Chloroplasts and mitochondria are both thought to have arisen through primary endosymbiosis (Kutschera and Niklas,2005).As each organelle evolved,most of its genome was transferred to the host nucleus resulting in the vast majority of its proteome being nuclear encoded and synthesized on cytosolic ribosomes (Shi and Theg,2013).Evolution of targeting mechanisms and molecular machinery to recognize and facilitate the import of all of these proteins was critical in the conversion of the ancient endosymbiont into an organelle.Most nuclear-encoded chloroplast and mitochondrial proteins are synthesized as precursors containing cleavable targeting peptides at the N-terminus,referred to here as chloroplast transit peptides (cTPs) and mitochondrial targeting peptides (mTPs),respectively (Wollman,2016).These targeting peptides help direct the precursor to the organellar surface where they are recognized by receptors of the TOC (chloroplast) or TOM (mitochondria) import machinery of the outer membrane.Following precursor recognition,a series of events leads to complete translocation into the stroma or matrix.