Increases in Absolute Temperature Stimulate Free Calcium Concentration Elevations in the Chloroplast

Abstract

Plants need to sense increases in temperature to be able to adapt their physiology and development to survive; however, the mechanisms of heat perception are currently relatively poorly understood. Here we demonstrate that in response to elevated temperature the free calcium concentration of the stroma of chloroplasts increases. This response is specific to the chloroplast, as no corresponding increase in calcium is seen in the cytosol. The chloroplast calcium response is dose-dependent above a threshold. The magnitude of this calcium response is dependent upon absolute temperature, not rate of heating. This response is dynamic: repeated stimulation leads to rapid attenuation of the response, which can be overcome by sensitisation at a higher temperature. More long-term acclimation to different temperatures resets the basal sensitivity of the system, such that plants acclimated to lower temperatures are more sensitive than those acclimated to higher temperatures. The heat-induced chloroplast calcium response was partially dependent upon the calcium-sensing receptor CAS which has been shown previously to regulate other chloroplast calcium signalling responses. Taken together our data demonstrate the ability of chloroplasts to sense absolute high temperature and produce commensurately quantitative stromal calcium response, the magnitude of which is a function of both current temperature and stress history.

Chloroplast Calcium Signaling in the Spotlight

“…In contrast to the above-mentioned stimuli, that have been demonstrated to trigger Ca 2+ transients in both chloroplasts and the cytosol, increases in absolute temperature were found to evoke Ca 2+ responses specific to chloroplasts, as no corresponding elevations were detected in the cytosol (Lenzoni and Knight, 2019). Interestingly, also in this case the chloroplast Ca 2+ response was found to be partially dependent on CAS (Lenzoni and Knight, 2019).…”

Recapitulation of the Function and Role of ROS Generated in Response to Heat Stress in Plants

“…The impediment to plant development is even more severe when the temperature rise is accompanied by other environmental stressors such as drought and pathogen infection [5,7,10]. In response to these harmful stresses, signaling molecules such as Ca 2+ and reactive oxygen species (ROS) accumulate in plants, as well as alter the intracellular levels of the phytohormones such as salicylic acid (SA) and jasmonic acid (JA), which are also associated with heat stress and defense [10][11][12][13][14][15][16][17].…”

Chloroplasts Use Calcium Signals to Call for Help under Heat Stress

“…In this issue, Lenzoni and Knight (2019) show that increases in the absolute temperature elevate the free calcium concentration in chloroplasts (Lenzoni and Knight 2019). This response is specific to chloroplasts as a similar response is not detectable in the cytosol after a sudden shift from 20�C to 40�C (Fig.�1).…”