Abstract
BackgroundParasitic plants, represented by several thousand species of angiosperms, use modified structures known as haustoria to tap into photosynthetic host plants and extract nutrients and water. As a result of their direct plant-plant connections with their host plant, parasitic plants have special opportunities for horizontal gene transfer, the nonsexual transmission of genetic material across species boundaries. There is increasing evidence that parasitic plants have served as recipients and donors of horizontal gene transfer (HGT), but the long-term impacts of eukaryotic HGT in parasitic plants are largely unknown.ResultsHere we show that a gene encoding albumin 1 KNOTTIN-like protein, closely related to the albumin 1 genes only known from papilionoid legumes, where they serve dual roles as food storage and insect toxin, was found in Phelipanche aegyptiaca and related parasitic species of family Orobanchaceae, and was likely acquired by a Phelipanche ancestor via HGT from a legume host based on phylogenetic analyses. The KNOTTINs are well known for their unique “disulfide through disulfide knot” structure and have been extensively studied in various contexts, including drug design. Genomic sequences from nine related parasite species were obtained, and 3D protein structure simulation tests and evolutionary constraint analyses were performed. The parasite gene we identified here retains the intron structure, six highly conserved cysteine residues necessary to form a KNOTTIN protein, and displays levels of purifying selection like those seen in legumes. The albumin 1 xenogene has evolved through >150 speciation events over ca. 16 million years, forming a small family of differentially expressed genes that may confer novel functions in the parasites. Moreover, further data show that a distantly related parasitic plant, Cuscuta, obtained two copies of albumin 1 KNOTTIN-like genes from legumes through a separate HGT event, suggesting that legume KNOTTIN structures have been repeatedly co-opted by parasitic plants.ConclusionsThe HGT-derived albumins in Phelipanche represent a novel example of how plants can acquire genes from other plants via HGT that then go on to duplicate, evolve, and retain the specialized features required to perform a unique host-derived function.
Highlights
Parasitic plants, represented by several thousand species of angiosperms, use modified structures known as haustoria to tap into photosynthetic host plants and extract nutrients and water
We show that a gene encoding albumin 1 KNOTTIN-like protein, closely related to the albumin 1 genes, only known from papilionoid legumes, serving dual roles in food storage and as insect toxins, was found in Phelipanche aegyptiaca and related parasitic species of family Orobanchaceae, and was likely acquired by a Phelipanche ancestor via horizontal gene transfer (HGT) from a legume host based on phylogenetic analyses
According to genomic sequences from nine related parasite species, 3D protein structure simulation tests, and evolutionary constraint analyses, the broomrape xenogene we identified here retains the intron structure, six highly conserved cysteine residues necessary to form a KNOTTIN protein, and displays levels of purifying selection like those seen in legumes
Summary
Parasitic plants, represented by several thousand species of angiosperms, use modified structures known as haustoria to tap into photosynthetic host plants and extract nutrients and water. Two recent reports by Yoshida et al [19] and Xi et al [25] were the first indications that nuclear protein coding sequences, likely obtained from their respective host species, could be integrated into the genomes of parasitic plants by HGT. These were important advances, but they provided few clues as to the long term impact of HGT, how the transgenes evolve, and how they may function.
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