Proving the mode of action of phytotoxic phytochemicals

Authors: DUKE, STEPHEN - Retired ARS Employee; Pan, Zhiqiang - Peter; Bajsa-Hirschel, Joanna

Submitted to: Plants
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2020
Publication Date: 12/11/2020
Citation: Duke, S., Pan, Z., Bajsa Hirschel, J.N. 2020. Proving the mode of action of phytotoxic phytochemicals. Plants. https://www.doi.org/10.3390/plants9121756.
DOI: https://doi.org/10.3390/plants9121756

Interpretive Summary: Knowledge of the mode of action of an allelochemical can be valuable for several reasons, such as proving and elucidating the role of the compound in nature and evaluating its potential utility as a pesticide. However, discovery of the molecular target site of a natural phytotoxin can be challenging. Because of this, we know little about the molecular targets of relatively few allelochemicals. There is interest in allelochemicals as templates for new classes of herbicides with new MOAs. Herbicides with new modes of action are needed badly as a tool to manage the rapidly growing evolution of weed resistance to herbicides with the twenty-five or so MOAs currently used by commercial herbicides.

Technical Abstract: This review describes the many approaches to molecular target site discovery, with an attempt to honestly point out the pitfalls of each approach. Clues from molecular structure, phenotypic effects, physiological effects, omics studies, genetic approaches, and use of artificial intelligence are discussed. All these approaches can be confounded if the phytotoxin has more than one molecular target at similar concentrations or is a prophytotoxin, requiring structural alteration to create an active compound. Unequivocal determination of the molecular target site requires proof of activity on the function of the target protein and proof that a resistant form of the target protein confers resistance to the target organism. In this paper, we define mode of action (MOA) of a phytotoxin as the process by which it affects a plant, including its primary target site. Understanding the MOA of phytotoxic phytochemicals (PPs) has both academic and practical utility. On the academic side, this knowledge can be useful in determination of whether a putative allelochemical is actually functioning as an allelochemical. For example, if the MOA of a compound is known from laboratory studies and there is a biological marker for this MOA, one can determine if the marker occurs when target plants are exposed to the levels of the compound found in soil.