Secrets within stems: The cryptic Apodanthes caseariae (Apodanthaceae), a rare neotropical holoendoparasite

Societal Impact Statement Holoendoparasites are extremely rare plants that live entirely hidden inside their hosts, with only flowers and fruits visible. We studied Apodanthes caseariae , found in central and South America, parasitizing Casearia sylvestris . Little is known about its life cycle. From field and anatomical studies, we describe its flowers, fruits, and seeds, which have features that suggest ants may help both pollinate the flowers and disperse the seeds. We also sequenced its tiny, non‐functional chloroplast genome, reflecting the absence of photosynthesis in this leafless plant. The single living species of Apodanthes offers unique opportunities for evolutionary and ecological studies while requiring urgent conservation. Summary Whereas most of the research on the holoendoparasitic family Apodanthaceae has focused on the 11 species of the genus Pilostyles , the monotypic Apodanthes remains largely overlooked. Here we provide novel data on the natural history, morpho‐anatomy, and plastome reduction of A. caseariae , one of the most intriguing holoendoparasitic plants in the Neotropics. Apodanthes caseariae has a two‐phase life cycle: an endophytic stage in which a cryptic, mycelium‐like body develops inside the stems of its primary host Casearia sylvestris (Salicaceae), and an exophytic stage marked by the emergence of flowers and fruits. This holoendoparasitic plant completely lacks roots, stems, or leaves. Floral buds develop internally, emerging as bright yellow, unisexual, fully formed flowers with secretory bracts. Fruits are fleshy and polyspermous. Floral visitors include bees, wasps, flies, and butterflies, but heavy secretion and frequent visits suggest ants could play a role as pollinators, as in other holoparasites like Balanophora (Balanophoraceae) and Cytinus (Cytinaceae). Thick floral and fruit secretions, along with funicle‐derived elaiosomes in Apodanthes , suggest adaptations for ant‐mediated seed dispersal (myrmecochory). Ants may both pollinate Apodanthaceae flowers and disperse their seeds. We report for the first time the plastome of Apodanthes caseariae as a circular small molecule of 13,348 bp similar in size and gene content to Pilostyles , suggesting plastome reduction predated their divergence. Conserved genes include 16S , 23S , and accD . Unique to A. caseariae are GAPDH and ATPase β , although most plastid genes show no transcriptional activity.