Root Exudates as Integral Part of Belowground Plant Defence

Abstract

Root exudates comprise a heterogeneous group of compounds that display various effects on soilborne organisms, including stimulation, attraction, but also repellence and inhibition. Therefore, root-secreted chemicals can assist belowground plant defence through direct and/or indirect mechanisms. Direct defence strategies exploited by roots include the secretion of phytochemicals with antimicrobial, insecticide, or nematicide properties. In contrast, other root exudates recruit or influence beneficial organisms to serve as biological weapons against plant aggressors, a mechanism termed indirect plant defence. Since rhizosecretion fundamentally shapes the composition of soil-inhabiting organisms and contributes to plant survival, the quality and quantity of defence root exudates are tightly controlled. Various environmental and endogenous factors can stimulate the release of phytochemicals that exhibit precisely targeted bioactivities. On the molecular level, several primary active transport proteins have been demonstrated to affect the composition of defence root exudates in the rhizosphere. In this chapter, we will focus our attention on direct and indirect defence strategies mediated by root exudates. In addition, we will shed light on regulatory mechanisms of defence-related root exudation that prevent belowground disease and ensure optimal plant performance.