Calreticulin is enriched in the crystal idioblasts of Pistia stratiotes

Calreticulin (CRT) has the highest calcium-binding capacity of the endoplasmic reticulum (ER) proteins found thus far in plants. In this study, we isolated cDNAs encoding CRT from Pistia stratiotes L. (Araceae) and analyzed the temporal and spatial patterns of CRT expression in leaves. Northern analysis showed young leaves had the highest CRT transcript levels, which decreased as the leaves matured. To investigate differences in the expression and cellular localization of CRT, in situ hybridization and immunolocalization were performed. We observed an enrichment of both CRT transcript and protein in a unique cell type called the crystal idioblast. Crystal idioblasts are specialized cells that act as calcium sinks in tissues such as leaves. Sub-cellular localization showed that the CRT protein partitioned into the ER of the crystal idioblasts. At higher resolution, using transmission electron microscopy (TEM), it appeared that the CRT protein was more abundant in “dilated” regions of the ER. Although the specific role(s) CRT plays in the idioblast remains to be determined, we hypothesize that this high-capacity calcium-binding protein functions in the regulation of intracellular idioblast calcium levels.