Spermidine (Spd) is one of the predominant polyamines in higher plants and plays a crucial role in combating various abiotic stresses. However, the molecular functions and underlying regulatory mechanisms associated with plant Spd synthase (SPDS) genes in cold tolerance remain poorly understood. In this study, cold treatment markedly induced Spd accumulation and enhanced SPDS activity in Ichang papeda (Citrus ichangensis), a cold-hardy plant in Citrus genus. Exogenous Spd supply led to dramatically improved cold tolerance. Two SPDS genes (CiSPDS1 and CiSPDS2) were identified in Ichang papeda, but only CiSPDS1 was substantially upregulated by cold. Overexpressing of CiSPDS1 in both tobacco (Nicotiana tabacum) and lemon (Citrus limon), a cold-sensitive Citrus species, promoted Spd synthesis and enhanced cold tolerance in the transgenic plants. In contrast, knockdown of CiSPDS1 in Ichang papeda by virus-induced gene silencing (VIGS) repressed Spd synthesis and greatly impaired the cold tolerance, which was restored by exogenous replenishment of Spd. In addition, we demonstrated that WRKY27 of Ichang papeda (CiWRKY27) directly bound to and activated the CiSPDS1 promoter through interacting with a W-box cis-acting element. Meanwhile, VIGS-mediated silencing of CiWRKY27 resulted in marked reduction of CiSPDS1 transcript levels and Spd contents and significantly impaired the cold tolerance in Ichang papeda. Taken together, this study illustrated the role of CiSPDS1 in cold tolerance and identified it as a direct target of CiWRKY27. These findings provide insight into the regulatory mechanism by which the molecular module CiWRKY27-CiSPDS1 regulates Spd accumulation for modulation of cold tolerance.

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