Effects of exogenous phytohormones on intracellular pH of Petunia hybrida pollen grains

Abstract

The effects of exogenous phytohormones (IAA, ABA, and GA3) on the intracellular (cytoplasmic) pH (pHc) in ungerminating and germinating petunia (Petunia hybrida L.) pollen grains were studied. The pHc values were measured with fluorescein diacetate. In ungerminating pollen grains, all phytohormones reduced pHc relatively rapidly; after 10–15 min, initial value was restored. In germinating pollen grains, IAA and ABA induced a relatively rapid cytosol alkalization, which was not reversed during experiment. GA3 acidified the cytosol, i.e., exerted the effect similar to that in ungerminating pollen grains. Sodium orthovanadate suppressed completely the hormone-induced pHc shift toward alkaline values in germinating pollen grains, whereas this inhibitor did not affect pHc in the absence of phytohormones. Sodium orthovanadate also slowed the recovery of pHc after hormone-induced cytoplasm acidification in ungerminating pollen grains, reduced pHc in control ungerminating grains, and weakened substantially the effects of all phytohormones on these pollen grains. On the basis of these results, we suggested that physiological activities of phytohormones in this system were mediated by pHc modulation, namely, a transient disturbance in the cytosolic pH homeostasis, which could trigger further phytohormone-induced cell responses. We concluded that a hormone-induced cytoplasm alkalization in pollen grains was mediated by the activity of their plasma membrane H+-ATPase and that this proton pump was involved in the control of pHc in both germinating and ungerminating pollen grains.