Inhibition of putrescine biosynthesis enhanced salt stress sensitivity and decreased spermidine content in rice seedlings

The effect of polyamine biosynthesis inhibitors on the salt stress response of rice seedlings was investigated. For this, DL-α-difluoromethylarginine (DFMA) and DL-α-difluoromethylornithine (DFMO), two competitive inhibitors of arginine decarboxylase (ADC) and ornithine decarboxylase (ODC), were used. The ADC and ODC are rate-limiting enzymes involved in synthesis of putrescine. The effective quantum yield of photosynthetic energy conversion (ΦPSII) decreased with the salt stress, and this decrease was highly significant in the treatments with DFMA and DFMO. Interestingly, addition of exogenous putrescine reduced the decline of ΦPSII. Putrescine content strongly decreased after one day of the inhibitor treatment. Although the content of spermidine (converted from putrescine) also showed an initial decrease in response to the inhibitors, it recovered to a similar level to that in the control after 3 d of treatment. Under the salt stress, the effect of the inhibitors on the different compounds was similar. Moreover, the addition of exogenous putrescine partially suppressed the decrease in spermidine and spermine content. A positive correlation between the spermidine and spermine content and the ΦPSII was observed. The results suggest that, under salt stress, a decrease in polyamine biosynthesis and/or polyamine content has a strong negative effect on leaves and increases salt stress sensitivity.