Heat signaling and stress responses in photosynthesis

Abstract

High temperature represents one of the most serious abiotic stress factors limiting plant photosynthesis, biomass production, and crop productivity. Photosynthetic apparatus is an important heat sensor in plants, sensing a wide range of air temperatures, from moderate to extreme. In this chapter we offer current knowledge on both photochemical and metabolic changes occurring within the photosynthetic apparatus in conditions of heat stress associated with signaling and stress response. The heat stress directly affects the heat- sensitive sites, mainly oxygen-evolving complex of photosystem II and Rubisco activase. It leads to subsequent indirect effects, such as changes of the redox status of individual components on thylakoid membrane in chloroplast and increase in production of reactive oxygen species (ROS). Hence, the redox signaling plays the crucial role in enhancement of alternative electron pathways such as cyclic electron flow as well as triggering the signal transduction pathways resulting to heat-stress response. The redox signaling in chloroplast is closely associated with ROS signaling, which interferes with regulation also out of chloroplast. The stress response involves mainly production of specific proteins (mostly heat shock proteins or antioxidants) or protective compounds (osmoprotectants) leading to increase of thermostability of sensitive sites or protection against ROS. Different signal molecules contribute in photosynthesis-related heat-stress signaling pathways, such as reactive oxygen species with hydrogen peroxide, nitric oxide, calcium, and abscisic acid. The specific roles of cytokinins and isoprene in heatstress response are also reviewed.