Ultrastructural features of seed and seedling of Cuscuta pedicellata

Abstract

This light and electron microscope study of the liverwort Asterella reveals that, as in other Marchantiales, the cells lining the dorsal air chambers are highly vacuolate with numerous amylochloroplasts in the peripheral cytoplasm. The ventral parenchyma in the midrib of the thallus contains aseptate fungal hyphae surrounded by an interfacial matrix and host cytoplasm forming transvacuolar strands. These are lined with microtubules, rarely seen in other fungal-hepatic associations or in mycorrhizas. Numerous lipid bodies found in all the thallus cells are thought to be associated with perennation during the winter dry season.Elongated, thick walled inner thallus cells, between the dorsal air chambers and the fungus-containing tissue, have a cytological organization not previously recorded in land plants. Initially highly vacuolate, with numerous microtubules of random orientation lining the tonoplast, these cells subsequently show interdigitation of vacuoles and cytoplasm producing a labyrinth of spherical and elongate tonoplast profiles lined by longitudinal arrays of microtubules. At the same time the cytoplasm becomes increasingly electron-lucent and the ribosomes, progressively lost from the ER, clump together. At maturity the inner thallus cells are highly polarized with most of the vacuoles lying nearer the thallus apex. In pits in the end walls, numerous plasmodesmata, with expanded cytoplasmic annuli recall the plamodesmatal fields in the mesophyll and phloem of the leaves in vascular plants.Far from being supporting parenchyma or sclerenchyma as assumed hitherto, the inner thallus cells of Asterella are clearly highly differentiated. Their vacuole microtubule associations are highly suggestive of a microtubule-based translocation system akin to that seen in many animal cells and perhaps fungal hyphae, but very different from bulk flow in sieve elements and actin-based cytoplasmic streaming.