Plant Calreticulins - isoforms, localization and function

Abstract: Calreticulin (CRT) is an endoplasmic reticulum (ER)-located protein involved in quality control of newly synthesized proteins as well as in calcium homeostasis of the cell. Most of what is known about CRT arises from studies of mammalian systems. However, using a Arabidopsis thaliana CRT gene expressed in mouse fibroblasts deficient of endogenous CRT, the wild type phenotype could be rescued with respect to protein folding and calcium homeostasis, suggesting conserved functions of CRT in plants. Phylogenetic analysis of higher plant CRTs divides CRTs into two groups. In Arabidopsis, there are three CRT isoforms, CRT1a and CRT1b, belonging to one group, and CRT3, belonging to the other group. CRT1a and CRT1b genes exhibited similar expression patterns. They were expressed in all tissues investigated, but mainly expressed in different flower parts and in roots. Localization studies using isoform specific antibodies, however, revealed that the protein CRT1a was present throughout the tissue in Arabidopsis root tips, while CRT1b only was present in the root cap, epidermal cells, and in the protoxylem cells. Mutation studies further revealed that if the CRT1a gene was silenced, the plant only exhibited a conditional tunicamycin stress-induced phenotype, while a crt1b mutant was significantly smaller compared to the wild type even without stress exposure, indicating different functions of the two isoforms in plant development. With regard to seed production, however, both mutants produced fewer seeds compared to the wild type. The function of the CRT3 isoform is still unknown, although the CRT3 gene exhibited a different expression pattern compared to CRT1a and CRT1b. Preliminary data indicate that CRT3 cannot complement for CRT1a and CRT1b, supporting that they indeed play different cellular roles. Taken together, it is here shown that overlapping as well as specific functions characterize the different CRT isoforms in Arabidopsis.