Submitted to: Plant Physiology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/22/2003
Publication Date: 1/22/2003
Citation: NAKATA, P.A., KOSTMAN, T.A., FRANCESCHI, V.R. CALRETICULIN IS ENRICHED IN THE CRYSTAL IDIOBLASTS OF PISTIA STRATIOTES. PLANT PHYSIOLOGY AND BIOCHEMISTRY. 2003. v. 41. p. 425-430. Interpretive Summary: We are interested in the regulation of calcium, an important nutrient, in plants. Therefore, we chose a particular type of protein called calreticulin, which is able to bind calcium, and, using a sophisticated microscope, we looked at where this calreticulin was expressed within water lettuce leaves at different stages of growth. We noticed there was much more calreticulin in young than old leaves. After seeing this protein in tissue form, this made us curious to find out where the calreticulin was expressed at the cellular level. We noticed there was a lot of calreticulin in a particular cell type known as a crystal idioblast. Using a high-powered microscope to pinpoint this protein's location within this cell type, we determined that the calreticulin was located within certain dilated areas in the endoplasmic reticulum. Now that we have this information, we can use it in future studies to evaluate some of our newly formed hypotheses about the role of this protein in the cell type. One theory is that calreticulin is involved in calcium regulation in that cell, because that cell accumulates a lot of calcium. In addition, that cell type might present a good system in which to study calcium regulation, in general. These laboratory pursuits are oriented toward improving human nutrition because at some point, we might be able to figure a way to boost the amount of absorbable calcium in plant food, like lettuce, that is eaten by children. Children need a lot of calcium to help build lifelong strength into their growing bones, but they often don't get enough of this basic nutrient in their contemporary diets.
Technical Abstract: 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 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. Subcellular 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.