The Regulation of Actin Organization by Actin-Depolymerizing Factor in Elongating Pollen Tubes[W]

Abstract

Pollen tube elongation is a polarized cell growth process that transports the male gametes from the stigma to the ovary for fertilization inside the ovules. Actomyosin-driven intracellular trafficking and active actin remodeling in the apical and subapical regions of pollen tubes are both important aspects of this rapid tip growth process. Actin-depolymerizing factor (ADF) and cofilin are actin binding proteins that enhance the depolymerization of microfilaments at their minus, or slow-growing, ends. A pollen-specific ADF from tobacco, NtADF1, was used to dissect the role of ADF in pollen tube growth. Overexpression of NtADF1 resulted in the reduction of fine, axially oriented actin cables in transformed pollen tubes and in the inhibition of pollen tube growth in a dose-dependent manner. Thus, the proper regulation of actin turnover by NtADF1 is critical for pollen tube growth. When expressed at a moderate level in pollen tubes elongating in in vitro cultures, green fluorescent protein (GFP)-tagged NtADF1 (GFP-NtADF1) associated predominantly with a subapical actin mesh composed of short actin filaments and with long actin cables in the shank. Similar labeling patterns were observed for GFP-NtADF1-expressing pollen tubes elongating within the pistil. A Ser-6-to-Asp conversion abolished the interaction between NtADF1 and F-actin in elongating pollen tubes and reduced its inhibitory effect on pollen tube growth significantly, suggesting that phosphorylation at Ser-6 may be a prominent regulatory mechanism for this pollen ADF. As with some ADF/cofilin, the in vitro actin-depolymerizing activity of recombinant NtADF1 was enhanced by slightly alkaline conditions. Because a pH gradient is known to exist in the apical region of elongating pollen tubes, it seems plausible that the in vivo actin-depolymerizing activity of NtADF1, and thus its contribution to actin dynamics, may be regulated spatially by differential H ؉ concentrations in the apical region of elongating pollen tubes.

Cytoskeleton-Associated Large RNP Complexes in Tobacco Male Gametophyte (EPPs) Are Associated with Ribosomes and Are Involved in Protein Synthesis, Processing, and Localization

“…33 To examine whether they also contained other transcripts, RT-PCR analysis was performed. Several tobacco pollen-specific genes were tested including profilin, 39 actin depolymerizing factor 1 (ADF1), 40 eukaryotic translation initiation factors eIF4E, eIFiso4E 41 and R-tubulin A1 (accession number AJ421411; D. Brevario, direct submission). RT-PCR analysis revealed that all tested transcripts were present in the EPP fractions isolated by ultracentrifugation as well as directly from SEC ( Figure 3).…”

Actin bundling in plants

“…These actin phenotypes are accompanied by either a reduction or stimulation of cell growth, respectively. In another study, over-expression of a pollen-specific ADF from tobacco, NtADF1, noticeably reduces the number of long axially oriented actin bundles in the pollen tube shank [Chen et al, 2002]. Regarding tip-growth, the subapical cortical fringe composed of parallel short bundles is unable to form in tip-growing moss protonema cells lacking ADF function [Augustine et al, 2008].…”

“…The apex of the pollen tube is submitted to constant changes in ionic conditions, including a fluctuating apical Ca 21 gradient and a subapical alkaline region [Holdaway-Clarke and Hepler, 2003;, which regulate actin cytoskeleton dynamics and organization by the activation/inactivation of ABPs [Ren and Xiang, 2007]. As an example, ADF predominantly localizes to the subapical region, where its fragmenting activity is stimulated by alkaline pH conditions [Chen et al, 2002;Lovy-Wheeler et al, 2006]. Given the high degree of actin cytoskeleton remodeling in the subapical region, the series of subapical actin structures reported so far has been suggested to belong to a continuum of structural configurations that interconvert from one to another during pollen tube growth.…”

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

“…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). At the apex, actin filaments are generally less abundant but are highly dynamic with dense actin structures, which mediates the interaction between the apical/subapical PM and the cytoplasm during rapid tube elongation (Fu et al, 2001; Lee and Yang, 2008;Cai and Cresti, 2009;Cheung et al, 2010;Qu et al, 2013;Rounds et al, 2014;Zhou et al, 2015;Qu et al, 2017).…”