Not Just Sweet Talkers

Recent studies showed that plant roots influence the composition of the bacterial population present in their rhizosphere. This observation implies that roots can affect their own colonization by various bacteria, and notably by plant growth-promoting rhizobacteria (PGPR). Since the plant-beneficial activities depend largely on the presence of PGPR on the root, it appears that the root's capacity to stimulate its own colonization is a great advantage. The exudates secreted by the root contain molecules acting as chemoattractants for many PGPR, and these molecules sometimes act specifically to attract certain cognate bacteria. In some cases, such as infection by a pathogen, the root can also secrete a higher amount of an attractant to recruit a higher quantity of PGPR. The roots exudates can also serve as a carbon source, providing energy and nutrient to the bacteria to multiply in the rhizosphere. Importantly, roots can influence biofilm formation and thus colonization of the roots. Various molecules are involved in this process such as malate, plant polysaccharides and quorum-sensing mimics. In addition, root-produced molecules can affect various cellular pathways, via modulation of transcription, translation or both. Many of these plant-stimulated pathways provide beneficial activities, such as antimicrobials and auxins production. Research on this topic is at its beginning, and most of the molecules and bacterial receptors involved are still to be found.