Abstract
Photosystems of higher plants alleviate heat-induced damage in the presence of light under moderate stressed conditions; however, in the absence of light (i.e., in the dark), the same plants are damaged more easily. (Yamauchi and Kimura, 2011) We demonstrate that regulating photochemical energy transfer in heat-treated wheat at 40 °C with light contributed to heat tolerance of the photosystem. Chlorophyll fluorescence analysis using heat-stressed wheat seedlings in light showed increased non-photochemical quenching (NPQ) of chlorophyll fluorescence, which was due to thermal dissipation that was increased by state 1 to state 2 transition. Transmission electron microscopy revealed structural changes in thylakoid membranes, including unstacking of grana regions under heat stress in light. It was accompanied by the phosphorylation of thylakoid proteins such as D1 and D2 proteins and the light harvesting complex II proteins Lhcb1 and Lhcb2. These results suggest that heat stress at 40 °C in light induces state 1 to state 2 transition for the preferential excitation of photosystem I (PSI) by phosphorylating thylakoid proteins more strongly. Structural changes of thylakoid membrane also assist the remodeling of photosystems and regulation of energy distribution by transition toward state 2 probably contributes to plastoquione oxidation; thus, light-driven electrons flowing through PSI play a protective role against PSII damage under heat stress.
Highlights
High temperatures under recent climate changes is a major environmental constraint for plant production because photosynthesis, which includes photochemical reactions as well as carbon assimilation, is a very heat-sensitive process [1]
Fv/Fm did not decrease under a 40 °C heat treatment in light [19], indicating that light drives a system that protects Photosystem II (PSII) from damage caused by over-reduction of PQ at high temperatures
To obtain a better understanding of heat tolerance of the photosystem, we focused on the regulating photochemical energy transfer in heat-treated wheat at 40 °C with light
Summary
High temperatures under recent climate changes is a major environmental constraint for plant production because photosynthesis, which includes photochemical reactions as well as carbon assimilation, is a very heat-sensitive process [1]. A change in energy transfer in photosystems is believed to alleviate heat-induced damage, e.g., thermal dissipation and state transition [10]. It has been recently demonstrated that state transition is a long-term acclimation to various natural light conditions in higher plants and that a part of the light-harvesting chlorophyll-binding protein II (LHCII) is phosphorylated and behaves as an effective PSI antenna [14]. Various environmental stressors in higher plants, including heat stress, tend to affect state transition through a change in energy distribution between the photosystems [15]. The results revealed that heat stress induces state 2; we observed increased non-photochemical quenching (NPQ) of chlorophyll fluorescence, LHCII phosphorylation and unstacked grana regions in thylakoid membranes in response to heat stress
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