Transcriptomes of the parasitic plant family Orobanchaceae reveal surprising conservation of chlorophyll synthesis

Abstract

Parasitism in flowering plants has evolved at least 11 times [1]. Only one family, Orobanchaceae, comprises all major nutritional types of parasites: facultative, hemiparasitic (partially photosynthetic), and holoparasitic (nonphotosynthetic) [2]. Additionally, the family includes Lindenbergia, a nonparasitic genus sister to all parasitic Orobanchaceae [3-6]. Parasitic Orobanchaceae include species with severe economic impacts: Striga (witchweed), for example, affects over 50 million hectares of crops in sub-Saharan Africa, causing more than $3 billion in damage annually [7]. Although gene losses and increased substitution rates have been characterized for parasitic plant plastid genomes [5, 8-11], the nuclear genome and transcriptome remain largely unexplored. The Parasitic Plant Genome Project (PPGP; http://ppgp.huck.psu.edu/) [2] is leveraging the natural variation in Orobanchaceae to explore the evolution and genomic consequences of parasitism in plants through a massive transcriptome and gene discovery project involving Triphysaria versicolor (facultative hemiparasite), Striga hermonthica (obligate hemiparasite), and Phelipanche aegyptiaca (Orobanche [12]; holoparasite). Here we present the first set of large-scale genomic resources for parasitic plant comparative biology. Transcriptomes of above-ground tissues reveal that, in addition to the predictable loss of photosynthesis-related gene expression in P. aegyptiaca, the nonphotosynthetic parasite retains an intact, expressed, and selectively constrained chlorophyll synthesis pathway.