Indole is an essential molecule for plant interactions with herbivores and pollinators

Indole is a well-characterized molecule that is used as a building block for a multitude of natural compounds. It can be utilized directly as a free indole or it can serve as a substrate for indole-derived metabolites. Besides serving as a substrate for the essential amino acid tryptophan, it is also involved in a variety of plant functions. One of the most important processes in which involving indole is the chemical interaction between plants and insects. These insects are classified into two groups: i) herbivores, which feed on plant tissue (e.g. caterpillars) or consume nutrients from the phloem (e.g. aphids), and ii) pollinators, which feed on plant nectar and pollen and serve as vectors fortransferring male gametes between flowers. The nature of indole and indole-derived metabolite usage may differ based on the type of interactions. While indole is a volatile compound emitted to the plant's surrounding, functioning as a remotesignal, indole-derived metabolites are mainly non-volatile and mostly function asdeterrents that harm herbivores by “direct contact”. In this review, we discuss the major role of indole in determining plant fitness by attracting pollinators and repelling herbivores.