Sensitivity and Responses of Chloroplasts to Heat Stress in Plants

Abstract

Increased temperatures caused by global warming threaten agricultural production, as warmer conditions can inhibit plant growth and development or even destroy crops in extreme circumstances. Extensive research over the past several decades has revealed that chloroplasts, the photosynthetic organelles of plants, are highly sensitive to heat stress, which affects a variety of photosynthetic processes including chlorophyll biosynthesis, photochemical reactions, electron transport, and CO₂ assimilation. Important mechanisms by which plant cells respond to heat stress to protect these photosynthetic organelles have been identified and analyzed. More recent studies have made it clear that chloroplasts play an important role in inducing the expression of nuclear heat-response genes during the heat stress response. In this review, we summarize these important advances in plant-based research and discuss how the sensitivity, responses, and signaling roles of chloroplasts contribute to plant heat sensitivity and tolerance.

Keywords: chloroplasts; heat stress; photosynthetic; response; retrograde signals.

FIGURE 1

Sensitivity and responses of chloroplasts under heat stress. Major effects of heat stress on chloroplasts include heat inactivation of PSII, Chl breakdown, inactivation of Rubisco, and impairment of protein translation. In response to heat stress, chloroplasts generate a large number of protein chaperones to protect PSII. Meanwhile, chloroplast protein quality control plays a role in maintaining proteostasis under conditions of environmental stress. Chloroplasts also participate in retrograde signal pathways that protect cellular integrity and the normal growth of plants.

FIGURE 2

The activity of chlorophyllase and Chl-degrading peroxidase under heat stress. Under normal growth conditions, biosynthesis and degradation of Chl are maintained at steady rates. Under heat stress, the activity of chlorophyllase and chlorophyll-degrading peroxidase increases and the content of Chl is severely reduced. Exogenous application of ZR maintains a higher content of Chl.

FIGURE 3

Retrograde signal pathways in chloroplasts under heat stress. Under heat treatment, ROS including H₂O₂ as a signaling molecule, induce the activation of HSEs and subsequently increase the expression of HSPs. RPS1 may activate the expression of the HsfA2-dependent heat-responsive gene in the nucleus. Tyr regulates the production of miRNAs by producing downstream metabolite tocopherols (vitamin E). The generation of tocopherols and PAP prevents the degradation of primary miRNAs and promotes the formation of miRNAs to increase the heat tolerance of plants.