Parasitic plants. form follows function. Background. Parasitism occurs in at least 17 different families. 8 of which are considered weedy pests

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1 Parasitic plants form follows function Background Parasitism occurs in at least 17 different families. 8 of which are considered weedy pests degrees of parasitism: hemi-parasite or semi-parasite holo-parasite Scrophulariaceae Striga (witchweeds) green hemi-parasite attack roots of tropical cereals and legumes Orobanchaceae Orobanche (broomrapes) hemi- or holo-parasites that attach to roots affect legumes and vegetables Santalaceae Thesium (Sandalwoods) woody, green hemi-parasites most affect trees, but some attack roots of agricultural plants Balanophoraceae Balanophora perrenial root parasites Convolvulaceae Cuscuta (Morning Glory) holo-parasitic stem parasites with vining yellow stems

2 affects mostly herbaceous plants Lauraceae Cassytha (the Laurel family) resemble Cuscuta stem parasites Viscaceae & Loranthaceae Mistletoes green hemi-parasites attack aerial part of trees morphology of parasitism all true parasites require a way to attach to the host haustorium - bridge of tissue connecting the parasite to the host, allows for transport of water and nutrients from host development regime of parasites is different from that normal plants early development the early stages of angiosperm development Dodder development In Cuscuta, the early stages of development are much like that of other angiosperms. Dodder root development this leads to short, swollen roots with no root cap and root hairs present all the way to the end. Swollen morphology Morphology of dodder roots is similar to roots of normal angiosperms treated to disrupt the cytoskeleton.

3 Day 1 - Ultrastructure Cytoplasm has a high density of ribosomes. Golgi and secretory vesicles are abundant. The plasma membrane is tightly appressed to the cell wall. Day 2-3 Ultrastructure Osmiophilic particles associated with cell wall (CW) loosening are found in both the wall space and in membrane-bound vesicles just inside of the plasmalemma. Vacuole has begun to break up. Plasmalemma is detached from the cell wall in numerous locations (*). By 3 days lytic vesicles (L) are abundant in the cytosol. Day 5 - Ultrastructure In tissues near the root apex, cell structure is badly degenerated. Cells have collapsed. Organelles are absent or badly damaged. Summary of root adaptations The swollen appearance is due to the low level of microtubules thus neither mitotic divisions nor cell elongation may occur and the loosening of the cell wall allowing for isodiametric expansion. The radicular end of the dodder seedling should be considered a highly modified basal portion of stem tissue used as a food reserve and basal support rather than a root. This degeneration of the root end of the dodder may allow a flow of carbon from this organ to sustain the continued growth of the shoot. Chloroplast development and photosynthesis Ultrastructure Ultrastructure similar to other plants but reduced in degree of development Photosynthesis light reactions can be detected biochemically

4 Photosynthetic proteins proteins for both light and dark reactions are present photosynthesis across the Cuscuta line Summary The chloroplasts Cuscuta pentagona contain a number of the proteins required for a successful fixation of CO2 and these proteins in the thylakoids are organized much like other higher plants, with the exception of the large percentage of the thylakoids organized into grana. This genus is comprised of a number of species of varying photosynthetic capability, ranging from none [Cuscuta europaea (Machado and Zetsche 1990)] to fully competent Cuscuta reflexa [Choudhury and Sahu 1999]. Photosynthetic capacity seems to be limited to tissues of pre-parasitic seedlings, inflorescences, and haustorial regions. the haustorium Haustorial development in Mistletoe Dodder haustorium haustorial formation Invasion of the host tissue is necessary to connection to the vascular tissue. Attachment to the host plant requires a dramatic changes in the cells that lead to the haustorial connection. Haustorium formation begins with the extension of epidermal cells on the surface proximal to the host. Ultrastructure of the dodder haustorial cell This extension necessitates loosening of the cell walls, which is mediated via cell wall loosening complexes found in abundance in the trichome cells. The loosened wall are then cemented to the host was via pectinaceous materials Cementing the bond In the attachment of the haustorium to the relatively flat leaf surface, the epidermal cells of the dodder (D) for a relatively flat interface to the host (H). The electron-opaque cement (c) forms a tight seal between the dodder and the host.

5 Hyphal cells lower haustorium (H) is a peg of tissue inserted into the host. From the peg, elongated epidermal cells called hyphae (h) appear to grow through the host cells; invading hyphae shows vesicles (v) and particles, some of which are membrane-bound accumulate in the cytoplasm. (N - nucleus) Phloic hyphae To establish a connection to the hosts phloem, the dodder hyphae (D) first grows to surround the host (H) phloem (mv - host microvasculature, N - nucleus) the mature phloic hyphae (p) contains extensive proliferation of endoplasmic reticulum (arrows). Xylic hyphae To establish a connection to the hosts xylem (x), the dodder hyphae differentiate into xylic hyphae (xh) xylic differentiation occurs from host xylem backward toward the lower haustorium, whereas phloic differentiation occurs from the lower haustorium toward the host phloem. Overall summary parasitic plants have specialized adaptations that allow them to do their jobs all possess hyphal connections to the host plant via the haustorium that allow for uptake of water and nutrients from the host plant many aerial parasites no longer have true roots capacity for photosynthesis varies widely a good source of info on parasitic plants: