Plant cell drawing with the main cell compartments for recombinant protein expression and targeting. Right: Nuclear encoded genes are expressed and targeted to different subcellular compartments using one of two different pathways, indicated by black or grey arrows. On one hand, translation occurs directly into the ER lumen by ribosomes associated with the ER. From here, proteins are transferred into vesicles via the Golgi apparatus (black arrow (1)) to the apoplast (black arrow (2a)) or vacuole (black arrow (2b)) and secreted. Other gene transcripts are translated by free ribosomes (grey arrow (1)) and targeted to the chloroplast (grey arrow (2a)) or mitochondrion (grey arrow (2b)) via specific transit peptides and protein import mechanisms (grey arrows). Left: Enlarged section of the plant cell wall which is primarily composed of cellulose, lignin and hemicellulose. ER: endoplasmic reticulum; TOC/TIC: translocon at the outer envelope of chloroplasts/translocon at the inner envelope of chloroplasts; TOM/TIM: translocase of the outer mitochondrial membrane/translocase of the inner membrane.