S. Persson et al., The Ca2+ status of the endoplasmic reticulum is altered by induction of calreticulin expression in transgenic plants, PLANT PHYSL, 126(3), 2001, pp. 1092-1104
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.