Increasing Polyamine Contents Enhances the Stress Tolerance via Reinforcement of Antioxidative Properties

Abstract

The diamine putrescine and the polyamines (PAs), spermidine (Spd) and spermine (Spm), are ubiquitously occurring polycations associated with several important cellular functions, especially antisenescence. Numerous studies have reported increased levels of PA in plant cells under conditions of abiotic and biotic stress such as drought, high salt concentrations, and pathogen attack. However, the physiological mechanism of elevated PA levels in response to abiotic and biotic stresses remains undetermined. Transgenic plants having overexpression of SAMDC complementary DNA and increased levels of putrescine (1.4-fold), Spd (2.3-fold), and Spm (1.8-fold) under unstressed conditions were compared to wild-type (WT) plants in the current study. The most abundant PA in transgenic plants was Spd. Under salt stress conditions, enhancement of endogenous PAs due to overexpression of the SAMDC gene and exogenous treatment with Spd considerably reduces the reactive oxygen species (ROS) accumulation in intra- and extracellular compartments. Conversely, as compared to the WT, PA oxidase transcription rapidly increases in the S16-S-4 transgenic strain subsequent to salt stress. Furthermore, transcription levels of ROS detoxifying enzymes are elevated in transgenic plants as compared to the WT. Our findings with OxyBlot analysis indicate that upregulated amounts of endogenous PAs in transgenic tobacco plants show antioxidative effects for protein homeostasis against stress-induced protein oxidation. These results imply that the increased PAs induce transcription of PA oxidases, which oxidize PAs, which in turn trigger signal antioxidative responses resulting to lower the ROS load. Furthermore, total proteins from leaves with exogenously supplemented Spd and Spm upregulate the chaperone activity. These effects of PAs for antioxidative properties and antiaggregation of proteins contribute towards maintaining the physiological cellular functions against abiotic stresses. It is suggested that these functions of PAs are beneficial for protein homeostasis during abiotic stresses. Taken together, these results indicate that PA molecules function as antisenescence regulators through inducing ROS detoxification, antioxidative properties, and molecular chaperone activity under stress conditions, thereby providing broad-spectrum tolerance against a variety of stresses.

Functional switching of NPR1 between chloroplast and nucleus for adaptive response to salt stress

“…1a). To investigate the physiological significances of NPR1 in response to salt stress, we established tobacco transgenic plants with p35S-driven NPR1 overexpression (NPR1-Ox) 22 . NPR1-Ox plants showed greater tolerance to salt stress, as determined by trypan blue staining for cell death 22 ( Fig.…”

“…To investigate the physiological significances of NPR1 in response to salt stress, we established tobacco transgenic plants with p35S-driven NPR1 overexpression (NPR1-Ox) 22 . NPR1-Ox plants showed greater tolerance to salt stress, as determined by trypan blue staining for cell death 22 ( Fig. 1b) as well as maximal photochemical efficiency of photosystem (PS) II (Fv/Fm) 23 based on chlorophyll fluorescence using a PAM 2000 Photosynthesis Yield Analyzer (Walz, Germany) ( Fig.…”

“…Most NPR1 studies have examined the intracellular localization of NPR1 after 24 h of pathogen infection or SA treatment, indicating that import of NPR1 into chloroplasts at an early time point after abiotic and biotic stresses has not been investigated. To better investigate the localization and functions of NPR1 under abiotic stress, we established stable transgenic tobacco plants expressing GFP fusion construct with tobacco NPR1 driven by a 0.8-kb region of the NPR1 promoter (pNPR1::NPR1-GFP) 22 . NPR1-GFP accumulated in chloroplasts of mesophyll protoplasts from this transgenic tobacco plant (Fig.…”

“…ing salt stress generally lead to accumulation of oxidized proteins and protein aggregation 22,44 , followed by interruption of protein homeostasis. Molecular chaperones are important components enhancing to the homeostasis and the quality control of proteins under stress conditions 22,45,46 . Chaperone proteins exist as multimeric structures consisting of oligomeric components 30 .…”

“…To investigate further physiological significances of chloroplast NPR1 in response to salt stress, we determined ROS levels in WT and NPR1-Ox transgenic plants under salt stress. Stress-induced intracellular levels of superoxide anion and hydrogen peroxide were investigated using the highly specific dyes benzene sulfonyl (BES)-So and BES-H 2 O 2 , respectively 22 . Stress-induced ROS levels were significantly inhibited in chloroplasts of NPR1-Ox compared to WT, suggesting that ROS generation might be closely related to the localization of NPR1 in chloroplasts ( Fig.…”