Anatomical and histochemical features of Taxillus sutchuenensis (Loranthaceae) are consistent with a parasitic lifestyle

Abstract Taxillus sutchuenensis (Lecomte) Danser parasitizes host Styrax japonicus Sieb. et Zucc. at the canopy. The aerial adventitious roots of T. sutchuenensis grow down the host trunk, producing haustoria that invade the host bark. Using brightfield and epifluorescence microscopy, we identified anatomical and histochemical features that allow this species to parasitize its host. The aerial adventitious roots are polyarch, with prominent metaxylem, initial secondary xylem in the stele, and an exodermis with Casparian bands and suberin lamellae; the roots do not have an evident endodermis. The cortex releases protein-rich secretions by decomposing the exodermis and the cork, and the phellogen produces cork to enclose these ruptures. The haustoria have tracheids and vessel elements that penetrate the host bark in all directions, forming xylematic concentric rings at maturity. The shoots have thick cuticles, and the leaf blade mesophyll is not differentiated in spongy and palisadic parenchyma. The root protein secretions may soften and dissolve host bark, facilitating its penetration by the haustoria. Apoplastic barriers consisting of the exodermis, cork, and thick cuticle, protect T. sutchuenensis while it parasitizes its host. Thus, several structures of T. sutchuenensis are consistent with a parasitic lifestyle: the proteins secreted by the aerial adventitious roots, the concentric rings in the haustorium, and the apoplastic barriers.