The Use of Arabidopsis to Study Interactions between Parasitic Angiosperms and Their Plant Hosts

Abstract

Parasitic plants invade host plants in order to rob them of water, minerals and nutrients. The consequences to the infected hosts can be debilitating and some of the world's most pernicious agricultural weeds are parasitic. Parasitic genera of the Scrophulariaceae and Orobanchaceae directly invade roots of neighboring plants via underground structures called haustoria. The mechanisms by which these parasites identify and associate with host plants present unsurpassed opportunities for studying chemical signaling in plant-plant interactions. Seeds of some parasites require specific host factors for efficient germination, thereby insuring the availability of an appropriate host root prior to germination. A second set of signal molecules is required to induce haustorium development and the beginning of heterotrophy. Later stages in parasitism also require the presence of host factors, although these have not yet been well characterized. Arabidopsis is being used as a model host plant to identify genetic loci associated with stimulating parasite germination, haustorium development, and parasite support. Arabidopsis is also being employed to explore how host plants respond to parasite attack. Current methodologies and recent findings in Arabidopsis - parasitic plant interactions will be discussed.

Figure 1: Life cycles of parasitic Scrophulariaceae and Orobanchaceae

Figure 2: Host recognition factors The naturally occurring strigolactone germination stimulants strigol, sorgolactone, and alectra are shown (Wigchert et al., 1999). Another sorgolactone stimulant for Orobanche germination is orobanchol, and apparent isomer of strigol (Yokota, 1998). Another class of natural germination stimulants is represented by dihydrosorgoleone (Fate and Lynn, 1996). Haustoria are induced by a variety of plant phenolics including flavonoids (Lynn et al., 1981; Albrecht et al., 1999), quinones (Chang and Lynn, 1986), and phenolic acids (Riopel and Timko, 1995).

Figure 3: Triphysaria haustoria on Arabidopsis root The photo on the left shows a haustorium (H) forming on the root of Triphysaria (T) upon contact with an Arabidopsis root (A) in vitro. The photo on the right is a free hand section through a haustorium and shows the xylem bridge (XB) (photo by Hugette Albrecht). The Arabidopsis root has been detached from the haustorium and is not seen in this section.

Figure 4: In vitro assay of Orobanche parasitism of Arabidopsis The polyethylene bag method is used to monitor the germination, attachment and invasion of O. aegyptiaca into Arabidopsis roots.

Figure 5: Xylem formation with Arabidopsis auxin variants Triphysaria were grown with different Arabidopsis mutants in agar. Haustoria that developed were cleared and examined for xylem bridges. The proportion of haustoria with xylem bridges is shown for each mutant host. The Standard Error was calculated as with n being the number of haustoria examined.

Figure 6: Orobanche growth on Arabidopsis in pots Arabidopsis supporting a flowering O. aegyptiaca

Figure 7: High throughput screen for germination stimulants A single Arabidopsis seedling is added to agar containing preconditioned Orobanche seeds in a 96 well microtiter plate. Germination is scored under a low power microscope.