Potential regulation of apple in vitro shoot growth via modulation of cellular polyamine contents

In the present research attempts were made to elucidate effects of d-arginine (d-arg), a potential inhibitor of arginine decarboxylase (ADC) involved in polyamine biosynthesis, on growth and polyamine biosynthesis of apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] in vitro shoots. Growth of the shoots cultured on proliferation medium added with 1 or 5 mM d-arg was inhibited over a 3-week treatment, with 5 mM being more effective. Activity of ADC was slightly or remarkably inhibited by 1 and 5 mM d-arg, respectively, in line with change of free putrescine (Put) content in the treated shoots. Activity of ornithine decarboxylase (ODC) was only repressed by 5 mM d-arg. Application of exogenous Put at 1 mM slightly recovered the growth of impaired apple shoots, while exogenous Put at 5 mM could resume the growth to a level similar with the control. Addition of exogenous Put led to notable increase in endogenous free Put, coupled with corresponding recovery of ADC and ODC activities. Transcriptional levels of several key genes involved in polyamine biosynthesis were more or less repressed by 5 mM d-arg relative to the untreated shoots, which were otherwise up-regulated by exogenously applied Put. The data presented herein showed that it is possible to regulate apple shoot growth via modulation of cellular polyamine contents.