We are studying how parasitic plants in the Orobanchaceae family, such as witchweed and broomrape, detect their hosts. It is well established that strigolactones trigger germination of these parasites, but the genes and mechanism of host-detection have been a longstanding mystery. Every year parasitic plants cause billions of dollars in agricultural losses in Africa, the Mediterranean, and parts of Asia. It is our hope that understanding how different strigolactones are recognized by parasites will allow us to create chemical or genetic methods to either block parasite germination or trigger suicidal germination.
KAI2 is a karrikin-responsive receptor that regulates seed germination in Arabidopsis. D14 is a strigolactone-specific receptor, but does not function at early stages of development in Arabidopsis. We found that parasitic species have extra gene copies of KAI2. These genes can be categorized into three groups that have undergone different rates of evolution. We found that a fast-evolving group of KAI2 genes in parasites (KAI2d) encode proteins with enlarged ligand-binding pockets, similar to D14, and are able to confer germination responses to strigolactones when introduced into Arabidopsis thaliana. These KAI2 genes allow parasitic plants to sense strigolactones from a host and grow. A similar evolutionary process may have given rise to the strigolactone receptor D14 in seed-bearing plants.