Role of abscisic acid and ethylene in the control of water transport-driving forces in germinating petunia male gametophyte

Data on ABA involvement in osmoregulation of in vitro germinating petunia (Petunia hybrida L.) male gametophyte were obtained. Two potential targets of ABA action in a pollen tube (PT) are identified. These are represented by (1) plasma membrane (PM) H⁺-ATPase, electrogenic proton pump participating in PM polarization, and (2) Ca-dependent K⁺-channels localized in the same membrane. It was established that a stimulatory effect of ABA on electrogenic activity of H⁺-ATPase is mediated by the increase in free Ca²⁺ level in the cytosol of a PT and reactive oxygen species (ROS) generation. Based on the results obtained on the role of K⁺ ions in the hormonal control of water transport-driving forces in a PT, the hypothesis suggesting that ABA stimulated pollen grains (PGs) germinating and PT growth by activating K⁺-channels was put forward. The revealed ABA-induced shift in cytoplasmic pH (pH_c) is suggested to be involved in a cascade of the events of the progamic phase of fertilization, including pH-dependent K⁺-channels functioning. It was established that ABA abolishes the inhibitory effects of ethylene receptors blocker, 1-methylcyclopropene (1-MCP), and blockers of ACC and ABA synthesis (aminooxyacetic acid, AOA, and fluridone, respectively) on PT germination and growth, whereas ethrel blocks the inhibitory effect of fluridone on PT growth. In stigmas pretreated with ABA and AOA before pollination, this phytohormone was found to suppress inhibitory effect of AOA on ACC synthesis in the pollen-pistil system. All these findings, taken together, led us to the conclusion that ABA is involved in petunia male gametophyte osmoregulation interacting with ethylene at the level of ACC synthesis in the progamic phase of fertilization.