The ultrastructure of dwarf mistletoe (Arceuthobium spp.) sinker cells in the region of the host secondary vasculature

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Abstract
Sinker cells showed ultrastructural similarities in 3 spp. of Arceuthobium [A. occidentale, A. tsugense, A. californicum] on 3 different hosts [Pinus sabiniana tsuga heterophylla, P. lambertiama] despite differences in season of collection and fixation. All species had abundant osmiophilic lipid bodies, plastids with prolamellarlike bodies, mitocondria with large nucleoids, chromocentric nuclei and peculiar saccules associated with plasmodesmatal fields. Xylem may not be continuous through sinkers. Apoplastic continuity between host and parasite is afforded by fused cellulosic cells walls. Pitlike wall thinnings and half-plasmodesmata are found between juxtaposed host and parasite cells. One-sided, imperforate sieve pores were noted between a sieve cell and a contiguous sinker cells. However, symplastic isolation of the host and parasite seems likely. Therefore, nutrients may be absorbed from the common host-parasite apoplast. Mobilization of nutrients out of the endophytic system to the aerial shoots is thought to be faciliated by differential starch storage in the parasite body.