Potato plants expressing antisense and sense S-adenosylmethionine decarboxylase (SAMDC) transgenes show altered levels of polyamines and ethylene: Antisense plants display abnormal phenotypes

A molecular approach has been used to study the role of polyamines in plant development by manipulating the expression of the S-adenosylmethionine decarboxylase (SAMDC) gene. SAMDC is a key enzyme involved in the biosynthesis of the polyamines spermidine and spermine and is also known to influence the rate of biosynthesis of ethylene. Previously, a cDNA clone of the SAMDC gene of potato has been isolated and characterized. This cDNA done has been used to make antisense and sense SAMDC constructs under the control of the 35S CaMV or tetracycline-inducible promoters. Agrobacterium-mediated transformation has been used to produce transgenic potato plants with the engineered antisense and sense SAMDC genes in order to downregulate or overexpress the SAMDC transcript, respectively. Decreases or increases in the level of SAMDC transcript in the antisense and sense transgenic plants were observed, respectively. Antisense transgenic plants which expressed the engineered SAMDC gene constitutively under the control of the 35S CaMV promoter with a duplicated enhancer region showed a range of stunted phenotypes with highly branched stems, short internodes, small leaves and inhibited root growth. The abnormal characteristics of the antisense plants correlated with the altered levels of SAMDC transcript, SAMDC activity, polyamine content and rate of ethylene evolution. Attempts to produce sense transgenic plants with the 35S SAMDC sense construct were unsuccessful indicating that a constitutive overexpression of the engineered SAMDC is lethal to the plants. When antisense and sense SAMDC transgenes were expressed under the control of a tetracycline-inducible promoter a number of transgenic plants were produced. In these antisense and sense plants, significant changes in the level of SAMDC transcript, SAMDC activity and free polyamine content were observed on tetracycline-induction of detached leaves.