Isolation of two differentially expressed wheat ACC synthase cDNAs and the characterization of one of their genes with root-predominant expression.

Two partial 1-aminocyclopropane-1-carboxylic acid (ACC) synthase cDNA clones (pWAS1, 1089 bp; and pWAS3, 779 bp) were isolated by polymerase chain reaction (PCR) using cDNA to total mRNA purified from etiolated wheat seedlings as template and degenerate oligonucleotides synthesized based on the regions of the ACC synthase amino acid sequence that are highly conserved among different plants. Northern analysis showed that the expression of the corresponding genes are differentially regulated. While the transcripts of pWAS1 were found in all the tissues of wheat that were tested with a maximum level at the early stages of spike development, pWAS3 mRNA was present almost exclusively in the root. A 5590 bp genomic clone, TA-ACS2, corresponding to pWAS3 cDNA has been isolated. The TA-ACS2 sequence consists of a 589-bp 5'-upstream region, 2743 bp of transcribed region with four exons and three introns and a 3'-downstream region of 2257 bp. Expression in Escherichia coli confirmed the ACC synthase activity of TA-ACS2 polypeptide. Sequence comparisons show that the two wheat ACC synthases are more similar to each other and to the rice ACC synthase, OS-ACS1, at the nucleotide level than at the amino acid level. The amino acid sequence of TA-ACS2 is most similar (66.1% identity) to that of broccoli. The chromosomal location of both wheat ACC synthase genes have been determined by aneuploid analysis. TA-ACS1 is located on the short arm of chromosomes 7A and 7D and on the long arm of chromosome 4A. TA-ACS2 is located on the long arm of homoeologous group 2 chromosomes.