Activation of actin-depolymerizing factor by CDPK16-mediated phosphorylation promotes actin turnover in Arabidopsis pollen tubes

Heat-stable protein PGSL1 enhances pollen germination and tube growth at high temperature

Global warming intensifies extreme heat events, threatening crop reproduction by impairing pollen development, germination, and tube growth. However, the mechanisms underlying pollen heat responses remain elusive. The actin cytoskeleton and actin-binding proteins (ABPs) are crucial in these processes, yet their roles under heat stress are poorly understood. Here, we identify a mutant, pollen germination sensitive to LatB (pgsl1), via forward genetic screening. PGSL1 encodes a heat-stable, plant-specific ABP that binds and stabilizes actin filaments (F-actin), preventing heat-induced denaturation. High temperatures reduce F-actin content but promote bundling in pollen tubes. Notably, pgsl1 mutants exhibit decreased F-actin abundance and bundling under heat stress compared to wild-type plants. These findings highlight PGSL1 as a key regulator of actin dynamics, essential for pollen heat tolerance, offering potential strategies to enhance crop resilience in a warming climate.

Functional Characterization of PeVLN4 Involved in Regulating Pollen Tube Growth from Passion Fruit

Passion fruit (Passiflora edulis), mainly distributed in tropical and subtropical regions, is popular for its unique flavor and health benefits. The actin cytoskeleton plays a crucial role in plant growth and development, and villin is a key regulator of actin dynamics. However, the mechanism underlying the actin filament regulation of reproductive development in passion fruit remains poorly understood. Here, we characterized a villin isovariant in passion fruit, Passiflora edulis VLN4 (PeVLN4), highly and preferentially expressed in pollen. Subcellular localization analysis showed that PeVLN4 decorated distinct filamentous structures in pollen tubes. We next introduced PeVLN4 into Arabidopsis villin mutants to explore its functions on the growing pollen tubes. PeVLN4 rescued defects in the elongation of villin mutant pollen tubes. Pollen tubes expressing PeVLN4 were revealed to be less sensitive to latrunculin B, and PeVLN4 partially rescued defects in the actin filament organization of villin mutant pollen tubes. Additionally, biochemical assays revealed that PeVLN4 bundles actin filaments in vitro. Thus, PeVLN4 is an important regulator of F-actin stability and is required for normal pollen tube growth in passion fruit. This study provides a new insight into the function of the actin regulator villin involved in the reproduction development of passion fruit.

Phosphorylation of ADF7-Mediated by AGC1.7 Regulates Pollen Germination in Arabidopsis thaliana

Actin depolymerizing factors (ADFs), like other actin-binding proteins (ABPs), are modified by phosphorylation to regulate the dynamics of the actin filaments, thereby functioning in various processes throughout the plant lifecycle. In this study, we found that the Arabidopsis thaliana cytoplasmic kinase AGC1.7 interacts with ADF7 in vitro and in vivo. AGC1.7 phosphorylates ADF7 at its Ser-6, Ser-103 and Ser-104 residues in vitro, while replacing these residues with alanine promotes ADF7-mediated actin depolymerization in vitro. Expression of the phosphorylation-mimetic mutant protein ADF7S6/103/104D driven by the pollen-specific LAT52 promoter fully rescues the defects in germination rate, silique length and seeds per silique in both adf7-2 and agc1.5 agc1.7 (agcdm) mutants. Our data establish a model whereby AGC1.7-mediated ADF7 phosphorylation plays an important role in pollen germination and pollen tube growth.

Autophagy modulates Arabidopsis male gametophyte fertility and controls actin organization

Autophagy, a crucial mechanism for cellular degradation, is regulated by conserved autophagy-related (ATG) core proteins across species. Impairments in autophagy result in significant developmental and reproductive aberrations in mammals. However, autophagy is thought to be functionally dispensable in Arabidopsis thaliana since most of the ATG mutants lack severe growth and reproductive defects. Here, we challenge this perception by unveiling a role for autophagy in male gametophyte development and fertility in Arabidopsis. A detailed re-assessment of atg5 and atg7 mutants found that reduced autophagy activity in germinated pollen accompanied by partial aberrations in sperm cell biogenesis and pollen tube growth, leading to compromised seed formation. Furthermore, we revealed autophagy modulates the spatial organization of actin filaments via targeted degradation of actin depolymerization factors ADF7 and Profilin2 in pollen grains and tubes through a key receptor, Neighbor of BRCA1 (NBR1). Our findings advance the understanding of the evolutionary conservation and diversification of autophagy in modulating male fertility in plants contrasting to mammals.

A RhoGAP controls apical actin polymerization by inhibiting formin in Arabidopsis pollen tubes

Formin is an important player in promoting apical actin polymerization in pollen tubes, but the mechanism regulating its activity remains unknown. We here identify REN1, a Rho GTPase-activating protein, as a negative regulator of formins in Arabidopsis pollen tubes. Specifically, we found that depletion of REN1 promotes apical actin polymerization and increases the amount of filamentous actin in pollen tubes. Interestingly, the effect of REN1 loss of function phenocopies the effect of formin gain of function, as it causes the formation of supernumerary membrane-derived actin bundles, which leads to tube swelling and membrane deformation. Importantly, inhibition of formins suppresses the phenotypic defects in ren1 mutant pollen tubes. We further demonstrate that REN1 physically interacts with the Arabidopsis formin protein AtFH5, predominantly with the C terminus, and inhibits the ability of AtFH5 to nucleate and assemble actin in vitro. Depletion of AtFH5 partially suppresses the phenotype in ren1 mutant pollen tubes, demonstrating that REN1 regulates apical actin polymerization at least partially through inhibiting AtFH5. We thus uncover a novel mechanism regulating formins and actin polymerization in plants.

Myosin XI, a model of its conserved role in plant cell tip growth

In eukaryotic cells, organelle and vesicle transport, positioning, and interactions play crucial roles in cytoplasmic organization and function. These processes are governed by intracellular trafficking mechanisms. At the core of that trafficking, the cytoskeleton and directional transport by motor proteins stand out as its key regulators. Plant cell tip growth is a well-studied example of cytoplasm organization by polarization. This polarization, essential for the cell's function, is driven by the cytoskeleton and its associated motors. This review will focus on myosin XI, a molecular motor critical for vesicle trafficking and polarized plant cell growth. We will center our discussion on recent data from the moss Physcomitrium patens and the liverwort Marchantia polymorpha. The biochemical properties and structure of myosin XI in various plant species are discussed, highlighting functional conservation across species. We further explore this conservation of myosin XI function in the process of vesicle transport in tip-growing cells. Existing evidence indicates that myosin XI actively organizes actin filaments in tip-growing cells by a mechanism based on vesicle clustering at their tips. A hypothetical model is presented to explain the essential function of myosin XI in polarized plant cell growth based on vesicle clustering at the tip. The review also provides insight into the in vivo localization and dynamics of myosin XI, emphasizing its role in cytosolic calcium regulation, which influences the polymerization of F-actin. Lastly, we touch upon the need for additional research to elucidate the regulation of myosin function.

Evolution of the thermostability of actin-depolymerizing factors enhances the adaptation of pollen germination to high temperature

Double fertilization in many flowering plants (angiosperms) often occurs during the hot summer season, but the mechanisms that enable angiosperms to adapt specifically to high temperatures are largely unknown. The actin cytoskeleton is essential for pollen germination and the polarized growth of pollen tubes, yet how this process responds to high temperatures remains unclear. Here, we reveal that the high thermal stability of 11 Arabidopsis (Arabidopsis thaliana) actin-depolymerizing factors (ADFs) is significantly different: ADFs that specifically accumulate in tip-growing cells (pollen and root hairs) exhibit high thermal stability. Through ancestral protein reconstruction, we found that subclass II ADFs (expressed specifically in pollen) have undergone a dynamic wave-like evolution of the retention, loss, and regeneration of thermostable sites. Additionally, the sites of AtADF7 with high thermal stability are conserved in ADFs specific to angiosperm pollen. Moreover, the high thermal stability of ADFs is required to regulate actin dynamics and turnover at high temperatures to promote pollen germination. Collectively, these findings suggest strategies for the adaptation of sexual reproduction to high temperature in angiosperms at the cell biology level.

Actin-depolymerizing factors 8 and 11 promote root hair elongation at high pH

A root hair is a polarly elongated single-celled structure that derives from a root epidermal cell and functions in uptake of water and nutrients from the surrounding environment. Previous reports have demonstrated that short periods of high pH inhibit root hair extension; but the effects of long-term high-pH treatment on root hair growth are still unclear. Here, we report that the duration of root hair elongation is significantly prolonged with increasing external pH, which counteracts the effect of decreasing root hair elongation rate and ultimately produces longer root hairs, whereas loss of actin-depolymerizing factor 8 and 11 (ADF8/11) function causes shortening of root hair length at high pH (pH 7.4). Accumulation of ADF8/11 at the tips of root hairs is inhibited by high pH, and increasing environmental pH affects the actin filament (F-actin) meshwork at the root hair tip. At high pH, the tip-focused F-actin meshwork is absent in root hairs of the adf8/11 mutant, actin filaments are disordered at the adf8/11 root hair tips, and actin turnover is attenuated. Secretory and recycling vesicles do not aggregate in the apical region of adf8/11 root hairs at high pH. Together, our results suggest that, under long-term exposure to high extracellular pH, ADF8/11 may establish and maintain the tip-focused F-actin meshwork to regulate polar trafficking of secretory/recycling vesicles at the root hair tips, thereby promoting root hair elongation.

Dehydration-responsive cytoskeleton proteome of rice reveals reprograming of key molecular pathways to mediate metabolic adaptation and cell survival

The plant cytoskeletal proteins play a key role that control cytoskeleton dynamics, contributing to crucial biological processes such as cell wall morphogenesis, stomatal conductance and abscisic acid accumulation in repercussion to water-deficit stress or dehydration. Yet, it is still completely unknown which specific biochemical processes and regulatory mechanisms the cytoskeleton uses to drive dehydration tolerance. To better understand the role of cytoskeleton, we developed the dehydration-responsive cytoskeletal proteome map of a resilient rice cultivar. Initially, four-week-old rice plants were exposed to progressive dehydration, and the magnitude of dehydration-induced compensatory physiological responses was monitored in terms of physicochemical indices. The organelle fractionation in conjunction with label-free quantitative proteome analysis led to the identification of 955 dehydration-responsive cytoskeletal proteins (DRCPs). To our knowledge, this is the first report of a stress-responsive plant cytoskeletal proteome, representing the largest inventory of cytoskeleton and cytoskeleton-associated proteins. The DRCPs were apparently involved in a wide array of intra-cellular molecules transportation, organelles positioning, cytoskeleton organization followed by different metabolic processes including amino acid metabolism. These findings presented open a unique view on global regulation of plant cytoskeletal proteome is intimately linked to cellular metabolic rewiring of adaptive responses, and potentially confer dehydration tolerance, especially in rice, and other crop species, in general.

Differential sensitivity of ADF isovariants to a pH gradient promotes pollen tube growth

The actin cytoskeleton is one of the targets of the pH gradient in tip-growing cells, but how cytosolic pH regulates the actin cytoskeleton remains largely unknown. We here demonstrate that Arabidopsis ADF7 and ADF10 function optimally at different pH levels when disassembling actin filaments. This differential pH sensitivity allows ADF7 and ADF10 to respond to the cytosolic pH gradient to regulate actin dynamics in pollen tubes. ADF7 is an unusual actin-depolymerizing factor with a low optimum pH in in vitro actin depolymerization assays. ADF7 plays a dominant role in promoting actin turnover at the pollen tube apex. ADF10 has a typically high optimum pH in in vitro assays and plays a dominant role in regulating the turnover and organization of subapical actin filaments. Thus, functional specification and cooperation of ADF isovariants with different pH sensitivities enable the coordination of the actin cytoskeleton with the cytosolic pH gradient to support pollen tube growth.