The key molecular event during the development of arbuscular mycorrhiza and the root nodule symbiosis is the activation of calcium- and calmodulin-dependent protein kinase (CCaMK). Its regulation is complex and involves positive as well as negative regulation facilitated by autophosphorylation of two conserved sites. Deregulated versions of CCaMK are sufficient for mediating both organogenesis and infection processes. Epistasis tests demonstrated that a main function of signaling components upstream of calcium spiking is the activation of CCaMK. Despite CCaMK being a central signaling hub, specificity for both symbioses exists, resulting in differential transcriptional gene expression patterns. While the specificity upstream of CCaMK can be conceptualized by the specific perception of rhizobial and fungal lipo-chitooligosaccharides via cognate LysM receptors, the mechanisms conferring transcriptional specificity downstream of CCaMK are likely conferred by a variety of transcriptional regulators, mediating symbiosis appropriate gene regulation.
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