Expression of the SAM recycling pathway in Nicotiana tabacum roots

Abstract

S-Adenosylmethionine (SAM)-dependent methylation of putrescine is a key step in the biosynthesis of nicotine in Nicotiana tabacum. Previous reports suggest that at least one step in SAM biosynthesis is upregulated during conditions that promote nicotine biosynthesis. This study investigated the expression of three genes involved in SAM biosynthesis/recycling (i.e. SAM synthetase (SAMS), S-adenosylhomocysteine hydrolase (SAHH), and methionine synthase (MS)) during conditions that increase the gene expression of nicotine biosynthetic genes. Removal of the synthetic auxin indolebutyric acid (IBA) increased SAMS, SAHH and MS mRNA levels in wild type Burley 21 root cultures. SAMS mRNA levels were significantly lower in LA21 root cultures, indicating that SAMS mRNA levels are controlled by the A and B regulatory loci. In contrast, MJA treatment did not alter SAMS, SAHH, or MS mRNA levels. Although IBA deprivation increased SAMS, SAHH, MS mRNA levels, no apparent changes in the corresponding steady state protein levels were observed. A MJA treatment-specific aabb genotype-dependent decrease in β-ATPase mRNA levels was also observed. Thus, the A– B regulon includes at least one target gene that is not associated with nicotine biosynthesis and the β-ATPase mRNA accumulation pattern was unlike that of other target genes regulated by the A– B loci.