Rhizosphere Signals for Plant–Microbe Interactions: Implications for Field-Grown Plants

Abstract

This review presents an analysis of rhizosphere signals important in plant–microbial interactions that have been studied in controlled conditions and how they may function on field-grown roots. We define rhizosphere signals to be molecules on or emitted from microorganism or root cells that are recognized by other cells and trigger a response. A well-known example are the flavonoids from legume roots, which bind to transcriptional activators in rhizobia bacteria triggering the release of Nod factors (lipochitin oligosaccharides) that bind to root hairs or cortical cells initiating nodule development. Many other signals are reported, e.g., phytohormones, quorum sensing signals (QSS) and their mimics, strigolactones, and exopolysaccharides. Some are involved in infection of roots by symbionts or pathogens; others in growth, physiological, and immune responses caused by commensal organisms that do not invade the root. The signals have so far been largely studied in the classical host–microbe framework in controlled conditions. Field rhizospheres, however, are host–microbe communities that change in space and time. Root surfaces, and thus binding sites for signals, change as the root ages and differentiates, as do chemicals released, which may be energy substrates, signals, or toxins to soil microorganisms in a local patch of soil. The nutritional status and disease susceptibility of the plant and the soil properties are also changing with space and time in the field. These dynamics explain in part why it has been so difficult to translate laboratory effects of signals to the field. Our analysis suggests that signal function in field rhizospheres would depend on (1) compatibility between microorganism populations and root tissue age and cell surfaces for signal–receptor binding, (2) proximity to the root, (3) moisture, and (4) plant nutrition and predisposition to response to microorganisms due to biotic or abiotic factors. These ideas need to be tested. Two major gaps in knowledge are the microorganisms that colonize the rhizosphere and their signals – only a small percentage have been sequenced, and how the plant genetics is regulating the perception and response to the diverse signals that may bind successfully to the root surfaces in the field.