Heat shock responses in Populus euphratica cell cultures: important role of crosstalk among hydrogen peroxide, calcium and potassium

Populus euphratica is an important woody species in north-west China that can survive extremely high temperatures. In the present study, the defensive responses of P. euphratica cells to moderate heat shock (HS, 48 °C) were investigated using fluorescence imaging technique and a non-invasive vibrating ion-selective microelectrode. HS induced a hydrogen peroxide (H2O2)—and caspase-like protease activity-dependent programmed cell death (PCD, <25 %) in P. euphratica cells. After HS, the total activities of antioxidant enzymes, such as catalase, ascorbate peroxidase and glutathione reductase, as well as those of non-enzymatic antioxidants, were significantly enhanced, leading to a decline in H2O2 accumulation. However, the upregulated antioxidant system was dependent on the activation of plasma membrane (PM) Ca2+-ATPase in HS-treated P. euphratica cells. In addition, H2O2, which was inhibited by the DPI (an inhibitor of PM NADPH oxidase as well as mitochondrial flavin-containing enzymes), was required for the activation of PM Ca2+-ATPase under HS condition. Moreover, HS induced an early influx of Ca2+ and an early efflux of K+ in P. euphratica cells. However, the inhibition of this Ca2+ influx and K+ efflux by GdCl3 and Tetraethylammonium chloride significantly decreased the production of H2O2. These results suggest that P. euphratica cells adapt to heat stress via the improvement of the antioxidant system. The upregulation of the antioxidant system is regulated by PM Ca2+-ATPase, H2O2, and the early activation of Ca2+ and K+ channels in the PM. Finally, a model was postulated to reveal the HS responses in P. euphratica cells.