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.

Phloem development: current knowledge and future perspectives.

Phloem, as a major tissue mediating long-distance communication, has been an object of extensive research ever since its structure was first reported in 1837. Functional phloem consists of sieve elements (SEs) and companion cells (CCs). While SEs are enucleated conducting cells in the phloem, CCs are cells with intact cellular components and are known to support the functioning of SEs. CCs are closely linked to SEs by symplastic connections mediated by plasmodesmata (PD). Sieve elements are notoriously sensitive to manipulation, which has hampered efforts to investigate their structure using microscopy or histology; phloem thus remains a mysterious tissue almost 200 yr after its discovery. Nevertheless, consistent efforts have overcome many of the technical barriers and generated considerable amounts of data about the structure and function of phloem. Advances in the 1950s and 1960s significantly improved our understanding of phloem anatomy and function. A major function of the phloem is to establish symplastic connections throughout the plant body, delivering nutrients and various signaling molecules, which play pivotal roles in growth and development. Despite the importance of phloem, details about the molecular mechanisms responsible for the establishment and maintenance of phloem continuity remain elusive.