Composite bundles, the host/parasite interface in the holoparasitic angiosperms Langsdorffia and Balanophora (Balanophoraceae)

Abstract

Composite bundles are not simply a type of vascular bundles, but an integrated host/parasite interface. We investigated their structure in tubers of Langsdorffia and Balanophora. Composite bundles in both genera have similar components: 1) a central mass of host vascular tissues among which are located large parasite transfer cells; 2) a sheath of parasite parenchyma surrounding the central host vascular tissues; 3) specialized conducting tissues in the sheath; and 4) apical meristems composed of both host and parasite meristematic cells. Sheath parenchyma is recognizable from parasite tuber matrix by having thinner cell walls, and, especially in Langsdorffia, by the presence of collapsed matrix cells between the bundle sheath and tuber matrix. Sheath‐conducting tissues consist of densely cytoplasmic transfer cells and small sieve tube members; in Langsdorffia, tracheary elements are also present. These sheath bundles connect with vascular bundles of the tuber matrix. Direct host/parasite contact only occurs by means of parasite transfer cells in the composite bundles. There is no xylem‐xylem contact at the host/parasite interface. Abundance of parasite transfer cells suggests that they play an important role in nutrient absorption and translocation.