Spermidine alleviates the growth of saline-stressed ginseng seedlings through antioxidative defense system

Highlights

• Here we described for the first time about the application of Spd to the salt-stressed ginseng.

• Exogenous Spd increased plant growth by increasing PA levels.

• During salinity stress, Spd lowered Put, with an increase in Spd and Spm.

• Exogenous Spd also increased antioxidant enzymes.

Abstract

Protective effects of exogenous spermidine (Spd), activity of antioxygenic enzymes, and levels of free radicals in a well-known medicinal plant, Panax ginseng was examined. Seedlings grown in salinized nutrient solution (150 mM NaCl) for 7 d exhibited reduced relative water content, plant growth, increased free radicals, and showing elevated lipid peroxidation. Application of Spd (0.01, 0.1, and 1 mM) to the salinized nutrient solution showed increased plant growth by preventing chlorophyll degradation and increasing PA levels, as well as antioxidant enzymes such as CAT, APX, and GPX activity in the seedlings of ginseng. During salinity stress, Spd was effective for lowering the accumulation of putrescine (Put), with a significant increase in the spermidine (Spd) and spermine (Spm) levels in the ginseng seedlings. A decline in the Put level ran parallel to the higher accumulation of proline (Pro), and exogenous Spd also resulted in the alleviation of Pro content under salinity. Hydrogen peroxide (H₂O₂) and superoxide (O₂⁻) production rates were also reduced in stressed plants after Spd treatment. Furthermore, the combined effect of Spd and salt led to a significant increase in diamine oxidase (DAO), and subsequent decline in polyamine oxidase (PAO). These positive effects were observed in 0.1 and 1 mM Spd concentrations, but a lower concentration (0.01 mM) had a very limited effect. In summary, application of exogenous Spd could enhance salt tolerance of P. ginseng by enhancing the activities of enzyme scavenging system, which influence the intensity of oxidative stress.

Introduction

Salinity is a major abiotic stressor that limits plant growth and yield. Growth inhibition of plants exposed to salinity is associated with the reduction of water availability and ion accumulation, causing an imbalance of minerals that leads to morphological, physiological, and metabolic modifications in plants. Salinity also induces the generation of reactive oxygen species (ROS), such as superoxide, hydrogen peroxide, singlet oxygen, and hydroxyl radicals in plants (Leshem et al., 2007). These ROS interrupt normal metabolism in plants by lipid peroxidation of membrane, denaturation of proteins and nucleic acids. Salt stress alters the critical balance between the production of ROS and the quenching activity of antioxidants, resulting in oxidative stress that causes damage to plants (Hernandez et al., 1999). Plants have evolved three main strategies to counteract salt stress: detoxification, re-establishment of homeostasis, and growth regulation. The relevant responses involve the accumulation of compatible solutes (proline, amino/organic acids, betaines, and polyamines), the up-regulation of antioxidant enzymes and Na⁺/H⁺ antiporters, the re-translocation of Na⁺ from the photosynthetic organs back to the roots, the K⁺ retention in root and leaf cells (Shabala and Cuin, 2007), the increased activity of plasma membrane (H⁺-ATPase) and tonoplast H⁺ pumps (Hasegawa and Bressan, 2000), and the expressions of different sets of genes that are part of plant signaling and defense system against salinity (Sairam and Tyagi, 2004). Antioxidant enzymes, namely superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione peroxidase (GPX), and non-enzymatic antioxidants, such as ascorbic acid (ASC), glutathione (GSH), α-tocopherol, carotenoids, phenolics, and proline, which play a key role in quenching ROS, are implicated in stress tolerance (Hernandez et al., 1999).

Polyamines (PAs), including spermidine (Spd, a triamine), spermine (Spm, a tetramine), and their obligate precursor, putrescine (Put, a diamine), are polybasic aliphatic amines that are ubiquitously distributed in all living organisms. Their positive charges at physiological pH levels enable PAs to interact electrostatically with polycationic macromolecules such as DNA, RNA, proteins, and phospholipids (Bouchereau et al., 1999). Most of these processes are positively correlated with changes in the intracellular levels of PAs. Put can be synthesized by two carboxylases in plants: ornithine decarboxylase (ODC) and arginine decarboxylase (ADC). Spermidine (Spd) and spermine (Spm) are formed from Put by the subsequent addition of an aminopropyl moiety from decarboxylated s-adenosylmethionine. PAs modulate numerous biological processes including cell proliferation, growth and development, morphogenesis, senescence, and response to abiotic stresses (Galston and Sawhney, 1990). Several mechanisms have been postulated for the protective nature of PAs, which include scavenging free radicals, stabilizing membranes and cellular structures, maintaining a cation–anion balance (Bouchereau et al., 1999). Both mono- and dicotyledonous plants increase the accumulation of endogenous PAs under salt stress, and the pattern of PA metabolism in response to salinity seems to be dependent on plant defense systems and/or duration of exposure to salt stress. Exogenous PA application has been proposed as a convenient and effective approach for combating the salt tolerance of plants and ultimately improving crop productivity under high salinity (Chattopadhayay et al., 2003). Put content is temporally correlated with the accumulation of Pro under salinity stress, because they are both competing for glutamate as a precursor. On the basis of these observations, our goal was to elucidate the protective role of exogenous PA, especially Spd, in salt stressed ginseng plant.

Panax ginseng (Korean ginseng) is a perennial herb of the family Araliaceae. Its dried roots are used for medicinal purposes, and the major active component of ginseng roots is ginsenosides, a triterpenoid saponin. Ginseng has pharmacological effects that can be used to normalize the human metabolic system and biological activities (Lee et al., 2011). However, ginseng cultivation is difficult, because ginseng is a shade loving crop and its cultivation requires a long period (4–5 years) to produce the highly valued roots. During long cultivation time, the ginseng plant can be easily exposed to different environmental stresses, which may cause drastic effects in its growth such as salt stress. Our earlier report showed that higher cellular levels of Spd facilitate the transcription of PgSPD (spermidine synthase gene in P. ginseng) in response to different abiotic stresses (Parvin et al., 2010). The present study is aimed at determining whether exogenous Spd enhanced salt tolerance, and whether Spd affects antioxidant properties in the seedlings of P. ginseng. In addition, changes in chlorophyll, carotenoid, PA, PA degradative enzyme activity, Pro, and lipid peroxidation were also assessed with regard to salt stress, along with or without Spd.