Bacterial biofilms as an essential component of rhizosphere plant-microbe interactions

Most plant-associated microorganisms inhabit the rhizosphere, the thin, well-structured, dynamic and highly competitive biofilm located between the roots and the soil. Rhizosphere development progresses through stages in which soil bacteria are first recruited by root exudates and then proliferate within a protective matrix from within which they influence the nutritional status of the plant, its growth, and its ability to resist pathogens and abiotic stresses. To circumvent the challenges that make characterization of rhizosphere biofilms in soil difficult, experiments are often conducted with bacteria cultured in root exudates within controlled environments in devices such as microtiter plates, flow chambers, drip flow reactors and microfluidic platforms. When coupled with microscopy and metabolic analyses, such cultures have revealed intimate details of root development, rhizosphere establishment and biofilm spatiotemporal microheterogeneity. Studies in soil have shown that introduced microorganisms must compete aggressively with indigenous populations to establish themselves within the biofilm matrix and form dense populations capable of quorum sensing and other signalling activities that mediate plant growth promotion and disease suppression via antibiotic production. Healthy biofilm matrices rich in water-binding components also help to relieve drought stress in plants growing in arid parts of the world. The chapter includes sections that discuss the close resemblance between the processes of rhizosphere colonization and biofilm formation, provide an overview of experimental approaches for studying rhizosphere biofilms, and cover the practical importance of the rhizosphere microbiome.