1 INTRODUCTION
Wheat (Triticum aestivum ) is a significant staple crop, which is widely grown worldwide. Drought and the rising global temperature are important factors affecting wheat production. According to one report, when the temperature increases by 1°C, wheat production is reduced by 1-4% (Wiegand & Cuellar,1981). A single stress, such as heat or drought, does not usually occur alone during natural production (Howarth & Ougham,1993).
PS II is extremely vulnerable to heat and drought stress and is regarded as the original site and principal component of photoinhibition (Dongsansuk, Lütz, & Neuner,2013). The first PS II component to be damaged is the oxygen evolving complex (OEC) (Takahashi, Milward, Fan, Chow & Badger, 2009), which leads to damage to the entire PS II reaction center. A damaged PS II is rapidly repaired at room temperature due to rapid synthesis of the PS II core protein D1, which can be assembled together with undamaged subunits to form a new PS II complex (Huang, Yang, Zhang, Zhang & Cao, 2012).
It has been reported that photoinhibition of PS II due to abiotic stresses can be alleviated by trehalose (Karim et al., 2007). Trehalose changes hydration of the PS II complex, which results in a conformational transition in PS II and more effectively protects the PS II complex (Yanykin, Khorobrykh, Mamedov & Klimov, 2015). Trehalose also significantly stimulates the stabilized oxygen evolution rate in the PS II complex (Mamedov, Petrova, Yanykin, Zaspa & Semenov,2015). However, the specific protective mechanism of trehalose against stress remains unclear. The trehalose level in plants is normally very low. Nevertheless, applying exogenous trehalose increases the internal level of trehalose and has been suggested as an alternative approach to improve stress tolerance (Chen & Murata,2002).
Activation of cyclic electron flow (CEF) effectively protects PS II against abiotic stressors (Wang et al.,2013). Huang et al. (2010) reported that the PS II complex in a tropical rosewood species rapidly recovers under low light conditions due to excitation of CEF (Huang, Zhang & Cao, 2010) The main feature of this repair process is replacement of the D1 protein in the photodamaged PS II by newly synthesized D1 and reassembly of active PS II (Andersson & Aro, 2001). CEF is necessary to stabilize OEC activity under high intensity light in the tropical tree species Erythrophleum guineense and thus prevent PS II from serious photodamage (Huang, Yang, Hu, Zhang & Cao, 2016). CEF generates an extra proton gradient across the thylakoid membrane, which promotes ATP synthesis to meet the demand for more ATP under stress(Zhang, Huang, Zhang & Cao, 2016) Synthesis of the D1 protein is enhanced by synthesis of ATP, which plays a key role in the photodamage and repair process of PS II (Allakhverdiev, Nishiyama, Takahashi, Miyairi, Suzuki & Murata, 2005).
Plants activate protective mechanisms, such as CEF, for PS II under stress. However, it is unknown whether trehalose can protect PS II by promoting this protective mechanism. Therefore, this study focused on the relationship between trehalose and CEF under heat and drought stress. Our results provide new insight into the effects of exogenous trehalose on PS II photoprotective mechanisms in wheat leaves under heat and drought stress.
2 MATERIALS AND METHODS