Transformation and Nucleic Acid Delivery to Mitochondria

Two types of mt DNA are present in the embryophytes (land plants). The mitochon-dria of the three clades of bryophytes (liver-worts, hornworts and mosses, see chapter " Promiscuous Organellar DNA ") possess circular genomes of around 105–185 kb resembling that of their green algal relatives like Chara and even the early-branching nongreen eukaryote, Reclinomonas ameri-cana (Li et al. 2009). They are therefore considered to be of the ancestral type despite the acquisition of several additional features such as moderate size increase, intron gain and RNA editing, which are generally absent in algal genomes (Wang et al. 2009). On the contrary, the mitochondria of seed plants (see Chap. 10 " Horizontal Gene Transfer in Eukaryotes: Fungi-to-Plant and Plant-to-Plant Transfers of organellar DNA ") harbor Abbreviations: mt – Mitochondrial; tRNA – Transfer RNA; VDAC – Voltage-dependent anion channel Summary Genomic, transcriptomic and proteomic approaches have yielded considerable information, which impacted our understanding of the interactions between the nucleus and the mitochon-dria. Plant mitochondrial (mt) genomes are very large (220–2,000 kb) and often occur as complex pools of recombined molecules whose stoichiometry is tightly controlled by the nucleus. Unlike their mammalian and fungal counterparts, plant mt transcripts undergo complex post-transcriptional modifi cations such as editing and trans-splicing. Due to the impossibility to stably transform plant mitochondria and hence to manipulate mt gene expression, the genetic regulation of plant mt genomes has remained poorly understood. In this chapter, we will review the experimental data concerning the unicellular green alga Chlamydomonas reinhardtii, the only photosynthetic organism for which mt transformation has been achieved. Although Chlamydomonas harbors an extremely compact linear mt genome (15.8 kb) that differs from the one typically found in vascular plants, this system could bring novel insights on the role of the few subunits of the respiratory chain that are encoded in the mt genome. This is particularly relevant for the nd genes, which encode sub-units of complex I since the yeast Saccharomyces cerevisiae, the other unicellular organism where mt transformation is performed nearly at will, is deprived of complex I. Moreover, because the Chlamydomonas mt genome only encodes three tRNAs, genetic manipulation of the organellar genome is a promising avenue to dissect the import of cytosolic tRNAs, a process that is now known to take place in plant and also human mitochondria. We also present alternative approaches such as the in vitro import of DNA or RNA and electroporation of isolated …