THE BROWN MIDRIB3 (BM3) MUTATION IN MAIZE OCCURS IN THE GENE ENCODINGCAFFEIC ACID O-METHYLTRANSFERASE

The brown midrib mutations are among the earliest described in maize. Plants containing a brown midrib mutation exhibit a reddish brown pigm entation of the leaf midrib starting when there are four to six leaves . These mutations are known to alter lignin composition and digestibil ity of plants and therefore constitute prime candidates in the breedin g of silage maize. Here, we show that two independent brown midrib3 (b m3) mutations have resulted from structural changes in the COMT gene, which encodes the enzyme O-methyltransferase (COMT; EC 2.1.1.6), invol ved in lignin biosynthesis. Our results indicate that the bm3-1 allele (the reference mutant allele) has arisen from an insertional event pr oducing a COMT mRNA altered in both size and amount. By sequencing a C OMT cDNA clone obtained from bm3-1 maize, a retrotransposon with homol ogy to the B5 element has been found to be inserted near the junction of the 3' coding region of the COMT gene intron. The second bm3 allele , bm3-2, has resulted from a deletion of part of the COMT gene. These alterations of the COMT gene were confirmed by DNA gel blot and polyme rase chain reaction amplification analyses. These results clearly demo nstrate that mutations at the COMT gene give a brown midrib3 phenotype . Thus, the gene genetically recognized as bm3 is the same as the one coding for COMT.