Abstract
A population of dynamic apical actin filaments is required for rapid polarized pollen tube growth. However, the cellular mechanisms driving their assembly remain incompletely understood. It was postulated that formin is a major player in nucleating apical actin assembly, but direct genetic and cytological evidence remains to be firmly established. Here we found that both Arabidopsis formin 3 (AtFH3) and formin 5 (AtFH5) are involved in the regulation of apical actin polymerization and actin array construction in pollen tubes, with AtFH3 playing a more dominant role. We found that both formins have plasma membrane (PM) localization signals but exhibit distinct PM localization patterns in the pollen tube, and loss of their function reduces the amount of apical actin filaments. Live-cell imaging revealed that the reduction in filamentous actin is very likely due to the decrease in filament elongation. Furthermore, we found that the rate of tip-directed vesicle transport is reduced and the pattern of apical vesicle accumulation is altered in formin loss-of-function mutant pollen tubes, which explains to some extent the reduction in pollen tube elongation. Thus, we provide direct genetic and cytological evidence showing that formin is an important player in nucleating actin assembly from the PM at pollen tube tips.
Highlights
The pollen tube is the passage for two non-motile sperm cells and its proper growth is essential for successful reproduction in flowering plants [1,2,3]
Using state-of-the-art live-cell imaging in combination with reverse genetic approaches, we demonstrate here that two class I formins, Arabidopsis formin 3 (AtFH3) and formin 5 (AtFH5), are involved in the regulation of apical actin polymerization and actin array construction in pollen tubes
We found that the pollen germination percentage was slightly but significantly reduced in pollen derived from fh3-1, fh3-2, fh5-3, fh3-1 fh5-2 and fh3-2 fh5-3 mutant plants (Fig 1F and 1G)
Summary
The pollen tube is the passage for two non-motile sperm cells and its proper growth is essential for successful reproduction in flowering plants [1,2,3]. Pollen tube growth is tightly regulated, and this raises many fascinating questions. Numerous studies suggest that the actin cytoskeleton is the core of the regulatory network of pollen tube growth, presumably by coordinating with various cellular events, such as the trafficking, docking and fusion of vesicles and the construction of the cell wall [4,5,6,7]. Actin filaments are arranged into distinct structures within different regions of growing pollen tubes, and these structures carry out distinct cellular functions [8,9,10,11,12,13]. Dynamic actin filaments within the apical region were demonstrated to be directly associated with the growth and turning of pollen tubes [14,15,16]. We still have an incomplete understanding of how those apical actin filaments are constantly generated in pollen tubes
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