MAP18 Regulates the Direction of Pollen Tube Growth in Arabidopsis by Modulating F-Actin Organization    

Abstract

For fertilization to occur in plants, the pollen tube must be guided to enter the ovule via the micropyle. Previous reports have implicated actin filaments, actin binding proteins, and the tip-focused calcium gradient as key contributors to polar growth of pollen tubes; however, the regulation of directional pollen tube growth is largely unknown. We reported previously that Arabidopsis thaliana MICROTUBULE-ASSOCIATED PROTEIN18 (MAP18) contributes to directional cell growth and cortical microtubule organization. The preferential expression of MAP18 in pollen and in pollen tubes suggests that MAP18 also may function in pollen tube growth. In this study, we demonstrate that MAP18 functions in pollen tubes by influencing actin organization, rather than microtubule assembly. In vitro biochemical results indicate that MAP18 exhibits Ca2+-dependent filamentous (F)-actin-severing activity. Abnormal expression of MAP18 in map18 and MAP18 OX plants was associated with disorganization of the actin cytoskeleton in the tube apex, resulting in aberrant pollen tube growth patterns and morphologies, inaccurate micropyle targeting, and fewer fertilization events. Experiments with MAP18 mutants created by site-directed mutagenesis suggest that F-actin-severing activity is essential to the effects of MAP18 on pollen tube growth direction. Our study demonstrates that in Arabidopsis, MAP18 guides the direction of pollen tube growth by modulating actin filaments.

CASEIN KINASE1-LIKE PROTEIN2 Regulates Actin Filament Stability and Stomatal Closure via Phosphorylation of Actin Depolymerizing Factor

“…For instance, actin dynamics have been implicated in mediating Ca 2+ concentration changes in the cytosol, K + channel activity, and vacuole shape determination (Hwang et al, 1997;Gao et al, 2009;Zhao et al, 2013), but the related molecular mechanisms remain to be established. One possibility is regulation of membrane recycling by actin reorganization Lee et al, 2008;Zhao et al, 2010;Bou Daher and Geitmann, 2011;Zhu et al, 2013), which would change the localization of ion channels/transporters and the area of vacuolar membrane. Actin dynamics may also act as an important component in signal transduction to mediate such changes (Gao et al, 2008;Zhao et al, 2011Zhao et al, , 2013Jiang et al, 2012).…”

Kinesins Have a Dual Function in Organizing Microtubules during Both Tip Growth and Cytokinesis in Physcomitrella patens

“…In apical cells expressing Lifeact-mEGFP, an actin focal point is also observed in the apical dome (Vidali et al, 2009). The microtubule-associated protein MAP18 functions in the direction of pollen tube growth by modulating actin filaments in Arabidopsis (Zhu et al, 2013). MAP18 can bind the MTs in vivo and is associated with cortical MTs in epidermis cells (Wang et al, 2007).…”

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

“…ric1-1 pollen tubes were more resistant to LatB than wild-type (Ws-2) tubes, whereas there was no significant difference between OX 9 and Col-0 pollen tubes. Movie 1, Supplemental Figure 4B, and previous reports (Qu et al, 2013;Zhu et al, 2013). Highly dynamic fine F-actin structures were observed at the apices, abundant and dense F-actin structures were present at the subapical regions, and parallel longitudinally orientated F-actin cables were found at the shank regions in Col-0, RIC1 OX 9, and ric1-1 (Col-0) pollen tubes.…”

“…In addition to RIC1, several actin regulators, such as VLN2, VLN5, MAP18, and MDP25, have been found to regulate pollen tube growth via actin filament severing activity in recent studies (Qu et al, 2013;Zhu et al, 2013;Qin et al, 2014). MAP18 is involved in regulating the growth direction of pollen tubes but has little effect on elongation rate.…”

“…Arabidopsis thaliana MAP18, a microtubule-destabilizing factor that modulates polarized cell growth in vegetative tissues (Wang et al, 2007), was recently demonstrated to regulate the direction of pollen tube growth via its Ca 2+ -dependent F-actin severing activity . VLN2, VLN5, and MAP18 were shown to sever F-actin in the cytoplasm at the tip of the pollen tube (Qu et al, 2013;Zhu et al, 2013), where a tip-focused Ca 2+ gradient is present. Qu et al (2013) proposed that VLN2 and VLN5 sever F-actin originating from the apical PM to facilitate its subsequent use in the construction of the actin collars/fringe.…”