ArabidopsisFormin3 Directs the Formation of Actin Cables and Polarized Growth in Pollen Tubes

Abstract

Cytoplasmic actin cables are the most prominent actin structures in plant cells, but the molecular mechanism underlying their formation is unknown. The function of these actin cables, which are proposed to modulate cytoplasmic streaming and intracellular movement of many organelles in plants, has not been studied by genetic means. Here, we show that Arabidopsis thaliana formin3 (AFH3) is an actin nucleation factor responsible for the formation of longitudinal actin cables in pollen tubes. The Arabidopsis AFH3 gene encodes a 785-amino acid polypeptide, which contains a formin homology 1 (FH1) and a FH2 domain. In vitro analysis revealed that the AFH3 FH1FH2 domains interact with the barbed end of actin filaments and have actin nucleation activity in the presence of G-actin or G actin-profilin. Overexpression of AFH3 in tobacco (Nicotiana tabacum) pollen tubes induced excessive actin cables, which extended into the tubes' apices.

Rice Morphology Determinant-Mediated Actin Filament Organization Contributes to Pollen Tube Growth

“…6, A and C), suggesting that the FH1FH2 domain of RMD without tip localization can cause defective polarized growth in pollen tubes. Consistently, overexpression of the FH1FH2 domains of Saccharomyces cerevisiae (yeast) and Arabidopsis formins caused defective polarity and growth arrest (Evangelista et al, 2002;Ye et al, 2009). Additionally, when pollen tubes were monitored individually during growth, the tubes with the transformed RMD-eGFP elongated at a higher rate (24.77 6 3.83 nm s 21 ; n = 16) compared with the control overexpressing only eGFP (20.15 6 2.91 nm s 21 ; n = 19; Fig.…”

“…Overexpression of the actin-nucleating domain of the type I formin AtFH1 increases the number of actin cables in tobacco pollen tubes, exhibiting an initial growth rate increase followed by subsequent growth inhibition due to the accumulation of F-actin cables (Cheung and Wu, 2004). Arabidopsis FH3 and FH5 are critical for pollen tube growth via regulating F-actin nucleation (Ye et al, 2009;Cheung et al, 2010). In Arabidopsis root hairs, AtFH4 and AtFH8 regulate tip growth by modulating the accumulation of F-actin and actin-nucleating activity (Deeks et al, 2005;Yi et al, 2005).…”

“…Overexpression of a lily pollen-specific type I formin, LlFH1, causes excessive actin cables in the pollen tube tip, and knockdown of LlFH1 specifically decreases F-actin in the tube apex and actin fringe formation and results in reduced germination rate of pollen and average length of pollen tubes (Li et al, 2017). Further investigations showed that formins function as nucleation factors to stimulate actin assembly and actin cables in pollen tubes (Ye et al, 2009;Cheung et al, 2010;van Gisbergen and Bezanilla, 2013;Li et al, 2017). These studies indicate that type I formins participate in generating actin structures that facilitate and maintain the tip growth of the pollen tube.…”

“…RNA interference (RNAi) of two type I formins in Arabidopsis and tobacco (Nicotiana tabacum) indicated their involvement in pollen tube growth. The reduction of Formin Homology3 (AtFH3) accumulation causes abolished actin cable formation in the shank region and altered cytoplasmic streaming, leading to shorter and wider cells (Ye et al, 2009). Decreased expression of NtFH5 disrupts actin organization near the apex regions and results in the wavy growth of tobacco pollen tubes (Cheung et al, 2010).…”

“…Distinct organization patterns of the actin cytoskeleton are present in different regions of the pollen tube to maintain the polarized growth of cells. In the shank region, longitudinal actin cables exist throughout the cytoplasm, facilitating the transport of organelles and vesicles between the shank region and the subapex to support the reverse-fountain cytoplasmic streaming pattern (Ye et al, 2009;Cheung et al, 2010;Zheng et al, 2013;Duckney et al, 2017;Qu et al, 2017). At the subapex, actin filaments form regular structures in different species, such as the collar, fringe, mesh, or funnel, and some observations indicated that subapical F-actin could regulate the accumulation of vesicles and the growth of the tip region (Gibbon et al, 1999;Fu et al, 2001;Vidali et al, 2001;Chen et al, 2002;LovyWheeler et al, 2005;Qu et al, 2017).…”