July 31, 2015

Conn et al. "Convergent evolution of strigolactone perception enabled host detection in parasitic plants"

Caitlin E. Conn1, Rohan Bythell-Douglas2, Drexel Neumann1, Satoko Yoshida3, Bryan Whittington4, 4, Ken Shirasu3, Charles S. Bond2, Kelly A. Dyer1, David C. Nelson1,*
James H. Westwood
1Department of Genetics, University of Georgia, Athens, GA 30602, USA
2School of Chemistry and Biochemistry, The University of Western Australia, Crawley, Western Australia 6009, Australia
3RIKEN Center for Sustainable Resource Science, Tsurumi-ku, Yokohama, Kanagawa 230-0045, Japan
4Department of Plant Pathology, Physiology, and Weed Science, Virginia Tech, Blacksburg, VA 24061, USA
Science 31 July 2015: Vol. 349 no. 6247 pp. 540-543 DOI: 10.1126/science.aab1140

Here's an interesting story on the perception of strigolactones (plant branching hormones) and how the normal physiological process of sensing these hormones has been co-opted  by sneaky parasites. Conn et al. find that a modern day clade of an ancient paralog of the Arabidopsis strigolactone receptor D14 is overrepresented in the genomes of certain parasitic plants. While derived from the same ancient paralog KAI2, the KAI2d clade members are used by obligate parasites to locate host plants. KAI2d induces germination in Orobanchaceae members by sensing strigolactones in host plant root exudates so that these parasites only germinate when a suitable host is near. In other words, while D14 is used by plants like Arabidopsis to sense their own strigolactone production and respond accordingly by initiating branching, parasitic plants use a very similar receptor with a common evolutionary origin to sense the same hormone but in this case to locate a host to parasitize. In a strange case of convergent evolution, both modern day clades (D14 and KAI2d) function as strigolactone sensors but for very different ends.

From the article:
Obligate parasitic plants in the Orobanchaceae germinate after sensing plant hormones, strigolactones, exuded from host roots. In Arabidopsis thaliana, the α/β-hydrolase D14 acts as a strigolactone receptor that controls shoot branching, whereas its ancestral paralog, KAI2, mediates karrikin-specific germination responses. We observed that KAI2, but not D14, is present at higher copy numbers in parasitic species than in nonparasitic relatives. KAI2 paralogs in parasites are distributed into three phylogenetic clades. The fastest-evolving clade, KAI2d, contains the majority of KAI2 paralogs. Homology models predict that the ligand-binding pockets of KAI2d resemble D14. KAI2d transgenes confer strigolactone-specific germination responses to Arabidopsis thaliana. Thus, the KAI2 paralogs D14 and KAI2d underwent convergent evolution of strigolactone recognition, respectively enabling developmental responses to strigolactones in angiosperms and host detection in parasites.

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