Detect thy neighbor: identity recognition at the root level in plants.

Some plant species increase root allocation at the expense of reproduction in the presence of non-self and non-kin neighbors, indicating the capacity of neighbor-identity recognition at the root level. Yet in spite of the potential consequences of root identity recognition for the relationship between plant interactions and community structure and functioning, this phenomenon still remains poorly understood. We first critically assess the evidence for the existence of self/non-self and kin recognition at the root level in plants. While root identity recognition most likely exists to some degree, there remain valid points of criticism regarding experiments that have documented this, particularly concerning the effects of pot volume in self/non-self recognition experiments and the roles of size inequality and asymmetric competition in kin recognition studies. Subsequently we review and propose some plausible physiological mechanisms that may underlie these responses. Finally we briefly discuss the relation between under- and aboveground interactions and the potential consequences of root identity recognition for agriculture, and conclude with raising several questions for future studies.