Parasitic Plants Striga and Phelipanche Dependent upon Exogenous Strigolactones for Germination Have Retained Genes for Strigolactone Biosynthesis

Abstract

Strigolactones are plant hormones with multiple functions, including regulating various aspects of plant architecture such as shoot branching, facilitating the colonization of plant roots by arbuscu-lar mycorrhizal fungi, and acting as seed germination stimulants for certain parasitic plants of the family Orobanchaceae. The obligate parasitic species Phelipanche aegyptiaca and Striga hermon-thica require strigolactones for germination, while the facultative parasite Triphysaria versicolor does not. It has been hypothesized that P. aegyptiaca and S. hermonthica would have undergone evolutionary loss of strigolactone biosynthesis as a part of their mechanism to enable specific de-tection of exogenous strigolactones. We analyzed the transcriptomes of P. aegyptiaca, S. hermon-thica and T. versicolor and identified genes known to act in strigolactone synthesis (D27, CCD7, CCD8, and MAX1), perception (MAX2 and D14) and transport (PDR12). These genes were then analyzed to assess likelihood of function. Transcripts of all strigolactone-related genes were found M. Das et al. 1152 in P. aegyptiaca and S. hermonthica, and evidence points to their encoding functional proteins. Gene open reading frames were consistent with homologs from Arabidopsis and other strigolac-tone-producing plants, and all genes were expressed in parasite tissues. In general, the genes re-lated to strigolactone synthesis and perception appeared to be evolving under codon-based selec-tive constraints in strigolactone-dependent species. Bioassays of S. hermonthica root extracts in-dicated the presence of strigolactone class stimulants on germination of P. aegyptiaca seeds. Tak-en together, these results indicate that Phelipanche aegyptiaca and S. hermonthica have retained functional genes involved in strigolactone biosynthesis, suggesting that the parasites use both en-dogenous and exogenous strigolactones and have mechanisms to differentiate the two.