Conversion of the Searching Hyphae of Dodder into Xylic and Phloic Hyphae: A Cytochemical and Immunocytochemical Investigation

Dodders are obligate parasites, requiring both water and carbohydrate to be supplied by the host plant. To achieve the successful flow of both carbohydrate and water, the searching hyphae of the dodder must penetrate the host and transform into xylic or phloic hyphae, depending on the cell type that the hyphae encounter. The phloic hyphae develop a massive handlike appendage that surrounds the host phloem, whereas the xylic hyphae are directly connected to the host. In this study, changes in wall composition and structure are monitored via structural, cytochemical, and immunocytochemical techniques that allow one to discriminate changes in the hyphae as they make the transition into functional xylem and phloem. Although the terminal structure of the phloic hyphae has been termed a transfer cell, it bears little resemblance to those cell types; few if any wall ingrowths are noted. Rather, an extensive array of smooth endoplasmic reticulum (ER) is aligned at right angles and adjacent to the host phloem cells; the wall of the phloic hyphae becomes reduced in cellulose and xyloglucans and enriched in pectins. This loosening of its own wall (by loss of cellulose-xyloglucan and increased pectin) and the presence of abundant smooth ER facilitates the transfer of saccharides apoplastically into the parasite. In contrast to the phloic hyphae, xylic hyphae achieve a direct connection between the host and parasite, so that a flow of water is maintained between the host and the parasite. Xylic hyphae do not form secondary walls as do normal xylem elements. Rather, areas of the xylic hyphae have areas where the secondary wall is produced evenly along their length as well as in less regular true tracheid-type secondary walls. Similarly, the xylans that are characteristic of secondary walls are found both in the secondary thickenings and along extensive stretches of the mature xylic hyphae. In some xylic hyphae, both continuous and sporadic thickenings are observed. The host xylem and the xylem of the haustoria have xylans limited to secondary thickenings only, however. These data indicate that although the xylic and phloic hyphae mimic xylem and phloem elements, they retain some unique qualities to facilitate their parasitic function.