Beneficial Roles of Melatonin on Redox Regulation of Photosynthetic Electron Transport and Synthesis of D1 Protein in Tomato Seedlings under Salt Stress.

Xiaoting Zhou,Lipan Hu,Hailiang Zhao,Zhirong Zou,Kai Cao,František Baluška,Tianhao Du

doi:10.3389/fpls.2016.01823

Abstract

Melatonin is important in the protection of plants suffering various forms of abiotic stress. The molecular mechanisms underlying the melatonin-mediated protection of their photosynthetic machinery are not completely resolved. This study investigates the effects of exogenous melatonin applications on salt-induced damage to the light reaction components of the photosynthetic machinery of tomato seedlings. The results showed that melatonin pretreatments can help maintain growth and net photosynthetic rate (PN) under salt stress conditions. Pretreatment with melatonin increased the effective quantum yield of photosystem II (ΦPSII), the photochemical quenching coefficient (qP) and the proportion of PSII centers that are “open” (qL) under saline conditions. In this way, damage to the photosynthetic electron transport chain (PET) in photosystem II (PSII) was mitigated. In addition, melatonin pretreatment facilitated the repair of PSII by maintaining the availability of D1 protein that was otherwise reduced by salinity. The ROS levels and the gene expressions of the chloroplast TRXs and PRXs were also investigated. Salt stress resulted in increased levels of reactive oxygen species (ROS), which were mitigated by melatonin. In tomato leaves under salt stress, the expressions of PRXs and TRXf declined but the expressions of TRXm1/4 and TRXm2 increased. Melatonin pretreatment promoted the expression of TRXf and the abundances of TRXf and TRXm gene products but had no effects on the expressions of PRXs. In summary, melatonin improves the photosynthetic activities of tomato seedlings under salt stress. The mechanism could be that: (1) Melatonin controls ROS levels and prevents damaging elevations of ROS caused by salt stress. (2) Melatonin facilitates the recovery of PET and D1 protein synthesis, thus enhancing the tolerance of photosynthetic activities to salinity. (3) Melatonin induces the expression of TRXf and regulates the abundance of TRXf and TRXm gene products, which may facilitate repair of the light reaction parts of the photosynthetic machinery.

Highlights

In many parts of the world, salt-affected soils cause considerable economic loss through unacceptable levels of yield reduction
We evaluate redox regulation by the TRXs, to explore the effects of melatonin on the synthesis of critical proteins, using photosystem II (PSII) repair under salt stress as an example
Our aim was determined whether increases in photosynthesis under salt stress following melatonin supplement are related to melatonin-induced protection of the photosynthetic machinery

Summary

Introduction

In many parts of the world, salt-affected soils cause considerable economic loss through unacceptable levels of yield reduction. PSII is sensitive to such abiotic stresses. ROS damages the photosynthetic apparatus (Hu et al, 2014) and blocks the synthesis of PSII proteins in the chloroplasts (Nishiyama et al, 2011). A repair system continuously restores PSII activity, so that photoinhibition of PSII becomes apparent only when the rate of light-induced inactivation exceeds the rate of repair. Salt stress increases the severity of photoinhibition (Ohnishi and Murata, 2006) because turnover of D1 protein (an integral part of the reaction center of PSII) is inhibited under salt stress. Studies have shown that salt stress impedes D1 protein synthesis by direct inactivation of the translation machinery at translation level (Allakhverdiev et al, 2002; Nishiyama and Murata, 2014). To improve PSII tolerance to abiotic stress, it will necessary somehow to increase the rate of PSII repair

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