Systematic Review on Raphide Morphotype Calcium Oxalate Crystals in Angiosperms

Calcium oxalate (CaOx) crystals are biominerals present in a wide variety of plants. Formation of these crystals is a biomineralization process occurring in vacuoles within specialized cells called crystal idioblasts. This process is dependent on two key components: deprotonated oxalic acid, and calcium ions (Ca), and can result in multiple crystal morphologies. Raphides are needle-like CaOx crystals found in various plant organs and tissues. Though their function is highly debated, they can potentially store calcium, sequester heavy metals, protect against herbivory and possibly programmed cell death. The last review of the taxonomic and anatomical distribution of raphides across the plant kingdom dates back to 1980, in a review by Franceschi and Horner, prompting an updated systematic review of raphides in plants. We conduct a broad literature search to record plant taxa and tissue locations containing raphides. We provide an overview of raphide-forming plant taxa, discussing phylogenetic distribution of raphides at the order level, and report on the specific locations of raphides within plants. Our review reveals raphide occurrence has been studied in 33 orders, 76 families and 1305 species, with raphides presence confirmed in 24 orders, 46 families and 797 species. These taxa represented less than 1 % of known species per family. Leaves are the most prominent raphide-containing primary location in all three major angiosperm clades investigated: Eudicots, Magnoliids, and Monocots. Roots are least reported to contain raphides. The collation of such information lays the groundwork to unveil the genetic origin and evolution of raphides in plants, and highlights targets for future studies of the presence and role of plant raphides.