Haustorial structure of the holoparasitic angiosperm Cynomorium coccineum L. invading host roots

The perennial angiosperm holoparasite Cynomorium coccineum L. (Cynomoriaceae) was found infecting the roots of halophytes in a Mediterranean coastal salt marsh in Egypt. The parasite consists of a subterranean rhizome, which carries haustoria and sends aboveground annual inflorescences. Although the published research on the botany and taxonomy of C. coccineum dates back to approximately 150 years ago (Weddell, 1860; Engler, 1889; Baccarini and Cannarella, 1899), it lacked sufficient information on the anatomy of the haustorium parasitizing the host roots. Here, we described for the first time, the anatomy and ultrastructure of the haustoria of C. coccineum infecting host roots. The hosts recorded in this study were Limonium narbonense Mill. (Plumbaginaceae) and Atriplex portulacoides L. (Chenopodiaceae). The anatomy of haustoria was investigated by light microscopy, by histochemical techniques to stain lignin and callose and by scanning and transmission electron microscopy.The invasion of the host root by the haustoria of C. coccineum is largely accomplished via mechanical pressure. The presence of sieve tube elements in the haustorium at the interface between the phloem of the host roots and the parasite, was detected by light microscopy. The sieve element plastids of the host H. portulacoides are of the P-type, which is characteristic of Chenopodiaceae. The sieve elements in the haustorium are either differentiating or mature; the formers have thick undulating walls, disintegrated protoplast, and callose occluded sieve plate pores. The mature ones have fibrillar nacreous walls, clear lumen and fine sieve plate pores occluded with non-callose material. Sieve element plastids and P-protein bodies were not observed in the phloem of the haustorium. Plasmodesmatal connections are lacking at the contacts between the host and the parasite. The contacts between the tracheary elements of H. portulacoides and the parasite occur by the abutments of their walls, pits, or parenchyma cells.