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N-acyl homoserine lactone (AHLs) produced by bacteria play a unique role in altering the expression of plant defence genes. AHL signals are constitutively produced by the vast majority of the rhizosphere and other groups of bacteria; and also varied levels of plant response are elicited through different types of AHL signals. Moreover, the defence mechanism of AHL-induced ISR is distinct from other bacterial compound-mediated plant response. It was also evident that the response of plants to bacterial AHLs may depend on plant species and chemical structure of AHLs. However, the question of how plants perceive the AHLs and distinguish between those molecules remains open. To date, no information is available either about plant AHL receptors or how plant cells can incorporate AHL signal molecules. Even though plants produce compounds similar to AHL signals, the precise source, structure and biological significance of these AHL mimics from plants are currently unknown. The specificity of plant mimics to stimulate or inhibit different types of AHL signals needs to be addressed. A thorough understanding of how plants perceive and respond to AHLs needs to be investigated. Copious questions remain to be addressed for the better understanding of quorum sensing of bacteria and trans-kingdom interactions of AHLs with plant cells.