Overexpression of Spermidine Synthase Enhances Tolerance to Multiple Environmental Stresses and Up-Regulates the Expression of Various Stress-Regulated Genes in Transgenic Arabidopsis thaliana

Abstract

;Polyamines play pivotal roles in plant defense to environmental stresses. However, stress tolerance of genetically engineered plants for polyamine biosynthesis has been little examined so far. We cloned spermidine synthase cDNA from Cucurbita ficifolia and the gene was introduced to Arabidopsis thaliana under the control of the cauliflower mosaic virus 35S promoter. The transgene was stably integrated and actively transcribed in the transgenic plants. As compared with the wild-type plants, the T2 and T3 transgenic plants exhibited a significant increase in spermidine synthase activity and spermidine content in leaves together with enhanced tolerance to various stresses including chilling, freezing, salinity, hyperosmosis, drought, and paraquat toxicity. During exposure to chilling stress (5°C), the transgenics displayed a remarkable increase in arginine decarboxylase activity and conjugated spermidine contents in leaves compared to the wild type. A cDNA microarray analysis revealed that several genes were more abundantly transcribed in the transgenics than in the wild type under chilling stress. These genes included those for stress-responsive transcription factors such as DREB and stress-protective proteins like rd29A. These results strongly suggest an important role for spermidine as a signaling regulator in stress signaling pathways, leading to build-up of stress tolerance mechanisms in plants under stress conditions.

Speculation: Polyamines are important in abiotic stress signaling

“…The overexpression of a spermidine synthase gene up-regulated the expression of various putative stress-regulated genes in chilled transgenic Arabidopsis compared to the corresponding wild type. These genes putatively encode transcription factors such as WRKY, MYB, B-box zinc finger proteins, DREB2B, sucrose non-fermenting protein 1 (SNF1)-related protein kinase 2 (SnRK2), calmodulin-related protein and stress-protective proteins, such as rd29A [11] (Table 1). SnRK2, characterized as OSPDK in rice, is Ca 2+ independent and has a role in the activation of various stress regulatory molecules, such as the phosphorylation of OSBZ8, a bZIP class of ABRE-binding transcription factors.…”

“…Plants overexpressing genes encoding enzymes involved in polyamine biosynthesis accumulate higher levels of polyamines and show enhanced tolerance to various stresses [9][10][11][12][13][14]. This suggests that an increase in polyamine synthesis is effective against all types of stress.…”

“…However, the real situation is more complicated, as in some cases the excess accumulation of polyamines due to the overexpression of these genes or to the absorbed exogenous polyamines is harmful to plant cells [3,11].…”

Polyamines: potent modulators of plant responses to stress

“…The damage to the plasma membrane and plasma membrane-bound H ' -ATPase activity in salt-treated rice seedlings was revoked by Spd (Roy et al 2005). Spd may serve two functions in plant stress tolerance, one as a direct stress-protecting compound and the other as a stresssignaling regulator (Kasukabe et al 2004). Moreover, exogenous Spd treatment caused a substantial reduction in ROS content and thereby lowered the oxidative stress in barley leaves under water deficit stress (Kubis 2005).…”

“…The application of inhibitor is limited due to their stability and specificity therefore alternative and efficient approaches were also made possible to shed light on PA metabolism using diverse genetic and molecular strategies, especially through the use of mutants of PAs metabolism and transgenic approaches during recent years (Kasukabe et al 2004;Urano et al 2004). …”