Structure and development of the upper haustorium in the parasitic flowering plant Cuscuta japonica (Convolvulaceae)

Abstract

The anatomical and ultrastructural development of the haustorium of the Cuscuta japonica, a holoparasitic angiosperm, growing on the host plant Impatiens balsamina was studied. After the shoot tips of light-grown parasite seedlings contacted the host, the upper haustorium (external to the host organ) developed through three main successive stages of the haustorial initials, the meristem, and the endophyte primoridium (EP) within the middle layer of the cortex of the parasite stem. The haustorial initial cells were characterized by abundant starch-bearing amyloplasts and mitochondria with an expanded intermembrane space. The meristem cells had numerous large chloroplasts with well-developed thylakoids, reflecting the capability for photosynthesis. Commonly, all three stages of haustorial cells contained conspicuous, large nuclei with enlarged nucleoli and dense cytoplasm including many other organelles, indicating a very active metabolism. In the final stage of upper haustorium development, the meristem cells differentiated into the EP, a host-penetrating tissue. The primordium had smaller file cells at the proximal end and elongate digitate cells at the distal end. The file cells divided actively, while the digitate cells contained abundant chloroplasts, dictyosomes, rough endoplasmic reticulum, and other organelles, suggesting that the EP was cytohistologically well organized for penetration into the host tissue.

Interspecific RNA Interference of SHOOT MERISTEMLESS-Like Disrupts Cuscuta pentagona Plant Parasitism

“…Previous literature interprets the dodder haustorium as a highly modified adventitious root (Truscott, 1958;Kuijt, 1976;Lee and Chai, 1989;Lee, 2007). Ultrastructural analysis of the origin of adventitious roots in cuttings of the distant relative petunia (Petunia hybrida) show their origin from cell divisions within the cambium layer itself (Ahkami et al, 2009) in a manner distinct from the dedifferentiation of cortical parenchyma in dodder.…”

“…Subsequent growth is supported by continued cell division activity in this region, and in the developing prehaustorium, three zones are visible. The vascular proximal cells are followed by a zone of dividing cells called file cells (marked with an asterisk in Figure 1E) (Lee, 2007). A distal-most zone of large cells with prominent nuclei called digitate cells (Lee, 2007) is evident at the tip of the prehaustoria (arrowhead in Figure 1E).…”

“…The vascular proximal cells are followed by a zone of dividing cells called file cells (marked with an asterisk in Figure 1E) (Lee, 2007). A distal-most zone of large cells with prominent nuclei called digitate cells (Lee, 2007) is evident at the tip of the prehaustoria (arrowhead in Figure 1E). These cells are likely to be the precursors of searching hyphae.…”

“…The haustorium initial originates from dedifferentiation of cortical parenchyma between the vascular stele and the epidermis (Lee, 2007), on the side of the dodder strand in contact with the host, in response to mechanical, light, and hormonal cues (Haidar et al, 1998;Furuhashi et al, 2011). The cells of the epidermis in contact with the host become elongated and branched (Lee, 2007), forming an adhesive junction above a region of meristematic cells (Lee, 2007).…”

“…The origin and development of the haustorium has previously been studied in detail in Cuscuta australis and Cuscuta japonica (Lee, 2007). The haustorium initial originates from dedifferentiation of cortical parenchyma between the vascular stele and the epidermis (Lee, 2007), on the side of the dodder strand in contact with the host, in response to mechanical, light, and hormonal cues (Haidar et al, 1998;Furuhashi et al, 2011).…”