A species-specific recognition system directs haustorium development in the parasitic plant Triphysaria (Scrophulariaceae).

Parasitic plants use host molecules to trigger development programs essential for parasitism. One such program governs the initiation, development, and function of haustoria, parasite-specific organs responsible for attachment and invasion of host tissues. Haustoria development can be initiated by several different molecules produced by appropriate host species. We are interested in understanding how these signals are interpreted by two related facultative parasites, Triphysaria eriantha (Benth). Chuang and Heckard, and T. versicolor Fischer and C. Meyer, to distinguish their own roots from those of potential hosts. We used an in vitro bioassay to determine what proportion of different Triphysaria populations formed haustoria in the presence and absence of closely related and unrelated host species. We found that the proportion of plants with haustoria was the same whether the plants were grown in isolation or with a conspecific host. In contrast, a significantly higher proportion of plants made haustoria when the host was a congeneric Triphysaria. Plants with haustoria neither enhanced nor inhibited other plants' propensity to form haustoria. Together these results indicate that qualitative differences exist in haustorium-inducing factors exuded by closely related species. The highest proportion of Triphysaria had haustoria when growth with Arabidopsis thaliana (L.) Heynh. Even in this case, however, some Triphysaria failed to develop haustoria. Interestingly, the percentage of haustoria that had vessel elements was higher when connections were made with Arabidopsis than with another Triphysaria. These results demonstrate that host recognition can be manifested at multiple points in haustorium development.