BT1, A PROTEIN CRITICAL FOR IN-VIVO STARCH ACCUMULATION IN MAIZE ENDOSPERM, IS NOT DETECTED IN MAIZE ENDOSPERM SUSPENSION-CULTURES

Maize Bt1 is a structural gene for the major 39-44-kDa amyloplast memb rane polypeptides. Loss of BT1 in the brittle1 (bt1) mutant results in an 80% reduction in kernel starch. As an initial step toward the unde rstanding of BT1 function, we investigated the relation of BT1 to star ch accumulation in the two well-characterized maize endosperm suspensi on-cultured cell lines which were derived from 10 days post pollinatio n (DPP) kernels of inbred A636 and 12-DPP kernels of the waxy mutant i n 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 cul tured eels. Bt1 transcripts were detected in 10- and 20-DPP A636 kerne ls, but only a trace of Bt1 transcripts was detected in the suspension -cultured cells. Southern blotting indicated that the Bt1 gene was pre sent in the genome of the cultured cells. The Bt1 gene products in A63 6 endosperm cultures were not increased by treatments which enhance st arch accumulation such as sucrose supplementation, chlorocholine chlor ide (CCC) addition to the medium, or both. A similar pattern of differ ential Bt1 gene expression was found in the waxy endosperm cultures. T hese results indicate that starch accumulation in the cultured cells i s not correlated with Bt1 gene expression. The significance of these f indings is discussed.