Do nonasterid holoparasitic flowering plants have plastid genomes?

Abstract

Past work involving the plastid genome (plastome) of holoparasitic plants has been confined to Scrophulariaceae (or Orobanchaceae) which have truncated plastomes owing to loss of photosynthetic and other genes. Nonasterid holoparasites from Balanophoraceae (Corynaea), Hydnoraceae (Hydnora) and Cytinaceae (Cytinus) were tested for the presence of plastid genes and a plastome. Using PCR, plastid 16S rDNA was successfully amplified and sequenced from the above three holoparasites. The sequence of Cytinus showed 121 single base substitutions relative to Nicotiana (8% of the molecule) whereas higher sequence divergence was observed in Hydnora and Corynaea (287 and 513 changes, respectively). Secondary structural models for these 16S rRNAs show that most changes are compensatory, thus suggesting they are functional. Probes constructed for 16S rDNA and for four plastid-encoded ribosomal protein genes (rps2, rps4, rps7 and rpl 16) were used in Southern blots of digested genomic DNA from the three holoparasites. Positive hybridizations were obtained using each of the five probes only for Cytinus. For Smal digests, all plastid gene probes hybridized to a common fragment ca. 20 kb in length in this species. Taken together, these data provide preliminary evidence suggestive of the retention of highly diverged and truncated plastid genome in Cytinus. The greater sequence divergence for 16S rDNA and the negative hybridization results for Hydnora and Corynaea suggests two possibilities: the loss of typically conserved elements of their plastomes or the complete absence of a plastome.