The positive regulation of putrescine on light-harvesting complex II and excitation energy dissipation in salt-stressed cucumber seedlings

https://doi.org/10.1016/j.envexpbot.2019.02.027Get rights and content

Highlights

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    Put increases heat dissipation capacity of salt stressed cucumber leaves.

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    Put maintains the stability of the light-harvesting complexs of PSII.

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    Put can affects the level of translation and transcription of LHCII protein subunits.

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    Put promotes the transformation of free PAs into conjugated PAs by regulating TGase.

Abstract

Non-photochemical quenching (NPQ) is an important mechanism employed by higher plants to protect themselves against the adverse effects of excess excitation energy. The regulatory mechanism of exogenous putrescine (Put) on the status of light-harvesting complex II (LHCII) and its dissipation of excess excitation energy in cucumber seedlings (Cucumis sativus L.) were investigated under salt stress in this study. Put significantly increased the maximum quantum photochemical efficiency in photosystem II (Fv/Fm), NPQ, and the actual yield of PSII (ΦPSII) under salt stress. Quantum yield of regulated energy dissipation in PSII (ΦNPQ) significantly increased and non-regulated energy dissipation (ΦNO) decreased when was sprayed on stressed plants. Put increased the expression levels of D1, D2, Lhcb4 and PsbS, thereby minimizing degradation to the monomeric and trimeric states of LHCII in the thylakoid membranes of salt-stressed cucumber leaves. In addition, Put up-regulated the activity of transglutaminase (TGase) and significantly increased the levels of total endogenous polyamines in monomeric and trimeric LHCII under salt stress. These results suggest that Put could play a crucial role in dissipating excess excitation energy by regulating endogenously increased polyamines to alter the adaptation of LHCII, which are most probably involved in an improvement of photochemical efficiency in salt stressed-cucumber seedlings.

Introduction

Soil salinity is one of the most adverse environmental stresses that reduces crop growth and yield through photosynthesis-related physiological processes (Parihar et al., 2015). In plants, salt stress induces the accumulation of excess excitation energy and the generation of reactive oxygen species (ROS). Photosynthesis can also cause a large increase in the production of ROS, including free radicals such as superoxide anion (O2radical dot−), as well as non-radical molecules like hydrogen peroxide (H2O2) and singlet oxygen (1O2) (Foyer, 2018). Increased ROS concentrations easily cause damage to photosystem II (PSII), and subsequently, repair cannot sustain the rate of damage leading to lower photochemical efficiency of PSII (Asada, 2006; Shu et al., 2012). Non-photochemical quenching (NPQ) is an important photo-protective process by which plants exposed to abiotic stress dissipate the harmful excess excitation energy as heat (Horton et al., 2005). It has been shown that plants involved in a process of NPQ by the light-harvesting complexes II aggregation (LHCII), mitigate damage to the structure of the photosynthetic apparatus (Zubik et al., 2013).

LHCII is a supercomplex of chlorophyll (Chl) and carotenoid (Car) binding proteins in the thylakoid membrane and consists of monomeric and trimer forms (Standfuss and Kühlbrandt, 2004). The most abundant trimeric major LHCII component contains three subunits, Lhcb1, Lhcb2 and Lhcb3, and is located at the periphery of PSII. Three minor LHCII components, CP29 (Lhcb4), CP26 (Lhcb5) and CP24 (Lhcb6), exist in monomeric form and bridge the major LHCII with the core complex (Horton et al., 1991). These protein complexes are not only responsible for the absorption of solar energy and transfer of excitation energy to the PSII reaction centers but also protect plants against the damaging effect of excessive energy under environmental stress conditions (Horton et al., 1999).

The tetraamine spermine (Spm), triamine spermidine (Spd), and diamine putrescine (Put), are the most common polyamines (PAs) in many organisms (Gill and Tuteja, 2010). It was hypothesized that PAs play a positive role in the regulation of structure and function of the photosynthetic apparatus (Kotzabasis et al., 1999). Several reports have shown the positive role of PAs in stabilizing protein structure and supporting the formation of more complex aggregates like the LHCII (Besford et al., 1993; Kotzabasis et al., 1993). PAs have been found in the PSII and LHCII of higher plants and have been implicated in a wide array of fundamental processes such as plant growth, senescence, and plant adaptation to abiotic stresses (Hamdani et al., 2011a; Ioannidis and Kotzabasis, 2015). Changes in the levels and types of Put and Spm could play key roles in regulating the LHCII size and enhance the tolerance of the photosynthetic apparatus to UV-B stress (Sfichi et al., 2004). PAs bound to thylakoid membranes induced by exogenous Put are involved in regulatiing PSII reaction center proteins under various environmental stresses (Navakoudis et al., 2007). The regulatory mechanisms of these PAs may be mediated by transglutaminase (TGase) (Sobieszczuk-Nowicka and Legocka, 2014). TGase can bind PAs to antenna proteins of the light-harvesting chlorophyll a/b-protein complex to modulate photosynthetic efficiency and the dissipation of excitation energy, thereby participating in photo-protection and the stabilization of LHCII (Sobieszczuk-Nowicka et al., 2008; Tang et al., 2018).

PAs scavenge ROS to inhibit oxidative damage in photosynthetic apparatuses by adjusting the dissipation of excess excitation energy thus alleviating the adverse effects on cucumber plants under salt stress (Hu et al., 2014; Yuan et al., 2014). In our previous study, exogenous Put improved the conversion of violaxanthin to the final product zeaxanthin and played a crucial role in dissipating excess excitation energy by enhancing the transcriptional level of violaxanthin deepoxidase (Yuan et al., 2018). However, it remained unclear, how Put regulates the aggregation of LHCII under salt stress. In the present study, we investigated the effects of exogenous Put on NPQ components, spectral characteristics of thylakoid membranes, LHCII-associated endogenous PAs levels, expression of LHCII protein and their encoding genes in salt stressed cucumber leaves. The aim was to elucidate Put-mediated protection mechanisms in the photochemical efficiency of cucumber seedlings under salt stress.

Section snippets

Plant material and treatments

Cucumber seeds (Cucumis sativus L., cv. Jingyou No. 4) were grown in a greenhouse in trays containing quartz sand at 28 ± 1 °C day/19 ± 1 °C night, under a maximum photosynthetic photon flux density (PPFD) of about 1200 μmol photons m−2∙s-1 with relative humidity of 75%˜80%. When two true leaves had expanded, the seedlings were transplanted to plastic containers (40 × 30 × 15 cm) containing half-strength Hoagland solution (pH 6.5 ± 0.1, EC 2.0˜2.2 dS m-1). The containers were arranged in a

Fv/Fm and ΦPSII

There were no significant differences in the maximum quantum yield of PS II (Fv/Fm), actual yield of PSII (ΦPSII), minimal fluorescence yield (Fo), and maximum fluorescence yield (Fm) between exogenous Put application and the control (Fig. 1). NaCl treatment for 7 days significantly decreased the values of Fv/Fm and ΦPSII by 11.70% and 64.78% compared with the control, respectively. Salt stress significantly increased the Fo value on days 3 and 7, but decreased the value of Fm on day 7. In

Discussion

Dissipation of excess excitation energy within LHCII by NPQ is an important photo-protective process in higher plants. Reported showed that when the rate of the energy-converting process within the reaction centers became insufficient to process the amount of harvested photons, the antenna complexes were able to switch to a protective function by dissipating the excess excitation energy as heat through the NPQ mechanism (Ruban et al., 2012; Kaňa et al., 2016). In the present study, the

Conclusion

The present study suggests that exogenous Put regulates NPQ and increases endogenous PAs levels in the monomeric and trimeric LHCII of salt-stressed cucumber seedlings. The increased binding of polyamines to the thylakoid membranes by TGase involved in the adaptational status of LHCII aggregation exerts a key regulatory role in dissipating excess excitation energy and confers protection against the inhibition of photochemical efficiency induced by salt stress.

Author statement

We thank the numerous individuals who participated in this study. Shirong Guo designed the experimental research content. Sheng Shu and Ruonan Yuan wrote the manuscript and performed the experiments. Jie Chen, Jianqiang Wu and Jianlan Shen prepared all the figures and performed some experiments. Jin Sun analyzed the data. Liangjie Wang and Yu Wang modified this manuscript. All authors reviewed and approved the manuscript.

Funding

This work was funded by the Fundamental Research Funds for the Central Universities (KYZ201738), the National Natural Science Foundation of China (No. 31471869 and 31672199) and was sponsored by the China Agriculture Research System (CARS-23-B12).

Disclosures

The authors have no conflicts of interest to declare.

Acknowledgements

We thank the numerous individuals who participated in this study. Shirong Guo designed the experimental research content. Sheng Shu and Ruonan Yuan wrote the manuscript and performed the experiments. Jie Chen, Jianqiang Wu and Jianlan Shen prepared all the figures and performed some experiments. Jin Sun analyzed the data. Liangjie Wang and Yu Wang modified this manuscript. All authors reviewed and approved the manuscript. We also sincerely thank Dr. Timothy J Hudelson of the University of

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