Distinct phenotypes generated by overexpression and suppression of S-adenosyl-L-methionine synthetase reveal developmental patterns of gene silencing in tobacco.

S-Adenosyl-L-methionine synthetase (SAM-S) catalyzes the conversion of L-methionine and ATP into S-adenosyl-L-methionine. Tobacco plants that were transformed with a construct allowing high transcription levels of an Arabidopsis sam-s gene could be grouped into two main classes based on their morphology. One class developed yellow-green leaves and had high SAM-S activity and transgene mRNA levels, whereas the other class was stunted and had leather-like leaves, very low SAM-S activity, and suppressed mRNA level of the transgene. Because both overexpression and silencing of transgene expression led to distinct, abnormal phenotypes, the developmental pattern of transgene silencing was visualized. In the lower leaves, the suppressed phenotype was associated with the veins. In successive leaves, the area of the suppressed tissue increased until all newly developed leaves displayed the suppressed phenotype. In this study, a hypothesis is presented for this developmental gene silencing. Furthermore, transgenic plants with suppressed SAM-S activity had a characteristic smell, a consequence of the accumulation of L-methionine that is converted into the volatile methanethiol.