Maize alpha-tubulin genes are expressed according to specific patterns of cell differentiation.

In the past few years many alpha- and beta-tubulin genes of different organisms have been cloned and studied, and in most systems studied they constitute multigene families. In plants, most studies have been done in Arabidopsis thaliana and Zea mays. In this paper, the study of mRNA accumulation by in situ hybridization and the activity of three maize alpha-tubulin gene promoters (tua1, tua2 and tua3) in transgenic tobacco plants are described. In maize, the expression of these three tubulin isotypes differ in the root and shoot apex and is associated with different groups of cells throughout the distinct stages of cell differentiation. In transgenic tobacco plants the promoters of the genes, fused to the uidA reporter gene (GUS), direct expression to the same tissues observed by in situ hybridization experiments. The tua1 promoter is mainly active in cortex-producing meristematic cells and in pollen, whereas tua3 is active in cells which are differentiating to form vascular bundles in the root and shoot apices. The accumulation of tua2 mRNA is detected by RNA blot in a similar form as tua1, but at a very much low level. In situ hybridization indicates that the tua2 mRNA specifically accumulates in the maize root epidermis. No GUS staining was detected in transgenic tobacco plants with the tua2 promoter. The difference in expression of the specific genes may be linked to processes where microtubules have different functions, suggesting that in plants, as in animals, there are differences in the function of the tubulin isotypes.

The maize caffeic acid O-methyltransferase gene promoter is active in transgenic tobacco and maize plant tissues.

The pattern of expression directed by the promoter of the maize caffeic acid O-methyltransferase (COMT) gene was studied by histochemical and fluorometric beta-glucuronidase (GUS) analysis in transgenic maize and tobacco plants. The COMT promoter directs GUS expression to the xylem and the other tissues undergoing lignification, and it responds to wounding and to elicitors. In transgenic maize plants, expression of GUS corresponds to the pattern of expression of the endogenous COMT gene as determined by northern analysis and in situ hybridization. The pattern in transgenic tobacco plants clearly shows that the maize promoter sequence is recognized by tobacco transcriptional factors, in spite of the anatomical differences and the evolutionary distance between these two species. The results suggest that the most significant promoter signals that induce the specific expression of the lignin COMT are conserved in different species.

The brown midrib3 (bm3) mutation in maize occurs in the gene encoding caffeic acid O-methyltransferase.

The brown midrib mutations are among the earliest described in maize. Plants containing a brown midrib mutation exhibit a reddish brown pigmentation of the leaf midrib starting when there are four to six leaves. These mutations are known to alter lignin composition and digestibility of plants and therefore constitute prime candidates in the breeding of silage maize. Here, we show that two independent brown midrib3 (bm3) mutations have resulted from structural changes in the COMT gene, which encodes the enzyme O-methyltransferase (COMT; EC 2.1.1.6), involved in lignin biosynthesis. Our results indicate that the bm3-1 allele (the reference mutant allele) has arisen from an insertional event producing a COMT mRNA altered in both size and amount. By sequencing a COMT cDNA clone obtained from bm3-1 maize, a retrotransposon with homology 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 polymerase chain reaction amplification analyses. These results clearly demonstrate 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.