Gibberellin mediates spermidine-induced salt tolerance and the expression of GT-3b in cucumber

Abstract Background Transcription factor GT-3b binds to the GT-1 element in the promoter of S -adenosylmethionine synthase ( SAMs ) gene, the key gene of spermidine (Spd) biosynthesis, to regulate its expression, thereby increasing the salt tolerance of cucumber plants. Furthermore, exogenous Spd increases the transcript level and protein abundance of SAMs, which promotes the accumulation of endogenous polyamine. However, whether Spd regulation of SAMs depending on GT-3b is largely unknown. Here, we investigated the potential mechanism of Spd in regulating of GT-3b in cucumber seedlings under salt stress.Results Exogenous Spd significantly increased the tolerance to salt stress in cucumber, while its effects were compromised when application of methylglyoxal bis-guanylhydrazone (MGBG), an inhibitor of Spd biosynthesis. GT-3b was significantly induced by Spd under salt stress. The promoter sequence of GT-3b was predicted that there contained cis-acting regulatory elements response to phytohormones, such as gibberellin (GA), salicylic acid (SA) and methyl jasmonate (MeJA). Foliar spray of GA 3 , SA and MeJA could induce the expression of GT-3b . Interestingly, exogenous Spd dramatically induced the expression of genes related to GA biosynthesis, increased the activity of gibberellin oxidase, and promoted the accumulation of GA 3 , but decreased in MGBG-treated plants. Furthermore, application of GA 3 increased the expression of GT-3b and salt tolerance, while blocked when treatment with paclobutrazol (PAZ), the GA biosynthesis inhibitor. In addition, Spd-induced salt tolerance was compromised in PAZ-treated plants.Conclusions Our results suggested that GA mediated Spd-induced salt tolerance and the expression of GT-3b in cucumber. These results provide new perspective for our understanding the molecular mechanism of Spd in regulation of salt tolerance in plants.