Fig. 6.

Proposed model for the SIREM root–root signaling pathway resulting in systemic root exudation. Specific root bacterial populations colonizing the local root induce systemic root exudation of metabolites. For example, Pseudomonadales (brown circles) and Bacillales (green circles) are associated with secretion of ferulic acid hexose (structure in brown box) and the acylsucrose S4:19 (6, 6, 5, 2) (green box), respectively. Local root microbiome also induces accumulation of azelaic acid glycosides (mono- or diglycosylated; structures in blue) in plants. Azelaic acid glycosides are potential signaling metabolites (blue dashed arrows) triggering the SIREM process while the aglycone form of azelaic acid is a SIREM exudate component. In nature, SIREM represents a long-distance communication between unshared microhabitats of a plant rhizosphere. SIREM is triggered by a microbial population at a specific root zone; the signal likely reaches the shoot (e.g., AzA-Hex) and descends to unshared areas of the root, inducing systemic exudation that finally mediates rhizosphere interactions.