Occurrence and light response of residual plastid genes in a Euglena gracilis bleached mutant strain OflB2

Euglena gracilis is a unicellular green eukaryotic microalga that features characteristics of both plants and animals. The photosynthetic function of its chloroplast is easily lost under stress resulting in bleached mutants, while the physiological role of their residual plastid DNAs remains unclear. In this study, we obtained five bleached mutants by ofloxacin (Ofl) treatment, identified 12 residual plastid genes in five bleached mutants, and determined the mRNA levels in the wild type E. gracilis (WT) and one bleached mutant (OflB2) under dark and light stimulation conditions by quantitative reverse transcribed PCR (qRT-PCR). Results show that the expression of all selected plastid genes in both WT and OflB2 mutant did not change significantly in darkness, while their responses to light stimulation were different. Under the light stimulation conditions, half of the genes did not change significantly, while most of the other genes were down-regulated in OflB2 mutant and up-regulated in WT. Therefore, the bleached mutant retains part of the plastid genome and the plastid relic is responsive to light. Our research will help to understand the functions of residual plastid DNA and evolution of chloroplasts.