Short-term blue light priming positively modulates rice thermotolerance

The natural coincidence of high intensity blue light and elevated temperatures raises the possibility that blue light may be associated with thermotolerance development. Evaluation of this hypothesis revealed that the short-term (14 h) blue light pre-treatment (200 µmol m −2 s −1 , B) stimulated defence mechanisms, namely up-regulation of the expression of stress-marker genes ( HSP26.2 and HSFA2d ) and promotion of the stress-related phytohormones - abscisic acid (ABA) signalling and cytokinin cis -zeatin content. The protective effect against subsequent heat stress (HS, 42°C, 6 h, white light: 200 µmol m −2 s −1 ) was demonstrated by an attenuated increase in leaf temperature, and maintenance of reactive oxygen species content and photosynthesis at non-stressed control levels. Simultaneously, higher activity of antioxidant enzymes and increased jasmonoyl-isoleucine level were found in leaves. Contrarily, combination of blue and white light (100 +200 µmol m −2 s −1 , WB) pre-treatment missed the thermopriming effect, characteristic for B. However, it exhibited a more effective response to HS compared with plants treated with the same intensity of white light alone, namely higher expression of some HSPs , an enhanced de-epoxidation state of the xanthophyll cycle, stimulated activity of antioxidant enzymes and increased content of jasmonic acid and auxin indole-3-acetic acid in leaves, of cis -zeatin in leaves and crowns and of ABA and salicylic acid in roots. The WB variant had the fastest growth during stress recovery (2 days, 27°C). In conclusion, blue light stimulated defence mechanisms, resulting in stress priming, which suggests that it can be used to increase stress tolerance in greenhouse cultivations.