Organization of Vascular Cells in the Haustorium of the Parasitic Flowering Plant Cuscuta japonica.

The stem parasite dodder, Cuscuta japonica, has evolved a specialized root-like organ, the haustorium, which is differentiated from the stem. In order to take up water and nutrients, C. japonica reprograms haustorial cells to vascular cells, connecting the host's vascular system to its own. However, little is known about vascular differentiation in haustoria. In this study, we first confirmed the temporal and spatial expression profiles of vascular cell type-specific genes, CjAPL, CjSEOR1, CjWOX4 and CjTED7, to examine whether phloem companion cells, developing sieve elements, procambial cells and differentiating xylem cells, respectively, are present in the haustoria. CjAPL and CjSEOR1 decreased, and CjWOX4 showed a transient increase before the onset of xylem vessel formation, and then decreased. CjTED7 increased coincidentally with xylem vessel formation. In situ hybridization demonstrated that CjWOX4-expressing cells and phloem-conducting cells are in close proximity, and occupied a domain distinguishable from xylem vessels, suggesting differentiation of a phloem/procambial domain and a xylem domain in the haustorium. Secondly, expression of regulatory genes that are involved in determination of the fate of procambial cells was investigated. Expression patterns of CjCLE41, CjGSK3 and CjBES1suggested that TDIF-TDR-GSK3-mediated signaling is activated in haustoria. The natural antisense transcript of CjCLE41 was detected in haustoria, implying the sense regulation of CjCLE41. Expression profiles of the regulatory genes, combined with those of cell type-specific marker genes, suggest that reprogramming of haustorial cells to vascular cells is regulated in a way that allows the immediate formation of xylem vessels by alleviating inhibition of xylem differentiation.