Strategies and future trends to identify the mode of action of phytotoxic compounds.

Small molecules affecting plant processes have been widely used as probes to study basic physiology. In agricultural practices some of these molecules have served as herbicides or plant growth regulators. Historically, most of the compounds were identified in large screens by the agrochemical industry, but also as phytoactive natural products. More recently, novel phytoactive compounds originated from academic research by chemical screens performed to induce specific phenotypes of interest. In the present review different approaches were evaluated for the identification of the mode of action (MoA) of phytoactive compounds. Based on the methodologies used for MoA identification, three approaches are differentiated: a phenotyping approach, an approach based on a genetic screen and a biochemical screening approach. Target sites of compounds targeting primary or secondary metabolism were identified most successfully with a phenotyping approach. Target sites for compounds that influence cell structure, such as cell wall biosynthesis or the cytoskeleton, or compounds that interact with the hormone system, were in most cases discovered by using a genetic approach. Examples showing the strengths and weaknesses of the different approaches are discussed in detail. Additionally, new techniques that could contribute to future MoA identification projects are reviewed. In particular, next-generation sequencing techniques may be used for the fast-forward mapping of mutants identified in genetic screens. Finally, a revised three-tiered approach for the MoA identification of phytoactive compounds is proposed. The approach consists of a 1st tier, which addresses compound stability, uniformity of effects in different species, general cytotoxicity and the effect on common processes such as transcription and translation. Advanced studies based on these findings initiate the 2nd tier MoA characterization, either with further phenotypic characterization, starting a genetic screen or establishing a biochemical screen. At the 3rd tier, enzyme assays or protein affinity studies should show the activity of the compound on the hypothesized target and should associate the in vitro effects with the in vivo profile of the compound.