The Ca2+ status of the endoplasmic reticulum is altered by induction of calreticulin expression in transgenic plants

To investigate the endoplasmic reticulum (ER) Ca2+ stores in plant cells, w e generated tobacco (Nicotiana tabacum; NT1) suspension cells and Arabidops is plants with altered levels of calreticulin (CRT), an ER-localized Ca2+-b inding protein. NT1 cells and Arabidopsis plants were transformed with a ma ize (Zea mays) CRT gene in both sense and antisense orientations under the control of an Arabidopsis heat shock promoter. ER-enriched membrane fractio ns from NT1 cells were used to examine how altered expression of CRT affect s Ca2+ uptake and release. We found that a 2.5-fold increase in CRT led to a 2-fold increase in ATP-dependent Ca-45(2+) accumulation in the ER-enriche d fraction compared with heat-shocked wild-type controls. Furthermore, afte r treatment with the Ca2+ ionophore ionomycin, ER microsomes from NT1 cells overproducing CRT showed a 2-fold increase in the amount of Ca-45(2+) rele ased, and a 2- to 3-fold increase in the amount of Ca-45(2+) retained compa red with wild type. These data indicate that altering the production of CRT affects the ER Ca2+ pool. Ln addition, CRT transgenic Arabidopsis plants w ere 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 wer e transferred from Ca2+-containing medium to Ca2+-depleted medium. Together these data are consistent with the hypothesis that increasing CRT in the E R increases the ER Ca2+ stores and thereby enhances the survival of plants grown in low Ca2+ medium.