A SPECIES-SPECIFIC RECOGNITION SYSTEM DIRECTS HAUSTORIUM DEVELOPMENT IN THE PARASITIC PLANT TRIPHYSARIA (SCROPHULARIACEAE)

Parasitic giants use host molecules to trigger developmental programs essential for parasitism. One such program governs the initiation, dev elopment, and function of haustoria, parasite-specific organs responsi ble for attachment and invasion of host tissues. Haustoria development can be initiated by several different molecules produced by appropria te host species. We are interested in understanding how these signals are interpreted by two related facultative parasites, Triphysaria eria ntha (Benth.) Chuang and Heckard, and T. versicolor Fischer and C. Mey er, to distinguish their own roots from those of potential hosts. We u sed an in vitro bioassay to determine what proportion of different Tri physaria populations formed haustoria in the presence and absence of c losely related and unrelated host species. We found that the proportio n 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 congene ric Triphysaria. Plants with haustoria neither enhanced nor inhibited other plants' propensity to form haustoria. Together these results ind icate that qualitative differences exist in haustorium-inducing factor s exuded by closely related species. The highest proportion of Triphys aria had haustoria when grown with Arabidopsis thaliana (L.) Heynh. Ev en 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 anoth er Triphysaria. These results demonstrate that host recognition can be manifested at multiple points in haustorium development.