Arabidopsis RIC1 Severs Actin Filaments at the Apex to Regulate Pollen Tube Growth

Abstract

Pollen tubes deliver sperms to the ovule for fertilization via tip growth. The rapid turnover of F-actin in pollen tube tips plays an important role in this process. In this study, we demonstrate that Arabidopsis thaliana RIC1, a member of the ROP-interactive CRIB motif-containing protein family, regulates pollen tube growth via its F-actin severing activity. Knockout of RIC1 enhanced pollen tube elongation, while overexpression of RIC1 dramatically reduced tube growth. Pharmacological analysis indicated that RIC1 affected F-actin dynamics in pollen tubes. In vitro biochemical assays revealed that RIC1 directly bound and severed F-actin in the presence of Ca2+ in addition to interfering with F-actin turnover by capping F-actin at the barbed ends. In vivo, RIC1 localized primarily to the apical plasma membrane (PM) of pollen tubes. The level of RIC1 at the apical PM oscillated during pollen tube growth. The frequency of F-actin severing at the apex was notably decreased in ric1-1 pollen tubes but was increased in pollen tubes overexpressing RIC1. We propose that RIC1 regulates F-actin dynamics at the apical PM as well as the cytosol by severing F-actin and capping the barbed ends in the cytoplasm, establishing a novel mechanism that underlies the regulation of pollen tube growth.

Signaling with Ions: The Keystone for Apical Cell Growth and Morphogenesis in Pollen Tubes

“…Conversely, AtRIC4 in response to Ca 2+ promotes F-actin assembly (Gu et al, 2005). Additionally, recent studies investigating the apical PMlocalized AtRIC1 add to the importance of RIC to regulate actin (Zhou et al, 2015b). Similar to AtRIC3, AtRIC1 binds to and severs F-actin in the presence of Ca 2+ .…”

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

“…Besides binding to actin filaments, our previous study showed that RMD also is a microtubule-stabilizing protein . Several previous investigations showed that treatment with the microtubuledepolymerizing chemical oryzalin causes no obvious effect on pollen tube growth (Bou Daher and Geitmann, 2011;Zhou et al, 2015), suggesting a minimal function of microtubules on pollen tube growth (Cai et al, 1997). In addition, unlike the role of formins in nucleating the unbranched actin filaments, the actin-related protein2/3 (ARP2/3) complex is known to form a branched actin network (Cheung and Wu, 2004).…”

“…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). Recently, Qu et al, (2017) demonstrated that F-actin originating from the apical membrane forms a specialized structure consisting of longitudinally aligned actin bundles at the cortex and inner cytoplasmic filaments with a distinct distribution, which plays a pivotal role in the regulation of the growth of the pollen tube tip in Arabidopsis (Arabidopsis thaliana).…”

“…VLN2, VLN5, AtFIM5, and LlFIM1 bundle apical PM-anchored F-actin and contribute to the formation of actin fringe or collar structures in the subapical region (Su et al, 2012;Qu et al, 2013;Zhang et al, 2016a). In the apex, several actin-binding proteins, including VLN and Ropinteractive CRIB Motif-containing Protein1, are implicated in the patterning of apical F-actin dynamics Zhou et al, 2015). However, the mechanisms that maintain the dynamic polarity of longitudinal actin cables and facilitate the polarized recycling of shank-apex actin still remain elusive.…”

ROP GTPases Structure-Function and Signaling Pathways

“…The severing activity of plasma membrane-localized RIC1 mediates the release of F-actin from the pollen tube apical plasma membrane into the cytoplasm. RIC1 also contributes to severing F-actin and capping the barbed ends in the cytoplasm (Zhou et al, 2015).…”