BT1, a protein critical for in vivo starch accumulation in maize endosperm, is not detected in maize endosperm suspension cultures

Abstract

Maize Bt1 is a structural gene for the major 39–44‐kDa amyloplast membrane polypeptides. Loss of BT1 in the brittle1 (bt1) mutant results in an 80% reduction in kernel starch. As an initial step toward the understanding of BT1 function, we investigated the relation of BT1 to starch accumulation in the two well‐characterized maize endosperm suspension‐cultured cell lines which were derived from 10 days post pollination (DPP) kernels of inbred A636 and 12‐DPP kernels of the waxy mutant in the A636 background. Starch in A636 endosperm cultures accounted for about 1.5% of the fresh weight of cells but BT1 was not detectable in amyloplast membranes or in microsomal membranes isolated from the cultured cells. Bt1 transcripts were detected in 10‐ and 20‐DPP A636 kernels, but only a trace of Bt1 transcripts was detected in the suspension‐cultured cells. Southern blotting indicated that the Bt1 gene was present in the genome of the cultured cells. The Bt1 gene products in A636 endosperm cultures were not increased by treatments which enhance starch accumulation such as sucrose supplementation, chlorocholine chloride (CCC) addition to the medium, or both. A similar pattern of differential Bt1 gene expression was found in the waxy endosperm cultures. These results indicate that starch accumulation in the cultured cells is not correlated with Bt1 gene expression. The significance of these findings is discussed.