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

Abstract

Actin cables in pollen tubes serve as molecular tracks for cytoplasmic streaming and organelle movement and are formed by actin bundling factors like villins and fimbrins. However, the precise mechanisms by which actin cables are generated and maintained remain largely unknown. Fimbrins comprise a family of five members in Arabidopsis thaliana. Here, we characterized a fimbrin isoform, Arabidopsis FIMBRIN5 (FIM5). Our results show that FIM5 is required for the organization of actin cytoskeleton in pollen grains and pollen tubes, and FIM5 loss-of-function associates with a delay of pollen germination and inhibition of pollen tube growth. FIM5 decorates actin filaments throughout pollen grains and tubes. Actin filaments become redistributed in fim5 pollen grains and disorganized in fim5 pollen tubes. Specifically, actin cables protrude into the extreme tips, and their longitudinal arrangement is disrupted in the shank of fim5 pollen tubes. Consequently, the pattern and velocity of cytoplasmic streaming were altered in fim5 pollen tubes. Additionally, loss of FIM5 function rendered pollen germination and tube growth hypersensitive to the actin-depolymerizing drug latrunculin B. In vitro biochemical analyses indicated that FIM5 exhibits actin bundling activity and stabilizes actin filaments. Thus, we propose that FIM5 regulates actin dynamics and organization during pollen germination and tube growth via stabilizing actin filaments and organizing them into higher-order structures.

Arabidopsis CROLIN1, a Novel Plant Actin-binding Protein, Functions in Cross-linking and Stabilizing Actin Filaments

“…Our experiments using cross-linked proteins and native gels demonstrated that CROLIN1 forms oligomers to bundle and cross-link F-actin. Moreover, the actin structure induced by CROLIN1 in vitro was similar to that of FIM1 and FIM5, which form higher order actin structures (12,13). These results reveal that the mechanism of actin bundling by CROLIN1 is different from the mechanism of other ABPs, such as formin (16) and villin (7), which mainly form actin cables and bundles.…”

“…For example, a VLN5 loss-of-function mutant displays delayed pollen tube growth and results in F-actin in both pollen grains and pollen tubes that are sensitive to latrunculin B (Lat B) (7). A loss-of-function mutant of Arabidopsis FIMBRIN5 also results in delayed pollen germination and inhibited tube growth, with both pollen germination and tube growth being hypersensitive to Lat B (13).…”

“…Tobacco NtWLIM1 and NtWLIM2 (8,9), all Arabidopsis AtLIMs (10), and Lilium LlLIM1 (11) are recognized as actin-bundling proteins. In addition, fimbrins (both Fimbrin1 and Fimbrin5) bundle or crosslink F-actin in vitro (12,13), and certain formins, such as rice OsFH5 (14,15) and Arabidopsis AtFH1, AtFH4, AtFH8, and AtFH14 (16 -19), also bundle actin in vitro.…”

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

“…The quantity and binding ratio of RIC1 and actin were measured using ImageJ software. The equilibrium K d value was determined as described by Wu et al (2010). RIC1 in the pellet ([RIC1] bound ) was plotted as a function of RIC1 in the supernatant ([RIC1] free ), and the data were fitted with a hyperbolic function using Kaleidagraph version 4.0 software (Synergy Software).…”

Plant Actin-binding Protein SCAB1 Is Dimeric Actin Cross-linker with Atypical Pleckstrin Homology Domain

“…Several classes of actin bundlers have been characterized in plants, including villins (7)(8)(9)(10)(11)(12), fimbrins (13,14), certain formins (15,16), and LIM proteins (17)(18)(19). These actin bundlers are also present in most other eukaryotes.…”