A proton pump (green) identifies a vacuole within the PSV (red).

A store is no good if its contents cannot be accessed. On page 991, Jiang et al. suggest that plant seeds might solve this problem by storing digestive enzymes in a membrane-bound structure called the globoid that is fully contained within the seed's protein storage vacuole (PSV). In theory, such a structure could protect the stored proteins from cleavage early in development, but then later allow access so the proteins can be mobilized for growth of the young seedling.

The globoid has been known for many years as a site where crystals of phytic acid (a phosphate source) are stored. Jiang et al. use permanganate staining to reveal that the compartment has a limiting membrane surrounding it. The membrane includes a vacuolar transmembrane protein, and the contents of the compartment include a lytic vacuolar protein. Thus, the authors suggest that this may be a degradative vacuole contained within the storage vacuole.

How this double vacuole structure might be generated remains a mystery. The lack of a double membrane suggests that the putative degradative compartment is probably not enveloped by the PSV. Choosing between other models for vacuolar formation will be easier once the relative orientation of globoid transmembrane proteins is known. ▪