Reply: Escaping a Low-Security Prison.

First, he states that a given protein will be predisposed to escape and move long distances when it is overexpressed on free ribosomes in the companion-cell (CC) cytosol. He suggests that retention rather than movement would be the default. This begs the question as to how the CC could retain all of its translated proteins when it is connected to the fast-flowing sieve element (SE) by numerous, large poreplasmodesmata units (PPUs). Perhaps it cannot, and small amounts of proteins may leak constantly into the translocation stream. Indeed, it seems very unlikely that every CC-protein is strongly anchored within the cytoplasm.