The cycad coralloid root: is there evidence for plant-microbe coevolution?

Cycads are survivors, ancient plants originating in the Carboniferous. We hypothesize that cycad resilience and recent diversification could be partially explained by their specialized coral-like (coralloid) roots and their microbiome and that these symbiotic partners are co-evolving. The coralloid root is unique in gymnosperms and rare in vascular plants. Coralloid roots and their associated microbes have been studied since the late 19th century, but a deeper understanding of their taxonomy and function has taken place only recently. And yet, we are at the ‘tip of the root’ as there are many open questions regarding this specialized organ and its evolutionary history. This review provides an overview of cycad coralloid roots and their microbiome, the technical limitations of their study to date, and the exciting questions that remain to be answered.