The structure and development of the haustorium in Cuscuta australis

Abstract

The structure and development of the haustorium of a parasitic angiosperm Cuscuta australis R. Brown growing on the host plant Trifolium repens L. was studied with light and electron microscopy. The upper haustorium, which lies external to the host organ, initiates endogenously from cortical cells of the middle layers of the parasite stem. The initial cells develop into a group of meristematic cells. As haustorial maturation progresses, the meristematic cells develop into an endophyte primordium that penetrates the host tissue. The endophyte primordium consists of three cell types: (i) remarkably enlarged elongate cells (digitate cells) with very dense cytoplasm and large nuclei at the central region; (ii) smaller file cells with prominent nuclei proximal to the digitate cells; and (iii) highly compressed cells distal to the digitate cells. Evidence suggests that the digitate cells are metabolically very active. The endophyte, which lies internal to the host tissue, consists of parenchymatous axial cells and elongate tip cells with dense cytoplasm and conspicuous nuclei. The axial and tip cells of endophyte are interpreted as originating from the file and digitate cells of the endophyte primordium, respectively. The tip cells independently penetrate the host tissues and transform into the filamentous hyphae. The hyphae reach the host vascular tissues and eventually differentiate into xylary or phloic conductive hyphae.