Signals do not convey information a manner that is absolute; their information depends on the context. Many biological signals are simple molecules or simple molecular events. It is the competence of the receiver cell or tissue that confers a specific, often diverse, “meaning” to these signals. In multicellular organisms, the spatial or temporal distribution of signalling is a key element of coordination. For instance, neural systems are based on a tight control of electrical fluxes along the axons that is complemented by tight spatial constraint of secretion events in the synapses. However, rather than patterning the signal itself, spatial and temporal patterns can also be generated by patterning the competence of the cells that perceive and interpret a signal. For the totipotent plant cells with their lack of hierarchy, such patterns of competence (Steinitz et al. 1976) are crucial. When signal competence is the key factor for coordination, there must be versatile and diversified ways to perceive signals. During plant evolution, the group of leucine-rich repeat (LRR) receptor-like kinases has been extended with more than a thousand members identified in the genomes of model plants such as Arabidopsis or rice. Three reviews in the current issue shed light on the versatility in molecular structure and biological function of this protein class and simultaneously draw the attention to a core element of signal competence.
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