The Ca Status of the Endoplasmic Reticulum Is Altered by Induction of Calreticulin Expression in Transgenic Plants

To investigate the endoplasmic reticulum (ER) Ca stores in plant cells, we generated tobacco (Nicotiana tabacum; NT1) suspension cells and Arabidopsis plants with altered levels of calreticulin (CRT), an ER-localized Ca-binding protein. NT1 cells and Arabidopsis plants were transformed with a maize (Zea mays) CRT gene in both sense and antisense orientations under the control of an Arabidopsis heat shock promoter. ER-enriched membrane fractions from NT1 cells were used to examine how altered expression of CRT affects Ca uptake and release. We found that a 2.5-fold increase in CRT led to a 2-fold increase in ATP-dependent Ca accumulation in the ER-enriched fraction compared with heat-shocked wild-type controls. Furthermore, after treatment with the Ca ionophore ionomycin, ER microsomes from NT1 cells overproducing CRT showed a 2-fold increase in the amount of Ca released, and a 2to 3-fold increase in the amount of Ca retained compared with wild type. These data indicate that altering the production of CRT affects the ER Ca pool. In addition, CRT transgenic Arabidopsis plants were used to determine if altered CRT levels had any physiological effects. We found that the level of CRT in heat shock-induced CRT transgenic plants correlated positively with the retention of chlorophyll when the plants were transferred from Ca-containing medium to Ca-depleted medium. Together these data are consistent with the hypothesis that increasing CRT in the ER increases the ER Ca stores and thereby enhances the survival of plants grown in low Ca medium.