Spotlight Received: 29 June 2011 Revised: 5 August 2011 Accepted: 11 August 2011 Published online in Wiley Online Library: (wileyonlinelibrary.com) DOI 10.1002/ps.2300 Physionomics and metabolomics – two key approaches in herbicidal mode of action discovery † Klaus Grossmann, a* Nicole Christiansen, b Ralf Looser, b Stefan Tresch, a Johannes Hutzler, a Stephan Pollmann c and Thomas Ehrhardt b Abstract BACKGROUND: For novel herbicides identified in greenhouse screens, efficient research is important to discover and chemically optimise new leads with new modes of action (MoAs). RESULTS: The metabolic and physiological response pattern to a herbicide can be viewed as the result of changes elicited in the molecular and biochemical process chain. These response patterns are diagnostic of a herbicide’s MoA. At the starting point of MoA characterisation, an array of bioassays is used for comprehensive physiological profiling of herbicide effects. This physionomics approach enables discrimination between known, novel or multiple MoAs of a compound and provides a first clue to a new MoA. Metabolic profiling is performed with the use of treated Lemna paucicostata plants. After plant extraction and chromatography and mass spectrometry, changes in levels of approximately 200 identified and 300 unknown analytes are quantified. Check for known MoA assignment is performed by multivariate statistical data analyses. Distinct metabolite changes, which can direct to an affected enzymatic step, are visualised in a biochemical pathway view. Subsequent target identification includes metabolite feeding and molecular, biochemical and microscopic methods. CONCLUSION: The value of this cascade strategy is exemplified by new herbicides with MoAs in plastoquinone, auxin or very-long-chain fatty acid synthesis. c  2011 Society of Chemical Industry Keywords: herbicides; mode of action discovery; metabolic profiling; physiological profiling 1 INTRODUCTION The strategic focus towards novel herbicides with new sites or modes of action (MoAs) is a priority assignment in crop protection research. Only six out of about 20 commercially used herbicidal MoAs in agriculture cover 78% of the market products. 1 Herbicides with novel MoAs are needed in the market to extend the features of herbicides with prospects of better efficiency against important weeds, wider and more selective activity spectrum and crop compatibility, extended weed resistance management and improved environmental behaviour. For active compounds identified in conventional greenhouse screens, efficient MoA research is important to discover and chemically optimise new leads. Owing to the complexity inherent in MoA discovery research, the constant challenge is to use technologies with the highest chance of success in order to reduce the time and work needed to develop a new market product. In recent years, the use of profiling methods, defined as the ‘omics’ technologies, has been the subject of particular discussion in MoA research for crop protection compounds (for a review, see elsewhere 2–7 ). These technologies are based on compre- hensive characterisation of molecular (gene expression profiling: transcriptomics), biochemical (protein profiling: proteomics), analytical (metabolic profiling: metabolomics) or physiological (physiological profiling: physionomics) changes in an organism, tissue or cell type at multiple levels from gene expression to the physiological phenotype. In particular, metabolic and physio- logical action profiles to a herbicide can be viewed as the result of alterations elicited in the molecular and biochemical process chain. Consequently, both techniques should be particularly pre- destined to generate action profiles that are highly diagnostic of a herbicide’s MoA. The present authors’ long-standing experience in the methodology of physiological and metabolic profiling confirms this assumption. Today, both methods play key roles in the technology platform for MoA discovery in herbicide research. Interestingly, the classical approach of MoA characterisation ∗ Correspondence to: Klaus Grossmann, BASF Agricultural Centre Limburgerhof, D-67117 Limburgerhof, Germany. E-mail: Klaus.grossmann@basf.com † This contribution is based on a paper presented at a symposium on the Science of Herbicide Discovery during the 51th Annual Meeting of the Weed Science Society of North America (2011). a BASF Agricultural Centre Limburgerhof, Limburgerhof, Germany b Metanomics GmbH, Berlin, Germany c Ctr Biotecnol and Genom Plantas, Campus Montegancedo, Madrid, Spain Pest Manag Sci (2011) www.soci.org c  2011 Society of Chemical Industry