Mitigation of gravitropic bending of snapdragon cut flowers through SNP treatment via ROS asymmetry elimination and auxin sensitivity reduction

Abstract

Snapdragons (Antirrhinum majus L.) are an important fresh-cut flower in the international flower market. During storage and transportation, snapdragons often undergo a gravitropic response that has a substantial adverse effect on their ornamental and market value. The gravitropic response is a complex physiological and biochemical process that is affected by signals such as auxins and reactive oxygen species (ROS). In this study, we found that the pretreatment of exogenous nitric oxide (NO) donor sodium nitroprusside (SNP, 0.2 mM) effectively inhibited the gravitropic bending of snapdragon cut flowers after horizontal placement. The gravitropic stimulus induced higher ROS (O2−∙ and H2O2) accumulation in the lower side compared to the upper side of the bending zone of the control flower stems. Compared to the control, a substantially higher accumulation of ROS was induced through the SNP treatment, correlating with the increased activity of catalase (CAT) and ascorbate peroxidase (APX), as well as the enhanced expression of AmAPX1 and AmCAT1. However, unlike the control, no ROS asymmetric distribution was found for the SNP-treated flowers. Four free auxins, IAA, IBA, ME-IAA and ICA, were detected in the flower stems, among which, only IBA showed asymmetric distribution after the gravitropic stimulus. An IBA content that was approximately 26% higher was detected in the lower side compared to the upper side of the control flower at 2 h, but no difference was found for the SNP-treated flowers. A higher expression of the AmPIN3 gene at the lower sides compared to the upper sides of the control may be correlated with the transient IBA asymmetric distribution. Interestingly, an expression of AmIAA2 and AmSUR50 that was around 2-fold was detected in the lower side compared to the upper side of the control, indicating that higher auxin sensitivity may be induced in the lower side. However, the expression of AmIAA2 and AmSUR50 was reduced by the SNP treatment, implying that high ROS levels may reduce auxin sensitivity. Furthermore, a higher AmExp1 and AmH+–ATPase1 expression in the lower side compared to the upper side was found for the control but not for the SNP-treated flowers, indicating the two genes may be involved in the bending of the flower stems. The above results reveal that SNP treatment eliminates the asymmetric distribution of ROS and reduced auxin sensitivity of snapdragon flower stems, thereby eliminating their asymmetric growth and further inhibiting their gravitropic response.