Characterization of the Polyamine Biosynthetic Pathways and Salt Stress Response in Brachypodium distachyon

To characterize polyamine (PA) biosynthetic pathways in Brachypodium distachyon, we analyzed the gene-expression patterns and PA contents in various organs. Three major PAs-putrescine (Put), spermidine (Spd), and spermine (Spm)-were detected, but thermospermine (T-Spm) was below the sensitivity limit of high-performance liquid chromatography in all tissues examined, although expression of the ACL5 gene, which is known as the T-Spm synthesis gene, was confirmed. In leaves, Put was the most abundant PA, and its level of accumulation was more than three times greater than those of Spd and Spm. In contrast, the primary PA in other organs was Spd. A high correlation was observed between arginine decarboxylase (ADC) gene-expression patterns and Put contents under normal growth conditions. These results indicate that Put is normally synthesized through the ADC pathway. We also analyzed salt stress responses and confirmed that all PA biosynthesis genes are induced by NaCl treatment. Moreover, Spm highly accumulated under salt stress conditions. Recently, T-Spm was shown to enhance the translation of Arabidopsis SAC51 family mRNAs, at least SAC51 and SACL1, by reducing the inhibitory effect of the conserved upstream open reading frame (uORF). Through database analysis, three and five SAC51 family genes were identified from B. distachyon and Zea mays, respectively. However, these genes are most closely related to SACL3 in Arabidopsis, and the commonly conserved uORF sequence was not confirmed for some of these genes. Therefore, the function of T-Spm in monocotyledonous plant species might be different from that in Arabidopsis.