Does salicylic acid regulate antioxidant defense system, cell death, cadmium uptake and partitioning to acquire cadmium tolerance in rice?

Summary

Salicylic acid (SA) may accelerate the cell death of cadmium-stressed roots to avoid cadmium (Cd) uptake by plants or may play positive roles in protecting the stressed roots from Cd-induced damage. To test these hypotheses, we performed a series of split-root hydroponic experiments with one-half of rice (Oryza sativa L. cv. Jiahua 1) roots exposed to 50 μM Cd and the other half not exposed. The objectives were to elucidate the effects of SA pretreatment on the time-dependent changes of H₂O₂ levels in roots, antioxidant defense system in different organs, root cell death and the dynamic distribution of Cd in the plants. In the split-root system, a higher Cd uptake rate was observed in the Cd-stressed portions of roots compared with the treatment with the whole roots exposed to Cd. Furthermore, an appreciable amount of Cd was translocated from the Cd-exposed roots to the unexposed roots and trace amounts of Cd were released into the external solution. The split-root method also caused the two root portions to respond differently to Cd stress. The activities of major antioxidant enzymes (superoxide dismutase, SOD; peroxidase, POD; and catalase, CAT) were significantly suppressed in the Cd-treated roots, hence leading to H₂O₂ burst, lipid peroxidation, cell death and growth inhibition. By contrast, in the non-Cd-treated roots, the activities of enzymes (SOD, CAT, and POD) and root growth were persistently stimulated during the experimental period. The H₂O₂ accumulation and lipid peroxidation were also induced in the non-Cd-treated roots, but they were significantly lower than those of the Cd-treated roots. The concentrations of glutathione (GSH) and non-protein thiols (NPT) in the Cd-treated roots were significantly higher than those of the untreated roots. SA pretreatment elevated enzymatic and non-enzymatic antioxidants, and the concentrations of GSH and NPT in roots and shoots, hence leading to alleviation of the oxidative damage as indicated by the lowered H₂O₂ and MDA levels. Furthermore, SA pretreatment mitigated the Cd-induced growth inhibition in both roots and shoots and increased transpiration compared with non-SA-pretreatment under Cd exposure. It is concluded that Cd can be partly transferred from the Cd-exposed roots to Cd-unexposed roots and that cell death can be accelerated in the Cd-stressed roots in response to Cd stress. The SA-enhanced Cd tolerance in rice can be attributed to SA-elevated enzymatic and non-enzymatic antioxidants and NPT, and to SA-regulated Cd uptake, transport and distribution in plant organs.

Introduction

Cadmium (Cd) is a ubiquitous element in the environment and is highly toxic to living organisms. In plants, Cd toxicity has been found to interfere with electron transport chains or block antioxidant enzymes structures, leading to accumulation of H₂O₂, and oxidative damage (e.g. lipid peroxidation), membrane leakage and finally cell death (Schützendübel et al., 2001; Schützendübel and Polle, 2002).

The H₂O₂-triggered cell death has been well-recognized and is an essential process to maintain tissue or organ homeostasis in concert with cell proliferation, growth, and differentiation (Greenberg, 1996; Mittler et al., 2004). Furthermore, under unfavorable conditions, cell death allows the plant to defend against biotic stress or to obtain more resources by reducing or even stopping growth of plant tissues. For example, H₂O₂-induced cell death has been best described during incompatible plant–pathogen interactions that form the basis for the hypersensitive response (HR) (Durner et al., 1997), and aerenchyma formation in root cortex for the toleration of low-oxygen soil environments (Drew et al., 2000). Studies with Nicotiana tabacum (TBY-2) (Fojtová and Kovařík, 2000) and Scots pine (Pinus sylvestris) (Schützendübel et al., 2001) have shown that Cd induced the morphology of cell death which was related to the H₂O₂ burst. However, to our knowledge, there have been no reports to show whether cell death in root tissues can build up a physical barrier to inhibit Cd uptake, and consequently benefit the whole plant through the avoidance of Cd toxicity as the mode of plant–pathogen interactions.

Salicylic acid (SA) acts as an important signaling element in plants, which has broad but divergent effects on damage development or stress acclimation of plants (Durner et al., 1997). Upon pathogen attack, SA accumulates to high levels at the site of pathogenic infection, binds and inhibits tobacco CAT activity in vitro and in vivo, thereby leading to an increase in the endogenous level of H₂O₂, which could then serve as a second messenger to induce cell death to create a physical barrier against pathogens. However, it is also reported that SA plays a key role in promoting plant resistance to various abiotic stresses. It has previously been reported that SA alleviated growth inhibition by Cd toxicity in barley (Hordeum vulgare) and soybean (Glycine max) (Metwally et al., 2003; Drazic and Mihailovic, 2005) and in rice (Guo et al., 2007a), although the underlying mechanism is not fully understood. Our more recent studies have shown that pretreatment of rice seeds with SA enhanced the antioxidant defense activities in Cd-stressed rice, thus alleviating Cd-induced oxidative damage and enhancing Cd tolerance. The possible mechanism involved was thought to be related to SA-induced H₂O₂ signaling in mediating Cd tolerance (Guo et al., 2007a). Thus, it is interesting to elucidate whether (1) SA has negative roles in accelerating partial root death to avoid Cd uptake, in an analog to the mode of action of SA-enhanced plant defense against pathogens through H₂O₂ bursts and consequent cell death, or (2) SA plays positive roles in protecting roots from damage in response to Cd stress. Therefore, we conducted a series of hydroponic experiments using a split-root system to investigate the time-dependent changes of H₂O₂ levels in roots, antioxidant defense system in different organs and root cell death under Cd stress and their relationships with the dynamic distribution of Cd following pretreatment of SA.