report more than physiological responses that are chosen without a good rationale to link them to the mode of action of the phytotoxin. Likewise, at sufficiently high concentrations, almost any compound will affect an in vitro activity of an enzyme or a physiological process such as respiration. If a compound is substantially active in vitro only at millimolar concentrations, it is unlikely to act as a phytotoxins in nature. So, we will focus on compounds that have in vitro effects at submillimolar concentrations. We use the word phytotoxin rather than allelochemical in this chapter, since, for many of the compounds discussed here, there is little or no proof that they act as allelochemicals in plant/plant interactions in nature. This chapter would be very short if we limited ourselves to compounds that are clearly established as allelochemicals in natural environments. There may be cases in which a phytotoxic molecule affects several molecular target sites. This greatly complicates establishing a mode of action, and may be another reason why the mode of action of relatively few plant-produced phytotoxins is well understood. We have divided this chapter into a section on sites of action for natural products that are apparently well understood and another section on compounds for which a significant amount of mode of action research has been done without fully resolving the molecular target site(s).