Cytoplasm and chloroplasts are not suitable subcellular locations for ß-zein accumulation in transgenic plants

Zeins, the main storage proteins of maize that accumulate in the endiplasmic reticulum of the endosperm cells, are particularly interesting because they are rich in the essential sulphur amino acids. Overexpression of certain zein genes in plants such as alfalfa would be expected to improve the nutritional characteristics of this crop. Recently, significant accumulation values have been reached, but still far from those considered useful for nutritional purposes. This study invstigates whether targeting to compartments other than the endoplasmic reticulum (cytosol and chloroplasts) could result in increasing ß-zein to the cytosol, the fragment which codes for the signal peptide has been removed. ß-zein has also been targeted to alfalfa and tobacco chloroplasts by a transit peptide signal. Both tobacco, as a model plant species, and alfalfa have been transformed with the assembled constructs. An alternative route to accumulate ß-zein in the chloroplasts is to synthesize ß-zein directly in the plastid lumen. Thus, the ß-zein gene has also been inserted into tobacco plastid DNA. The ß-zein gene in each different type of transformed plant was properly transcribed, as determined by northern blot analysis, but no accumulation of ß-zein was detected, either in the cytoplasm or in the chloroplasts of alfalfa and tobacco transformed plants. Therefore, it is concluded that chloroplasts and the cytosol are not favourable subcellular locations for zein protein accumulation.