Over-expression of Chinese cabbage calreticulin 1, BrCRT1, enhances shoot and root regeneration, but retards plant growth in transgenic tobacco

Calreticulin (CRT) is a ubiquitously expressed, high capacity Ca ²⁺-binding protein that is involved in intracellular Ca²⁺ homeostasis and molecular chaperoning in the endoplasmic reticulum (ER). A cDNA encoding a calreticulin, BrCRT1 (Brassica rapa Calreticulin 1), has been isolated from Chinese cabbage (B. rapa subsp. pekinensis) flower bud. Constitutive over-expression of the BrCRT1 gene promotes robust shoot production and root formation at sub-optimal concentrations of BA/NAA, which are important factors controlling plant regeneration in tissue culture. In contrast, the suppressed BrCRT1 line exhibited a slight reduction of shoot and root regeneration. In spite of enhanced regeneration in tissue culture, the seedling and plant growth rate was inhibited in soil. The steady state level of BrCRT1 transcripts was sensitive to exogenous auxins and cytokinins, and rapidly accumulated within 30 min, and this induction required de novo protein synthesis. Together with the results of transgenic tobacco plants and mRNA analysis in Chinese cabbage, our data suggest that BrCRT1 genes may up-regulate the competency of vegetative tissue to respond to hormonal signals involved in shoot and root regeneration processes.