Abstract
The angiosperm pollen tube delivers two sperm cells into the embryo sac through a unique growth strategy, named tip growth, to accomplish fertilization. A great deal of experiments have demonstrated that actin bundles play a pivotal role in pollen tube tip growth. There are two distinct actin bundle populations in pollen tubes: the long, rather thick actin bundles in the shank and the short, highly dynamic bundles near the apex. With the development of imaging techniques over the last decade, great breakthroughs have been made in understanding the function of actin bundles in pollen tubes, especially short subapical actin bundles. Here, we tried to draw an overall picture of the architecture, functions and underlying regulation mechanism of actin bundles in plant pollen tubes.
Highlights
In the sexual reproduction of angiosperms, firstly, the pollen grain hydrates and produces the pollen tube
Recent research has shown that the short actin bundles at the cortex of the sub-apex region exhibit uniform polarity with their barbed-end anchoring on the apical membrane [7,45], and, supposedly, they serve as tracks for the barbed end directed myosin XIs [46], while the internal fine actin filaments in the sub-apex are involved in regulating the backward movement of vesicles, presumably by acting as a physical barrier [26]
In adf7 pollen tube shanks, the severing frequency and depolymerization rate of filaments significantly decreased, while their maximum lifetime significantly increased [103]. These results suggest that actin depolymerizing factors (ADFs)-mediated severing regulates the turnover of longitudinal actin cables to promote pollen tube tip growth [103]
Summary
In the sexual reproduction of angiosperms, firstly, the pollen grain hydrates and produces the pollen tube It passes through the column and reaches the micropyle via apical growth. The double fertilization process is completed [1] In this process, the polar growth of pollen tubes is a key link in the sexual reproduction of higher plants. The microfilaments consist of long, rather thick bundles, which are parallel to the growth direction of the pollen tube and extend to the base of the subapical region [21]. Long-distance transport relies on the microfilament bundles in the pollen tube shank [22,23], while short-distance transport depends on the short and highly dynamic actin bundles in the subapical region [24,25,26]. This article aims to provide a comprehensive survey covering the distribution, organization, and regulation of actin bundles in pollen tubes
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