Parasitic plants belonging to the genera Orobanche
are obligate parasites that subsist on hosts’ roots. Phelipanche aegyptiaca
parasitizes a wide range of plants, including important crops such as tomato, melon, and legumes, and causes serious damage to crop production [1
]. P. aegyptiaca
develops a radicle upon germination when perceiving a stimulant exuded from the hosts’ roots. When the radicle contacts the host root, a haustorium (a specialized intrusive organ) differentiates from the terminus of the radicle and reaches the host’s vascular tissues, and the vascular conducting tissues then differentiate. After establishing the conducting tissues, P. aegyptiaca
initiates the development of a tubercle, which is a storage organ developed from the seedling remaining outside the host’s root [2
]. Formation of vascular conducting tissue in haustoria is necessary for obligate parasitic plants to absorb water and nutrients from the hosts, because they lack roots or photosynthetic organs.
Xylem and phloem differentiation in haustoria are pivotal steps in the successful establishment of a parasitic connection. Xylem differentiation is characterized by the formation of vessel elements with highly lignified cell walls [3
]. Elongated cells in the haustorial tips of Striga hermonthica
perforate cell walls of the host’s xylem vessel, and the invading part of the contact cell (the osculum) loses its cytoplasmic contents and differentiates into a water-conducting vessel element [3
]. In a P. aegyptiaca
parasitic complex, exogenous auxin application decreases the infection rate, suggesting that auxin flow plays an important role in xylem continuity [4
]. Open xylem connections at the parasite-host interface allow a flow of water and minerals to the parasitic plant.
Haustorial phloem differentiation has also been described. Studies on the haustiorial phloem of various parasitic plants showed that the presence of sieve elements (SEs) in haustoria depends upon the species in question. A morphological study using light microscopy on the structure of the haustorium of a stem parasitic plant, Cuscuta gronovii
, has demonstrated that most of the cells in sieve tubes have prominent nuclei and sieve plates, and no sieve-tube members reach the host phloem [5
]. In contrast, morphological studies of haustoria of a stem obligate parasitic plant, Cuscuta odorata
, using electron microscopy have demonstrated that there are sieve element (SE)-companion cell (CC) complexes in haustoria that are directly in contact to host SEs [6
]. A root facultative parasitic plant in the Orobancaceae
, does not develop phloem in haustoria, and it takes organic substances through xylem. Another root facultative parasitic plant, Alectra vogelii
, develops SEs in haustoria, but they do not have a direct contact to the host’s SE [6
]. In secondary haustoria of a root obligate parasitic plant, Phelipanche ramosa
, SEs appear to develop, however, a final cell that attaches to host’s SE is of parenchymatous nature [6
]. On the other hand, transport function of haustorial phloem has been demonstrated by using symplasmic tracers. Studies using fluorescent symplasmic tracers have demonstrated that carboxyfluorescein or CC-expressed green fluorescent protein (GFP) can be transported from host to parasite through haustoria in Cuscuta reflexa
], Phelipanche ramosa
], and P. aegyptiaca
]. In haustoria of a facultative parasitic plant, Phtheirospermum japonicum
, carboxyfluorescein was transported from the host’s sieve tubes to haustoria, but CC-expressed GFP was not, implying the lack of phloem-to-phloem connection between the host and parasite, or difficulty in GFP movement due to its size [11
]. Despite the exception of Lathraea
and P. japonicum
, these studies on the cell morphology and the transport function collectively suggested that it is likely that phloem contents should be transported from the host to parasitic plants by haustorial phloem. However, because few previous studies examined the cell morphology and transport function of the haustorial phloem simultaneously, it is still unclear whether the pathway in which phloem contents are transported occurs in SEs.
In this study, we observed the transport function of phloem-conducting cells in haustoria and their morphology at the same time. We identified phloem-conducting cells in haustoria by the plant-to-plant translocation of GFP from AtSUC2pro::GFP tomato sieve tubes. GFP-conducting cells contained nuclei but not callose-rich sieve plates, indicating that phloem-conducting cells in haustoria differ from conventional sieve elements (SE). However, normal SEs were present in phloem-conducting cells in tubercle protrusions. The retention of nuclei and lack of sieve plates suggest that SE maturation is retarded in the haustoria. We also investigated the expression profiles of genes involved in SE differentiation, including homologs of NAC-DOMAIN CONTAINING TRANSCRIPTION FACTOR (NAC45), NAC45/86-DEPENDENT EXONUCLEASE-DOMAIN PROTEIN 1 (NEN1), and NEN4. The relative expression levels of these genes were higher in haustoria than in the tubercle protrusions. This result was not consistent with the immaturity of sieve elements; therefore, nuclear degradation in haustorial GFP-conducting cells may not be controlled exclusively by the NAC45/86-NEN regulatory pathway. Our results also suggest that formation of plasmodesmata with a large size exclusion limit (SEL) is independent of nuclear degradation and callose deposition.