Regulation of actin dynamics by actin-binding proteins in pollen

Abstract

A dynamic network of polymers, the actin cytoskeleton, co-ordinates numerous fundamental cellular processes. In pollen tubes, organelle movements and cytoplasmic streaming, organization of the tip zone, vesicle trafficking, and tip growth have all been linked to actin-based function. Further, during the self-incompatibility response of Papaver rhoeas, destruction of the cytoskeleton is a primary target implicated in the rapid cessation of pollen tube growth and alterations in actin dynamics are associated with the initiation of programmed cell death. Surprisingly, these diverse cellular processes are accomplished with only a small amount of filamentous actin and a huge pool of polymerizable monomers. These observations hint at incredibly fast and complex actin dynamics in pollen. To understand the molecular mechanisms regulating actin dynamics in plant cells, the abundant actin monomer-binding proteins, a major filament nucleator, a family of bundling and severing proteins, and a modulator of growth at the barbed-end of actin filaments have been characterized biochemically. The activities of these proteins are generally consistent with textbook models for actin turnover. For example, the three monomer-binding proteins, profilin, ADF, and CAP, are thought to function synergistically to enhance turnover and the exchange of subunits between monomer and polymer pools. How individual actin filaments behave in living cells, however, remains largely unexplored. Actin dynamics were examined using variable angle epifluorescence microscopy (VAEM) in expanding hypocotyl epidermal cells. Our observations of single filament behaviour are not consistent with filament turnover by treadmilling, but rather represent a novel property called stochastic dynamics. A new model for the dynamic control of actin filament turnover in plant cells is presented.

Organisation and regulation of the cytoskeleton in plant programmed cell death

“…ABPs regulate F-actin polymerisation and organisation in plant cells 2 and are likely to have a role in mediating PCD. To date rather few ABPs have been examined in the context of PCD.…”

“…Alterations to the actin cytoskeleton are regulated by actin-binding proteins (ABPs) that stimulate formation of new filaments, promote filament elongation, bundle filaments together, strengthen interaction between actin/tubulin subunits, induce severing and depolymerisation. 2 Plant microtubules undergo dynamic instability 3 under the control of microtubule-associated proteins (MAPs; reviewed by Hamada 4 and Sedbrook 5 ). The dynamic nature of F-actin and microtubules enables flexibility of cytoskeletal organisation, making it an integral part of signalling networks, translating developmental and environmental cues into cellular responses.…”

ArabidopsisFIMBRIN5, an Actin Bundling Factor, Is Required for Pollen Germination and Pollen Tube Growth

“…Furthermore, imaging of actin filaments has shown that distinct actin structures form in pollen tubes across different species. The consensus view is that actin filaments can be divided into at least three distinct structures in pollen tubes: longitudinal actin cables found in the shanks of pollen tubes; dense actin structures, such as collar-like structures or actin fringes, found at the subapexes; and highly dynamic, but less abundant, actin filaments found at the extreme tips (Kost et al, 1998;Fu et al, 2001;Lovy-Wheeler et al, 2005;Cheung et al, 2008;Chen et al, 2009;Vidali et al, 2009;Staiger et al, 2010). These distinct actin structures are believed to perform distinct functions.…”

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

“…Actin shapes and dynamics are modulated directly by several types of actin-binding proteins during pollen tube growth (Ren and Xiang, 2007;Staiger et al, 2010;Fu, 2015). In Arabidopsis and lily (Lilium longiflorum), fimbrins (FIM), domain of Lin11, Isl-1, and Mec-3 proteins, actin-depolymerizing factors, several formins, and villins (VLN) have been shown to be involved in the formation and maintenance of longitudinal actin cables in the shank region of pollen tubes Zheng et al, 2013;Fu, 2015).…”