Maize BMS cultured cell lines survive with massive plastid gene loss

As part of developing an ex planta model system for the study of maize plastid and mitochondrial gene expression, a series of established Black Mexican Sweet (BMS) suspension cell lines was characterized. Although the initial assumption was that their organelle biochemistry would be similar enough to normal in planta cells to facilitate future work, each of the three lines was found to have plastid DNA (ptDNA) differing from control maize plants, in one case lacking as much as 70% of the genome. The other two BMS lines possessed either near-wild-type ptDNA or displayed an intermediate state of gene loss, suggesting that these clonal lines are rapidly evolving. Gene expression profiles of BMS cells varied dramatically from those in maize leaf chloroplasts, but resembled those of albino plants lacking plastid ribosomes. In spite of lacking most plastid gene expression and apparently mature rRNAs, BMS cells appear to import proteins from the cytoplasm in a normal manner. The regions retained in BMS ptDNAs point to a set of tRNA genes universally preserved among even highly reduced plastid genomes, whereas the other preserved regions may illuminate which plastid genes are truly indispensable for plant cell survival.