Insights from Structure-Function Studies into Perception of Fatty Acid-Derived Defense Signals

Studies that correlate the structure of a molecule with its biological function or activity are useful in identifying the structural components that determine how the molecule interacts with binding proteins. This enables the synthesis of structural analogs with desirable properties, such as agrochemicals that improve plant developmental traits or adaptations to environmental stress. This review highlights a group of plant defense-inducing small signaling molecules characterized by a fatty acid-derived molecular skeleton with different functional groups. These include medium chain 3-hydroxy fatty acids (mc-3-OH-FAs) derived from the bacterial cell wall; green leaf volatiles (GLVs), which comprise primary aldehydes, alcohols, and esters derived from plant membranes; insect-derived fatty acid-amino acid conjugates (FACs), caeliferins, and bruchins; and sphingoid bases from oomycete pathogens. These molecules are typically lipophilic, and their mechanism of action is likely determined by both specific structural hallmarks and physicochemical properties. They activate defense responses via signaling pathways and are therefore presumed to interact with extra- or intracellular receptor proteins. However, classical receptors have only been characterized for mc-OH-FAs, sphingoid bases, and FACs. Structure-function studies may reveal structural features of these molecules that are critical for binding to receptors and relevant to the specificity of these interactions. This is particularly significant for GLVs, which have been extensively investigated for their roles in plant stress signaling and interplant communication, yet no specific receptor has been identified to date. This comparative review aims to shed light on perception of GLVs and other small molecules.