Iron accumulation does not parallel the high expression level of ferritin in transgenic rice seeds

Abstract

To answer the question whether iron accumulation in transgenic rice seeds depends on the expression level of exogenous soybean ferritin, we generated two kinds of ferritin hyper-expressing rice lines by introducing soybean ferritin SoyferH-1 gene under the control of the rice seed storage glutelin gene promoter, GluB-1 and the rice seed storage globulin gene promoter, Glb-1, (GluB-1/SoyferH-1 and Glb-1/SoyferH-1, DF lines), and by introducing the SoyferH-1 gene under the control of Glb-1 promoter alone (Glb-1/SoyferH-1, OF lines). Ferritin expression was restricted to the endosperm in both lines and protein levels determined by western blot analysis were up to 13-fold higher than in a construct previously reported FK22 (GluB-1/SoyferH-1, in genetically Kitaake background); [corrected] however, the maximum iron concentrations in seeds of both of the new lines were only about 30% higher than FK22. The maximum iron concentration in the OF and DF lines was about threefold higher than in the non-transformant. The mean Fe concentration in leaves of ferritin over-expressing lines decreased to less than half of the non-transformant while that the plant biomasses and seed yields of the ferritin-transformed lines were not significantly different from those of the non-transformant, suggesting that accumulation of Fe in seeds of hyper-expression ferritin rice did not always depend on the expression level of exogenous ferritin but may have been limited by Fe uptake and transport. No obvious differences were observed for other divalent-metal concentrations (Ca, Cd, Cu, Mg, Mn and Zn) in the seeds among all experimental lines and non-transformant.