Chilling induces a decrease in pyrophosphate-dependent H+-accumulation associated with a DeltapH(vac)-stat in mung bean, a chill-sensitive plant

Chilling leads to cytoplasmic acidification in chill-sensitive plants. A possible explanation for this observation is that a ΔpH-stat between the cytosol and vacuole (ΔpH_vac-stat) is perturbed by chilling. To understand the nature of this ΔpH_vac-stat, the effect of temperature, between 20 and 0 °C, on pyrophosphate (PPi)- or ATP-dependent acidification of vacuolar vesicles, isolated from mung bean hypocotyls, was determined. Over the temperature range investigated, the H⁺-influx mediated by PPase was balanced with the H⁺-efflux, which was PPi-dependently suppressed, and consequently a constant pH in vesicles (pH_in) of ca. 5 was maintained against temperature changes. However, the ΔpH_in driven by ATP decreased as the temperature dropped. Thus, the PPi-dependent H⁺-accumulation may function as an essential factor to form a ΔpH_vac-stat against temperature changes. Next, to study the chilling sensitivity of PPi-dependent H⁺-accumulation, vacuolar vesicles were isolated from control seedlings or from seedlings chilled at 0 °C for 1 d. Chilling treatment resulted in a decrease in the H⁺-accumulation rate and in the steady-state ΔpH_in formed by PPi, the causes of which were enhanced by PPi-dependent H⁺-efflux and reduced by H⁺-influx driven by PPase. Together, the results suggest that the decrease of PPi-dependent H⁺-accumulation associated with the ΔpH_vac-stat could result in cytoplasmic acidification.