Salt stress-mediated changes in free polyamine titers and expression of genes responsible for polyamine biosynthesis of apple in vitro shoots

In the present paper, changes in contents of free polyamines and expression of genes responsible for polyamine biosynthesis of apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] in vitro shoots exposed to 100 mM or 200 mM NaCl for 10 days were investigated. Salt stress led to severe growth retardation, based on electrolyte leakage (EL), net increase in fresh weight and shoot length. Na concentration in salt-treated shoots was significantly higher than that of the control, and K was unexpectedly enhanced, particularly by 100 mM NaCl. Three commonly occurring polyamines, putrescine (Put), spermidine (SPd) and spermine (Spm), were detected in the shoots, in which Spd and Spm were the most and the least abundant pan, respectively. Both types of salt stress led to significant reduction of free Put, and Spm was significantly reduced by 100 mM NaCl, whereas Spd underwent negligible fluctuation. The genes encoding for polyamine biosynthetic enzymes showed diverse expression patterns in response to salt treatment. Salt stress led to induction of MdADC, MdSAMDCI, MdSPDSI and MdSPMS, whereas it appreciably inhibited transcript level of MdACL5 in a concentration-dependent manner, indicating that the genes were differentially affected by salt stress. The data presented herein showed that salt stress led to conspicuous alteration of free polyamine contents and that mRNA levels of some genes involved in polyamine biosynthesis were strongly influenced. Mechanism underlying the decrease in free Put and gene expression patterns following long-term salt stress were discussed. (c) 2007 Elsevier B.V. All rights reserved.