Cadmium Disrupts the Balance between Hydrogen Peroxide and Superoxide Radical by Regulating Endogenous Hydrogen Sulfide in the Root Tip of Brassica rapa.

Wenjing Lv,Zhiqi Shi,Jian Chen,Lifei Yang,Ming Xian,Cunfa Xu,Jinsong Shao

doi:10.3389/fpls.2017.00232

Abstract

Cd (cadmium) stress always alters the homeostasis of ROS (reactive oxygen species) including H2O2 (hydrogen sulfide) and (superoxide radical), leading to the oxidative injury and growth inhibition in plants. In addition to triggering oxidative injury, ROS has been suggested as important regulators modulating root elongation. However, whether and how Cd stress induces the inhibition of root elongation by differentially regulating endogenous H2O2 and , rather than by inducing oxidative injury, remains elusive. To address these gaps, histochemical, physiological, and biochemical approaches were applied to investigate the mechanism for Cd to fine-tune the balance between H2O2 and in the root tip of Brassica rapa. Treatment with Cd at 4 and 16 μM significantly inhibited root elongation, while only 16 μM but not 4 μM of Cd induced oxidative injury and cell death in root tip. Fluorescent and pharmaceutical tests suggested that H2O2 and played negative and positive roles, respectively, in the regulation of root elongation in the presence of Cd (4 μM) or not. Treatment with Cd at 4 μM led to the increase in H2O2 and the decrease in in root tip, which may be attributed to the up-regulation of Br_UPB1s and the down-regulation of their predicted targets (four peroxidase genes). Cd at 4 μM resulted in the increase in endogenous H2S in root tip by inducing the up-regulation of LCDs and DCDs. Treatment with H2S biosynthesis inhibitor or H2S scavenger significantly blocked Cd (4 μM)-induced increase in endogenous H2S level, coinciding with the recovery of root elongation, the altered balance between H2O2 and , and the expression of Br_UPB1s and two peroxidase genes. Taken together, it can be proposed that endogenous H2S mediated the phytotoxicity of Cd at low concentration by regulating Br_UPB1s-modulated balance between H2O2 and in root tip. Such findings shed new light on the regulatory role of endogenous H2S in plant adaptions to Cd stress.

Highlights

Reactive oxygen species (ROS), a set of active forms of molecular oxygen (O2) occurred in plant cells, comprise both free radical (e.g., O2−, superoxide radical; OH, hydroxyl radical) and nonradical forms (e.g., H2O2, hydrogen peroxide; 1O2, singlet oxygen) (Gill and Tuteja, 2010)
The expression of only one LCD (Bra001131) was downregulated by treatment with 4 μM of Cd (Figure 5C). These results suggested treatment with Cd at 4 μM stimulated the generation of endogenous H2S in root tip, which may resulted from the extensive up-regulation of LCDs and DCDs
Cd is able to induce the increase in H2O2 and the decrease in O2− in the roots of G. max and C. sativus (Heyno et al, 2008)

Summary

INTRODUCTION

Reactive oxygen species (ROS), a set of active forms of molecular oxygen (O2) occurred in plant cells, comprise both free radical (e.g., O2−, superoxide radical; OH, hydroxyl radical) and nonradical forms (e.g., H2O2, hydrogen peroxide; 1O2, singlet oxygen) (Gill and Tuteja, 2010). Cd-induced phytotoxicity has been closely linked to the over-generation of ROS, leading to oxidative injury, lipid peroxidation, cell death, and growth stunt (DalCorso et al, 2010; Lin and Aarts, 2012; Andresen and Küpper, 2013). Excessive Cd at toxic dosage induces remarkable increases in both H2O2 and O2− in plants (Xu et al, 2012; Pérez-Chaca et al, 2014). Cd induces two waves of ROS in the roots of Glycine max, which the maximum accumulation of H2O2 appears faster than that of O2− (Pérez-Chaca et al, 2014). Whether and how ROS act as signaling molecule rather than a trigger of oxidative stress to regulate root growth under Cd exposure remains obscured. The possible mechanisms driving these physiological processes, and their significance, were discussed

MATERIALS AND METHODS

RESULTS

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DISCUSSION