Significance of phenolic compounds in plant‐soil‐microbial systems

Abstract Numerous reports have been published on the occurrence, isolation, and characterization of phenolic compounds in plant‐soil systems. The low molecular weight phenolics are of great interest because of their effects as allelopathic compounds and plant growth regulators, and they have traditionally been considered as defense molecules in plant‐pathogen interactions. More recently, their role as signal molecules in plant‐microbe systems has become evident. Specific molecules can act either as inducers for virulent genes in plant‐pathogen systems, such as Agrobacterium, or as inducers and “repressors”; in Rhizobium‐legumc and probably other symbiosis. The overall regulation role of these compounds in compatible and incompatible host‐microbe interactions is discussed in this review.