Ethylene production is associated with alleviation of cadmium-induced oxidative stress by sulfur in mustard types differing in ethylene sensitivity Mohd. Asgher, Nafees A. Khan n , M. Iqbal R. Khan, Mehar Fatma, Asim Masood Plant Physiology and Biochemistry Division, Department of Botany, Aligarh Muslim University, Aligarh 202002, India article info Article history: Received 7 February 2014 Received in revised form 18 April 2014 Accepted 19 April 2014 Keywords: Cadmium Ethylene Photosynthesis Sulfur Sulfur assimilation abstract We studied the response of ethylene-sensitive (Pusa Jai Kisan) and ethylene-insensitive (SS2) mustard (Brassica juncea) cultivars to 0, 0.5, 1.0 and 2.0 mM SO 4 2 À , and the effect of 1.0 mM SO 4 2 À was studied in the amelioration of 50 mM cadmium (Cd). The Cd-induced oxidative stress and Cd accumulation were greater in SS2 than Pusa Jai Kisan, but sulfur (S) application alleviated Cd-induced oxidative stress more prominently in Pusa Jai Kisan by increasing S-metabolism and synthesis of reduced glutathione (GSH) and ethylene production; and promoted photosynthesis and plant dry mass under Cd stress. The ethylene-sensitive cultivar responded more to S treatment under Cd stress and showed increased activity of antioxidant system resulting in increased photosynthesis and growth. Cadmium treatment resulted in rapid increase in ethylene formation which adversely influenced photosynthesis and plant dry mass. However, S and ethephon application to Cd-treated plants lowered ethylene formation to optimal range responsible for maximal GSH synthesis and protection against Cd-induced oxidative stress. The similarity of the effectiveness of 1.0 mM SO 4 2 À with 200 mLL À1 ethylene source as ethephon in alleviation of 50 mM Cd further verifies that differential alleviation of Cd toxicity in the two cultivars by S was dependent on ethylene production. The results suggest that ethylene production determines Cd stress alleviation by S via regulatory interaction with antioxidant metabolism. Thus, ethylene production and sensitivity bear a prominent role in alleviation of Cd stress by S and can be used as a criterion for developing Cd tolerant genotypes. & 2014 Elsevier Inc. All rights reserved. 1. Introduction Cadmium (Cd) toxicity is one of the most widespread abiotic stresses responsible for limiting plant metabolism and productiv- ity worldwide (Nazar et al., 2012). The uptake and accumulation of Cd in plants can produce excess reactive oxygen species (ROS), having potential to displace essential elements such as zinc (Zn), calcium (Ca), and iron (Fe) from proteins and can inhibit electron transport chain in the chloroplast and mitochondria (Gallego et al., 2012). Cadmium negatively affects growth as a direct consequence of the inhibition of photosynthesis in plants (Mobin and Khan, 2007). It also can cause cell death due to the oxidative damage to membranes, proteins, and nucleic acids resulting in significant reductions of growth and productivity (Nazar et al., 2012). Recently, Dias et al. (2013) have reported that increasing Cd concentration reduces maximum efficiency of pigment system II (PSII) and net CO 2 assimilation rate together with the decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activ- ity. They have also shown that Cd 2 þ ions interfere with Rubisco activation, lower activity and damage its structure by substituting for magnesium (Mg 2 þ ) ions. In order to cope with the excess ROS induced by Cd stress in cells, plants develop several strategies including induction of enzymatic and non-enzymatic components of antioxidant defense system. Sulfur (S) is one of the essential mineral nutrient elements required for growth and development of plants. It regulates plant metabolism under optimal and stressful environments (Iqbal et al., 2013; Khan et al., 2013; Nazar et al., 2014). Study of Guo et al. (2009) have shown that the enzymes of S-assimilatory pathway were induced by Cd stress and plants showed tolerance to Cd stress. Sulfur assimilation pathway is linked to the synthesis of ethylene via cysteine (Cys) and methionine. In Cys synthesis, two enzymes of S-assimiltion, ATP-sulfurylase (ATP-S; EC. 2.7.7.4) and Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ecoenv Ecotoxicology and Environmental Safety http://dx.doi.org/10.1016/j.ecoenv.2014.04.017 0147-6513/& 2014 Elsevier Inc. All rights reserved. Abbreviations: ACS, 1-aminocyclopropane carboxylic acid synthase; APX, ascor- bate peroxidase; ATP-S, adenosine triphosphate sulfurylase; Cd, cadmium; Cys, cysteine; DAS, days after sowing; GSH, reduced glutathione; GR, glutathione reductase; ROS, reactive oxygen species; Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; SAT, serine acetyl transferase; S, sulfur; SOD, superoxide dismutase; TBARS, thiobarbituric acid reactive substances n Corresponding author. Fax: þ91 571 2702016. E-mail address: naf9@lycos.com (N.A. Khan). Ecotoxicology and Environmental Safety 106 (2014) 54–61