The chaperone calreticulin plays important roles in a variety of processes in the endoplasmic reticulum (ER) of animal cells, such as Ca2+ signaling and protein folding. Although the functions of calreticulin are well characterized in animals, only indirect evidence is available for plants. To increase our understanding of plant calreticulins we introduced one of the Arabidopsis isoforms, AtCRT1a, into calreticulin-deficient (crt-/-) mouse embryonic fibroblasts. As a result of calreticulin deficiency, the mouse crt-/- fibroblasts have decreased levels of Ca2+ in the ER and impaired protein folding abilities. Expression of the AtCRT1a in mouse crt-/- fibroblasts rescued these phenotypes, i.e. AtCRT1a restored the Ca2+-holding capacity and chaperone functions in the ER of the mouse crt-/- fibroblasts, demonstrating that the animal sorting machinery was also functional for a plant protein, and that basic calreticulin functions are conserved across the Kingdoms. Expression analyses using a beta-glucuronidase (GUS)-AtCRT1a promoter construct revealed high expression of CRT1a in root tips, floral tissues and in association with vascular bundles. To assess the impact of AtCRT1a in planta, we generated Atcrt1a mutant plants. The Atcrt1a mutants exhibited increased sensitivity to the drug tunicamycin, an inducer of the unfolded protein response. We therefore conclude that AtCRT1a is an alleviator of the tunicamycin-induced unfolded protein response, and propose that the use of the mouse crt-/- fibroblasts as a calreticulin expression system may prove useful to assess functionalities of calreticulins from different species.