Baskauf, dePamphilis, and Eickmeier, 1999

Baskauf, Carol J., William G. Eickmeier, and Claude W. dePamphilis, Abstracts for the 84th Annual Meetings of the Ecological Society of America, p. 48 (1999)

Parasitic Orobanche corymbosa lacks photosynthetic activity despite the presence of a conserved photosynthetic gene.

We compare in vivo gas-exchange data and in vitro RUBISCO activity for a holoparasite (Orobanche corymbosa), a hemiparasite (Pedicularis canadensis), and a nonparasite (Antirrhinum majus) in the Scrophulariaceae. Unlike hemiparasites, holoparasites appear to lack chlorophyll and depend entirely on their hosts for nutritional carbon. Freed from evolutionary constraints, photosynthetic genes (such as rbcL, coding for the large subunit of RUBISCO) have mutated freely or even been eliminated in some holoparasitic lineages. Surprisingly, the holoparasitic O. corymbosa maintains an apparently functional rbcL gene, with statistical analysis suggesting that the open reading frame (ORF) is being retained because of functional constraint rather than chance. Nonetheless, our study provides no evidence of in vitro RUBISCO activity or net photosynthetic CO2 uptake in the light for O. corymbosa. Similar levels of CO2 evolution occur under light and dark conditions. In contrast, photosynthetic activity is evident for both Antirrhinum and Pedicularis, with levels of CO2 uptake in the light and RUBISCO activity appearing to be higher for the nonparasite than for the hemiparasite. Evolutionary and functional evidence suggests that rbcL may encode a protein with an unexpected nonphotosynthetic function in O. corymbosa.

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