The late pollen actins are essential for normal male and female development in Arabidopsis

Control of the Actin Cytoskeleton Within Apical and Subapical Regions of Pollen Tubes

In flowering plants, sexual reproduction involves a double fertilization event, which is facilitated by the delivery of two non-motile sperm cells to the ovule by the pollen tube. Pollen tube growth occurs exclusively at the tip and is extremely rapid. It strictly depends on an intact actin cytoskeleton, and is therefore an excellent model for uncovering the molecular mechanisms underlying dynamic actin cytoskeleton remodeling. There has been a long-term debate about the organization and dynamics of actin filaments within the apical and subapical regions of pollen tube tips. By combining state-of-the-art live-cell imaging with the usage of mutants which lack different actin-binding proteins, our understanding of the origin, spatial organization, dynamics and regulation of actin filaments within the pollen tube tip has greatly improved. In this review article, we will summarize the progress made in this area.

Monomeric G-actin is uniformly distributed in pollen tubes and is rapidly redistributed via cytoplasmic streaming during pollen tube growth

Dynamic assembly and disassembly of the actin cytoskeleton has been implicated in the regulation of pollen germination and subsequent tube growth. It is widely accepted that actin filaments are arrayed into distinct structures within different regions of the pollen tube. Maintenance of the equilibrium between monomeric globular actin (G-actin) and filamentous actin (F-actin) is crucial for actin assembly and array construction, and the local concentration of G-actin thus directly impacts actin assembly. The localization and dynamics of G-actin in the pollen tube, however, remain to be determined conclusively. To address this question, we created a series of fusion proteins between green fluorescent protein (GFP) and the Arabidopsis reproductive actin ACT11. Expression of a fusion protein with GFP inserted after methionine at position 49 within the DNase I-binding loop of ACT11 (GFP_Met49 -ACT11) rescued the phenotypes in act11 mutants. Consistent with the notion that the majority of actin is in its monomeric form, GFP_Met49 -ACT11 and GFP fusion proteins of four other reproductive actins generated with the same strategy do not obviously label filamentous structures. In further support of the functionality of these fusion proteins, we found that they can be incorporated into filamentous structures in jasplakinolide (Jasp)-treated pollen tubes. Careful observations showed that G-actin is distributed uniformly in the pollen tube and is rapidly redistributed via cytoplasmic streaming during pollen tube growth. Our study suggests that G-actin is readily available in the cytoplasm to support continuous actin polymerization during rapid pollen tube growth.

Arabidopsis ACT11 modifies actin turnover to promote pollen germination and maintain the normal rate of tube growth

Actin is an ancient conserved protein that is encoded by multiple isovariants in multicellular organisms. There are eight functional actin genes in the Arabidopsis genome, and the precise function and mechanism of action of each isovariant remain poorly understood. Here, we report the characterization of ACT11, a reproductive actin isovariant. Our studies reveal that loss of function of ACT11 causes a delay in pollen germination, but enhances pollen tube growth. Cytological analysis revealed that the amount of filamentous actin decreased, and the rate of actin turnover increased in act11 pollen. Convergence of actin filaments upon the germination aperture was impaired in act11 pollen, consistent with the observed delay of germination. Reduction of actin dynamics with jasplakinolide suppressed the germination and tube growth phenotypes in act11 pollen, suggesting that the underlying mechanisms involve an increase in actin dynamics. Thus, we demonstrate that ACT11 is required to maintain the rate of actin turnover in order to promote pollen germination and maintain the normal rate of pollen tube growth.

Arabidopsis plants deficient in constitutive class profilins reveal independent and quantitative genetic effects

BACKGROUND

The actin cytoskeleton is involved in an array of integral structural and developmental processes throughout the cell. One of actin's best-studied binding partners is the small ubiquitously expressed protein, profilin. Arabidopsis thaliana is known to encode a family of five profilin sequence variants: three vegetative (also constitutive) profilins that are predominantly expressed in all vegetative tissues and ovules, and two reproductive profilins that are specifically expressed in pollen. This paper analyzes the roles of the three vegetative profilin members, PRF1, PRF2, and PRF3, in plant cell and organ development.

RESULTS

Using a collection of knockout or severe knockdown T-DNA single mutants, we found that defects in each of the three variants gave rise to specific developmental deficiencies. Plants lacking PRF1 or PRF2 had defects in rosette leaf morphology and inflorescence stature, while those lacking PRF3 led to plants with slightly elongated petioles. To further examine these effects, double mutants and double and triple gene-silenced RNAi epialleles were created. These plants displayed significantly compounded developmental defects, as well as distinct lateral root growth morphological phenotypes.

CONCLUSION

These results suggest that having at least one vegetative profilin gene is essential to viability. Evidence is presented that combinations of independent function, quantitative genetic effects, and functional redundancy have preserved the three vegetative profilin genes in the Arabidopsis lineage.

Formins: emerging players in the dynamic plant cell cortex

Formins (FH2 proteins) are an evolutionarily conserved family of eukaryotic proteins, sharing the common FH2 domain. While they have been, until recently, understood mainly as actin nucleators, formins are also engaged in various additional aspects of cytoskeletal organization and signaling, including, but not limited to, the crosstalk between the actin and microtubule networks. A surprising diversity of domain organizations has been discovered among the FH2 proteins, and specific domain setups have been found in plants. Seed plants have two clades of formins, one of them (Class I) containing mostly transmembrane proteins, while members of the other one (Class II) may be anchored to membranes via a putative membrane-binding domain related to the PTEN antioncogene. Thus, plant formins present good candidates for possible mediators of coordination of the cortical actin and microtubule cytoskeletons, as well as their attachment to the plasma membrane, that is, aspects of cell cortex organization likely to be important for cell and tissue morphogenesis. Although experimental studies of plant formin function are hampered by the large number of formin genes and their functional redundancy, recent experimental work has already resulted in some remarkable insights into the function of FH2 proteins in plants.

Double repression of soluble starch synthase genes SSIIa and SSIIIa in rice (Oryza sativa L.) uncovers interactive effects on the physicochemical properties of starch

Soluble starch synthases (SSs) are major enzymes involved in starch biosynthesis in developing rice (Oryza sativa L.) endosperm. Despite extensive studies of SSs in various plant species including rice, the functional modes of action among multiple SS genes are still not clear. Here, we generated transgenic RNA interference (RNAi) repressed lines for seven of the eight members of the rice SS gene family and studied their effects on starch synthesis and grain formation. Consistent with their expression domains, RNAi repression of genes that encode isozymes SSI, SSIIa, and SSIIIa had strong effects on grain development, whereas no obvious phenotypic changes were observed in transgenic plants with the other SS genes being RNAi repressed, indicating functional redundancies among the genes. To study the potential functional interactions of SS genes, we generated SSIIa/SSIIIa double repression lines whose kernels displayed a chalky kernel appearance and had increased amylose levels, increased pasting temperatures, and decreased viscosities. The double mutation also reduced short (degree of polymerization (DP) 5-6) and long (DP 12-23) amylopectin chain contents in the grain and increased the medium long types (DP 7-11). The nonadditive nature of the double mutation line suggests that SSIIa and SSIIIa interact with each other during starch synthesis. Such interaction may be physical via starch phophorylase as indicated by our pair-wise yeast two-hybrid assays on major starch synthesis enzymes. Collectively, the data showed that SSIIa and SSIIIa play distinctive, but partially overlapping, roles during rice grain starch synthesis. The possibility of extensive redundancy or complementarity among SS isozymes is discussed.

Computational identification of root hair-specific genes in Arabidopsis

Activated cortical domains (ACDs) are regions of the plant cell cortex performing localized membrane turnover, delimited by concerted action of the cortical cytoskeleton and endomembrane compartments. Arabidopsis thaliana rhizodermis consists of two cell types differing by a single ACD (trichoblasts, carrying tip-growing root hairs, and hairless atrichoblasts), providing a model for the study of ACD determination. We compiled a set of genes specifically upregulated in root hairs from published transcriptome data, and compared it with a "virtual Arabidopsis root hair proteome", i.e. a list of computationally identified homologs of proteins from the published soybean root hair proteome. Both data sets were enriched in genes and proteins associated with root hairs in functional studies, but there was little overlap between the transcriptome and the proteome: the former captured gene products specific to root hairs, while the latter selected those abundant in root hairs but not necessarily specific to them. Decisive steps in ACD specification may be performed by signaling proteins of high expression specifity and low abundance. Nevertheless, 73 genes specifically transcribed in Arabidopsis trichoblasts or root hairs encode homologs of abundant root hair proteins from soybean. Most of them encode "housekeeping" proteins required for rapid tip growth. However, among the "candidates" is also a generative actin isoform, ACT11. Preliminary characterization of an act11 mutant allele indeed suggests a hitherto unexpected role for this gene in root and root hair development.

Arabidopsis actin-depolymerizing factor AtADF4 mediates defense signal transduction triggered by the Pseudomonas syringae effector AvrPphB

The actin cytoskeleton has been implicated in plant defenses against pathogenic fungi and oomycetes with limited, indirect evidence. To date, there are no reports linking actin with resistance against phytopathogenic bacteria. The dynamic behavior of actin filaments is regulated by a diverse array of actin-binding proteins, among which is the Actin-Depolymerizing Factor (ADF) family of proteins. Here, we demonstrate that actin dynamics play a role in the activation of gene-for-gene resistance in Arabidopsis (Arabidopsis thaliana) following inoculation with the phytopathogenic bacterium Pseudomonas syringae pv tomato. Using a reverse genetics approach, we explored the roles of Arabidopsis ADFs in plant defenses. AtADF4 was identified as being specifically required for resistance triggered by the effector AvrPphB but not AvrRpt2 or AvrB. Recombinant AtADF4 bound to monomeric actin (G-actin) with a marked preference for the ADP-loaded form and inhibited the rate of nucleotide exchange on G-actin, indicating that AtADF4 is a bona fide actin-depolymerizing factor. Exogenous application of the actin-disrupting agent cytochalasin D partially rescued the Atadf4 mutant in the AvrPphB-mediated hypersensitive response, demonstrating that AtADF4 mediates defense signaling through modification of the actin cytoskeleton. Unlike the mechanism by which the actin cytoskeleton confers resistance against fungi and oomycetes, AtADF4 is not involved in resistance against pathogen entry. Collectively, this study identifies AtADF4 as a novel component of the plant defense signaling pathway and provides strong evidence for actin dynamics as a primary component that orchestrates plant defenses against P. syringae.

ACTIN DEPOLYMERIZING FACTOR9 controls development and gene expression in Arabidopsis

Actin depolymerizing factors (ADF/cofilin) modulate the rate of actin filament turnover, networking cellular signals into cytoskeletal-dependent developmental pathways. Plant and animal genomes encode families of diverse ancient ADF isovariants. One weakly but ubiquitously expressed member of the Arabidopsis ADF gene family, ADF9, is moderately expressed in the shoot apical meristem (SAM). Mutant alleles adf9-1 and adf9-2 showed a 95% and 50% reduction in transcript levels, respectively. Compared to wild-type, mutant seedlings and plants were significantly smaller and adult mutant plants had decreased numbers of lateral branches and a reduced ability to form callus. The mutants flowered very early during long-day light cycles, but not during short days. adf9-1showed a several-fold lower expression of FLOWERING LOCUS C (FLC), a master repressor of the transition to flowering, and increased expression of CONSTANS, an activator of flowering. Transgenic ADF9 expression complemented both developmental and gene expression phenotypes. FLC chromatin from adf9-1 plants contained reduced levels of histone H3 lysine 4 trimethylation and lysine 9 and 14 acetylation, as well as increased nucleosome occupancy consistent with a less active chromatin state. We propose that ADF9 networks both cytoplasmic and nuclear processes within the SAM to control multicellular development.

The ancient subclasses of Arabidopsis Actin Depolymerizing Factor genes exhibit novel and differential expression

The Actin Depolymerizing Factor (ADF) gene family of Arabidopsis thaliana encodes 11 functional protein isovariants in four ancient subclasses. We report the characterization of the tissue-specific and developmental expression of all Arabidopsis ADF genes and the subcellular localization of several protein isovariants. The four subclasses exhibited distinct expression patterns as examined by qRT-PCR and histochemical assays of a GUS reporter gene under the control of individual ADF regulatory sequences. Subclass I ADFs were expressed strongly and constitutively in all vegetative and reproductive tissues except pollen. Subclass II ADFs were expressed specifically in mature pollen and pollen tubes or root epidermal trichoblast cells and root hairs, and these patterns evolved from an ancient dual expression pattern comprised of both polar tip growth cell types, still observed in the monocot Oryza sativa. Subclass III ADFs were expressed weakly in vegetative tissues, but were strongest in fast growing and/or differentiating cells including callus, emerging leaves, and meristem regions. The single subclass IV ADF was constitutively expressed at moderate levels in all tissues, including pollen. Immunocytochemical analysis with subclass-specific monoclonal antibodies demonstrated that subclass I isovariants localize to both the cytoplasm and the nucleus of leaf cells, while subclass II isovariants predominantly localize to the cytoplasm at the tip region of elongating root hairs and pollen tubes. The distinct expression patterns of the ADF subclasses support a model of ADF s co-evolving with the ancient and divergent actin isovariants.

Class-specific interaction of profilin and ADF isovariants with actin in the regulation of plant development

Two ancient and highly divergent actin-based cytoskeletal systems have evolved in angiosperms. Plant genomes encode complex actin and actin binding protein (ABP) gene families, most of which are phylogenetically grouped into gene classes with distinct vegetative or constitutive and reproductive expression patterns. In Arabidopsis thaliana, ectopic expression of high levels of a reproductive class actin, ACT1, in vegetative tissues causes severe dwarfing of plants with aberrant organization of most plant organs and cell types due to a severely altered actin cytoskeletal architecture. Overexpression of the vegetative class actin ACT2 to similar levels, however, produces insignificant phenotypic changes. We proposed that the misexpression of the pollen-specific ACT1 in vegetative cell types affects the dynamics of actin due to its inappropriate interaction with endogenous vegetative ABPs. To examine the functionally distinct interactions among the major classes of actins and ABPs, we ectopically coexpressed reproductive profilin (PRF4) or actin-depolymerizing factor (ADF) isovariants (e.g., ADF7) with ACT1. Our results demonstrated that the coexpression of these reproductive, but not vegetative, ABP isovariants suppressed the ectopic ACT1 expression phenotypes and restored wild-type stature and normal actin cytoskeletal architecture to the double transgenic plants. Thus, the actins and ABPs appear to have evolved class-specific, protein-protein interactions that are essential to the normal regulation of plant growth and development.