Signaling in pollen tube growth: crosstalk, feedback, and missing links

Abstract

Pollen tubes elongate rapidly at their tips through highly polarized cell growth known as tip growth. Tip growth requires intensive exocytosis at the tip, which is supported by a dynamic cytoskeleton and vesicle trafficking. Several signaling pathways have been demonstrated to coordinate pollen tube growth by regulating cellular activities such as actin dynamics, exocytosis, and endocytosis. These signaling pathways crosstalk to form a signaling network that coordinates the cellular processes required for tip growth. The homeostasis of key signaling molecules is critical for the proper elongation of the pollen tube tip, and is commonly fine-tuned by positive and negative regulations. In addition to the major signaling pathways, emerging evidence implies the roles of other signals in the regulation of pollen tube growth. Here we review and discuss how these signaling networks modulate the rapid growth of pollen tubes.

Keywords: polarity; pollen development; reproductive biology; signal transduction..

Figure 1.

Structural Regulation of Pollen Tube Growth. A summary diagram shows the brief intracellular organization of a growing pollen tube, including actin cytoskeleton, exocytosis, endocytosis, and cell wall dynamics. CCV, clathrin-coated vesicle; EE, early endosome; LE, late endosome; RE, recycling endosome. For details, see the text.

Figure 2.

Simplified Model of the Discussed Mechanical Signaling Networks in Self-Organizing Pollen Tube Growth. For better-elucidated signaling modules, feedback regulations and pathway crosstalks are common. Their regulations of structural networks are partially revealed, whilst still needing further characterization. The role of emerging new pathways, such as hormone and small peptide signaling, remains elusive. For details, see the text.