Three-dimensional reconstructions of haustoria in two parasitic plant species in the Orobanchaceae.

Parasitic plants infect other plants by forming haustoria, specialized multicellular organs consisting of several cell types, each of which has unique morphological features and physiological roles associated with parasitism. Understanding the spatial organization of cell types is, therefore, of great importance in elucidating the functions of haustoria. Here, we report a three-dimensional (3-D) reconstruction of haustoria from two Orobanchaceae species, the obligate parasite Striga hermonthica infecting rice (Oryza sativa) and the facultative parasite Phtheirospermum japonicum infecting Arabidopsis (Arabidopsis thaliana). In addition, field-emission scanning electron microscopy observation revealed the presence of various cell types in haustoria. Our images reveal the spatial arrangements of multiple cell types inside haustoria and their interaction with host roots. The 3-D internal structures of haustoria highlight differences between the two parasites, particularly at the xylem connection site with the host. Our study provides cellular and structural insights into haustoria of S. hermonthica and P. japonicum and lays the foundation for understanding haustorium function.

Ultrastructure of hydathode trichomes of hemiparasitic Rhinanthus alectorolophus and Odontites vernus: how important is their role in physiology and evolution of parasitism in Orobanchaceae?

The Rhinanthoid clade of the family Orobanchaceae comprises plants displaying a hemiparasitic or holoparasitic strategy of resource acquisition. Some of its species (mainly Rhinanthus spp.) are often used as models for studies of hemiparasite physiology. Although there is a well-developed concept covering their physiological processes, most recent studies have neglected the existence of hydathode trichomes present on leaves of these hemiparasitic plants. As a first step for the proposed integration of these structures in the theory of physiological processes of the hemiparasites, we described the outer micromorphology and ultrastructure of the hydathode trichomes on leaves of hemiparasitic Rhinanthus alectorolophus and Odontites vernus with scanning and transmission electron microscopy (SEM and TEM, respectively). The TEM inspections of both types of trichome revealed typical ultrastructural features: labyrinthine cell wall, high content of cytoplasm in cells with numerous mitochondria and presence of plasmodesmata. All these features indicate high metabolic activity complying with their function as glandular trichomes actively secreting water. The active secretion of water by the hydathode trichomes (evidence for which is summarised here) also presents a possible mechanism explaining results of previous gas exchange measurements detecting high dark respiration and transpiration rates and a tight inter-correlation between them in hemiparasitic Orobanchaceae. In addition, this process is hypothesised to have allowed multiple evolutionary transitions from facultative to obligate hemiparasitism and unique xylem-feeding holoparasitism of Lathraea with a long-lived underground stage featuring a rhizome covered by scales of leaf origin.

Seed ultrastructure and water absorption pathway of the root-parasitic plant Phelipanche aegyptiaca (Orobanchaceae).

BACKGROUND AND AIMS: Obligate root parasitic plants of the Orobanchaceae do not germinate unless they chemically detect a host plant nearby. Members of this family, like Orobanche, Phelipanche and Striga, are noxious weeds that cause heavy damage to agriculture. In spite of their economic impact, only a few light microscopical studies of their minute seeds have been published, and there is no knowledge of their ultrastructure and of the role each tissue plays during the steps preceding germination. This paper describes the ultrastructure of Phelipanche seeds and contributes to our understanding of seed tissue function. METHODS: Seeds of P. aegyptiaca were examined under light, scanning electron, transmission electron and fluorescence microscopy following various fixations and staining protocols. The results were interpreted with physiological data regarding mode of water absorption and germination stimulation. KEY RESULTS AND CONCLUSIONS: The endothelium, which is the inner layer of the testa, rapidly absorbs water. Its interconnected cells are filled with mucilage and contain labyrinthine walls, facilitating water accumulation for germination that starts after receiving germination stimuli. Swelling of the endothelium leads to opening of the micropyle. The perisperm cells underneath this opening mediate between the rhizosphere and the embryo and are likely to be the location for the receptors of germination stimuli. The other perisperm cells are loaded with lipids and protein bodies, as are the endosperm and parts of the embryo. In the endosperm, the oil bodies fuse with each other while they are intact in the embryo and perisperm. Plasmodesmata connect the perisperm cells to each other, and the cells near the micropyle tightly surround the emerging seedling. These perisperm cells, and also the proximal embryo cells, have dense cytoplasmic contents, and they seem to represent the two seed components that are actively involved in transfer of reserve nutrients to the developing seedling during germination.