Exogenous Melatonin Mitigates Photoinhibition by Accelerating Non-photochemical Quenching in Tomato Seedlings Exposed to Moderate Light during Chilling.

Fei Ding,Bin Liu,Shuoxin Zhang,Meiling Wang

doi:10.3389/fpls.2017.00244

Fei Ding, Bin Liu + Show 2 more

Open Access

https://doi.org/10.3389/fpls.2017.00244

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Abstract

Melatonin plays an important role in tolerance to multiple stresses in plants. Recent studies have shown that melatonin relieves photoinhibition in plants under cold stress; however, the mechanisms are not fully understood. Non-photochemical quenching (NPQ) is a key process thermally dissipating excess light energy that plants employ as a protective mechanism to prevent the over reduction of photosystem II. Here, we report the effects of exogenous melatonin on NPQ and mitigation of photoinhibition in tomato seedlings exposed to moderate light during chilling. In response to moderate light during chilling, the maximum quantum yield (Fv/Fm) and the effective photochemical efficiency (F′v/F′m) of PSII were both substantially reduced, showing severe photoinhibition in tomato seedlings, whereas exogenous application of melatonin effectively alleviated the photoinhibition. Further experiment showed that melatonin accelerated the induction of NPQ in response to moderate light and maintained higher level of NPQ upon longer exposure to light during chilling. Consistent with the increased NPQ was the elevated de-epoxidation state of xanthophyll pigments in melatonin-pretreated seedlings exposed to light during chilling. Enzyme activity assay showed that violaxanthin de-epoxidase (VDE), which catalyzes the de-epoxidation reaction in the xanthophyll cycle, was activated by light and the activity was further enhanced by application of melatonin. Further analysis revealed that melatonin induced the expression of VDE gene in tomato seedlings under moderate light and chilling conditions. Ascorbic acid is an essential cofactor of VDE and the level of it was found to be increased in melatonin-pretreated seedlings. Feeding tomato seedlings with dithiothreitol, an inhibitor of VDE, blocked the effects of melatonin on the de-epoxidation state of xanthophyll pigments and the induction of NPQ. Collectively, these results suggest that exogenous melatonin mitigates photoinhibition by accelerating NPQ through the stimulation of VDE activity and the enhancement of de-epoxidation state of xanthophyll pigments.

Highlights

Plants are largely dependent on the efficient conversion of absorbed light energy to chemical energy to sustain growth and development
The evidence leading to the conclusion includes (1) application of melatonin accelerates Non-photochemical quenching (NPQ) by increasing rates of violaxanthin deepoxidase (VDE); (2) melatonin-mediated increase in NPQ is a consequence of elevated VDE activity; and (3) increased VDE activity is due to melatonin-induced expression of VDE and melatonin-mediated accumulation of VDE cofactor ascorbic acid
Information regarding the effects of melatonin on the xanthophyll cycle and the induced NPQ is still lacking in plants

Summary

Introduction

Plants are largely dependent on the efficient conversion of absorbed light energy to chemical energy to sustain growth and development. In the longterm evolution, plants have developed multiple mechanisms to balance excess light absorption with photochemical utilization in order to protect photosystems against photodamages (Horton et al, 1996; Niyogi et al, 2001). One important mechanism is to thermally dissipate excess absorbed light energy in the light-harvesting antenna complexes of photosystem II (PSII), which confers protection of PSII against inactivation and potential damages by excess light energy. This process of thermally dissipation is referred to as non-photochemical quenching (NPQ). NPQ involves energy-dependent quenching (qE), quenching associated with state transition (qT) and photoinhibition (qI), among which qE acts predominantly to dissipate excess excitation energy absorbed in the PSII antenna as heat and plays an important role in alleviation of PSII photoinhibition (Wraight and Crofts, 1970; Briantais et al, 1979; Horton et al, 1996; Nilkens et al, 2010)

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