Exogenous Melatonin Suppresses Dark-Induced Leaf Senescence by Activating the Superoxide Dismutase-Catalase Antioxidant Pathway and Down-Regulating Chlorophyll Degradation in Excised Leaves of Perennial Ryegrass (Lolium perenne L.).

Jing Zhang,Jing Li,Huibin Li,Bingru Huang,Bin Xu

doi:10.3389/fpls.2016.01500

Abstract

Leaf senescence is a typical symptom in plants exposed to dark and may be regulated by plant growth regulators. The objective of this study was to determine whether exogenous application of melatonin (N-acetyl-5-methoxytryptamine) suppresses dark-induced leaf senescence and the effects of melatonin on reactive oxygen species (ROS) scavenging system and chlorophyll degradation pathway in perennial grass species. Mature perennial ryegrass (Lolium perenne L. cv. ‘Pinnacle’) leaves were excised and incubated in 3 mM 2-(N-morpholino) ethanesulfonic buffer (pH 5.8) supplemented with melatonin or water (control) and exposed to dark treatment for 8 days. Leaves treated with melatonin maintained significantly higher endogenous melatonin level, chlorophyll content, photochemical efficiency, and cell membrane stability expressed by lower electrolyte leakage and malondialdehyde (MDA) content compared to the control. Exogenous melatonin treatment also reduced the transcript level of chlorophyll degradation-associated genes and senescence marker genes (LpSAG12.1, Lph36, and Lpl69) during the dark treatment. The endogenous O2- production rate and H2O2 content were significantly lower in these excised leaves treated with melatonin compared to the water control. Exogenous melatonin treatment caused increases in enzymatic activity and transcript levels of superoxide dismutase and catalase but had no significant effects on ascorbate peroxidase, glutathione reductase, dehydroascorbate reductase, and monohydroascorbate reductase. The content of non-enzymatic antioxidants, such as ascorbate and dehydroascorbate, were decreased by melatonin treatment, while the content of glutathione and oxidized glutathione was not affected by melatonin. These results suggest that the suppression of dark-induced leaf senescence by exogenous melatonin may be associated with its roles in regulating ROS scavenging through activating the superoxide dismutase-catalase enzymatic antioxidant pathway and down-regulating chlorophyll degradation in perennial ryegrass.

Highlights

Leaf senescence is a highly regulated natural process during leaf development and can be accelerated by abiotic stresses such as low light conditions (Causin et al, 2009; Brouwer et al, 2012)
Physiological analysis demonstrated that exogenous treatment of excised leaves with melatonin effectively suppressed darkinduced leaf senescence, as manifested by maintaining greater chlorophyll content and photochemical efficiency, and lower MDA content and electrolyte leakage (EL) in leaves of perennial ryegrass exposed to the dark treatment
Previous studies with other plant species have found the positive effects of melatonin dark-induced leaf senescence; for perennial ryegrass, the effective concentration was between 20 and 100 μM in this study

Summary

Introduction

Leaf senescence is a highly regulated natural process during leaf development and can be accelerated by abiotic stresses such as low light conditions (Causin et al, 2009; Brouwer et al, 2012) This process is characterized by the loss of net chlorophyll content due to chlorophyll degradation (Matile et al, 1999; Hörtensteiner, 2006). The stay-green gene (SGR), cloned in many plant species, has been found to regulate chlorophyll degradation by interacting with the six CCEs (Park et al, 2007; Sakuraba et al, 2012) The transcription of these seven chlorophyll degradation-associated genes is upregulated during the senescence process (Matile et al, 1999; Park et al, 2007; Jespersen et al, 2016). Similar to chlorophyll degradation-associated genes, the transcription of senescence marker genes, such as SAG12, h36, and l69, is upregulated during leaf senescence (Lee et al, 2001; Zhou et al, 2013)

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