Other types of storage plastids

In addition to coloured pigments and starch, plastids are capable of accumulating other types of storage material. These can include lipids, which accumulate in elaioplasts and proteins, which accumulate in proteinoplasts. In both cases such plastids are found often in specialised cells within complex tissues. For instance, elaioplasts are commonly formed in the tapetal cells of the anther where they accumulate large amounts of neutral esters (Ting et al. 1998), which are released by elaioplast breakdown and contribute to the lipid component of the pollen wall (Clement and Pacini 2001). Storage lipids in plastids occur in structures called plastoglobules, which are commonly found in all plastid types. It is the extent of plastoglobule production, which essentially defines an elaioplast from any other plastid type, since elaioplasts are generally packed full of plastoglobuli. A recent proteome analysis of plastoglobuli reveals they contain several proteins involved in metabolism of isoprenoid derived molecules as well as fibrillins, which form a protein coat around the exterior of the plastoglobulus preventing coalescence (Yt-terberg et al. 2006). This suggests that plastoglobuli have a metabolic role in the plastid rather than simply being a storage sac. It is unclear whether such a pro-

teome and metabolome profile varies significantly between plastoglobuli in elaio-plasts and those plastoglobuli, which appear less abundantly in other plastid types such as chloroplasts.

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