Regulation of Petunia Pollen Tube Growth by Phytohormones: Identification of Their Potential Targets

It is known that cytoskeleton-dependent trafficking of cell wall and membrane components to apical plasma membrane (PM) coupled with ion transport across pollen PM is crucial for maintaining polar pollen tube growth. To elucidate whether plant hormones are involved in these processes, the effects of exogenous phytohormones, indole-3-acetic acid (IAA), abscisic acid (ABA), gibberellin A₃ (GA₃) and cytokinin (kinetin) on the growth, PM polarization, actin cytoskeleton (AC) organization and cytoplasmic pH (pH_c) of in vitro 4 h-growing petunia pollen tubes were investigated. IAA, ABA and GA₃ displayed the growth-stimulating effects and these were accompanied by orthovanadate-sensitive hyperpolarization of the PM. Fluorescent labeling the enzyme with H⁺-ATPase antibodies exhibited IAA- and ABA-induced lateral PM redistribution of it into the subapical zone of pollen tube PM. Pollen cultivation on the medium with latrunculin B, the inhibitor of actin polymerization, resulted in inhibition of pollen tube growth and simultaneously in the drop of endogenous IAA content. The IAA-growth stimulating effect was correlated with increased content of actin filaments (AF) in both apical and subapical zones of tubes, while ABA and GA₃ exerted the same effect but it was accompanied by redistributing F-actin only to apical zone. In contrast, kinetin decreased the total F-actin content and inhibited pollen tube growth. It has been shown that the рН_c of growing pollen tubes is sensitive to the plant hormones. In the case of male gametophyte growing for 1, 2 and 4 h, IAA induced alkalinization of the cytosol, while ABA and GA₃ exerted qualitatively similar effect only after its growth for 1 h and 4 h, respectively. Kinetin, in contrast, resulted in acidification of the cytosol. All these results, taken together, indicate, for the first time, potential targets of the phytohormone action in pollen tubes.