The role of Cercospora zeae-maydis homologs of Rhodobacter sphaeroides ¹O2-resistance genes in resistance to the photoactivated toxin cercosporin.

The photosynthetic bacterium Rhodobacter sphaeroides and plant pathogenic fungus Cercospora nicotianae have been used as models for understanding resistance to singlet oxygen (¹O2), a highly toxic reactive oxygen species. In Rhodobacter and Cercospora, ¹O2 is derived, respectively, from photosynthesis and from the ¹O2-generating toxin cercosporin which the fungus produces to parasitize plants. We identified common genes recovered in transcriptome studies of putative ¹O2-resistance genes in these two systems, suggesting common ¹O2-resistance mechanisms. To determine if the Cercospora homologs of R. sphaeroides ¹O2-resistance genes are involved in resistance to cercosporin, we expressed the genes in the cercosporin-sensitive fungus Neurospora crassa and assayed for increases in cercosporin resistance. Neurospora crassa transformants expressing genes encoding aldo/keto reductase, succinyl-CoA ligase, O-acetylhomoserine (thiol) lyase, peptide methionine sulphoxide reductase and glutathione S-transferase did not have elevated levels of cercosporin resistance. Several transformants expressing aldehyde dehydrogenase were significantly more resistant to cercosporin. Expression of the transgene and enzyme activity did not correlate with resistance, however. We conclude that although the genes tested in this study are important in ¹O2 resistance in R. sphaeroides, their Cercospora homologs are not involved in resistance to ¹O2 generated from cercosporin. [ABSTRACT FROM AUTHOR]