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