Defects In Pollen Tube Growth Research Articles

The transcription factor LBD10 sustains pollen tube growth and integrity by modulating reactive oxygen species homeostasis via the regulation of flavonol biosynthesis in Arabidopsis.

Reproduction in angiosperms relies on the precise growth of pollen tubes, facilitating the delivery of sperm cells to the ovule for double fertilization. LATERAL ORGAN BOUNDARIES DOMAIN10 (LBD10), a plant-specific transcription factor, plays a pivotal role in Arabidopsis pollen development. Here, we uncovered LBD10's function in sustaining pollen tube growth and integrity. The lbd10 mutant exhibited elevated levels of reactive oxygen species (ROS) and hydrogen peroxide (H2O2) in both pollen grains and tubes, leading to compromised pollen tube growth. The inhibition of ROS synthesis and scavenging of excess ROS with an antioxidant treatment each alleviated these defects in lbd10. The lbd10 mutant displayed reduced flavonol accumulation in both pollen grains and tubes. All the altered phenotypes of lbd10 were complemented by expressing LBD10 under its native promoter. Exogenous application of flavonoids recused the defects in pollen tube growth and integrity in lbd10, along with reducing the excess levels of ROS and H2O2. LBD10 directly binds the promoters of key flavonol biosynthesis genes in chromatin and promotes reporter gene expression in Arabidopsis mesophyll protoplasts. Our findings indicate that LBD10 modulates ROS homeostasis by transcriptionally activating genes crucial for flavonol biosynthesis, thereby maintaining pollen tube growth and integrity.

Plasma membrane H+-ATPases sustain pollen tube growth and fertilization

Pollen tubes are highly polarized tip-growing cells that depend on cytosolic pH gradients for signaling and growth. Autoinhibited plasma membrane proton (H+) ATPases (AHAs) have been proposed to energize pollen tube growth and underlie cell polarity, however, mechanistic evidence for this is lacking. Here we report that the combined loss of AHA6, AHA8, and AHA9 in Arabidopsis thaliana delays pollen germination and causes pollen tube growth defects, leading to drastically reduced fertility. Pollen tubes of aha mutants had reduced extracellular proton (H+) and anion fluxes, reduced cytosolic pH, reduced tip-to-shank proton gradients, and defects in actin organization. Furthermore, mutant pollen tubes had less negative membrane potentials, substantiating a mechanistic role for AHAs in pollen tube growth through plasma membrane hyperpolarization. Our findings define AHAs as energy transducers that sustain the ionic circuit defining the spatial and temporal profiles of cytosolic pH, thereby controlling downstream pH-dependent mechanisms essential for pollen tube elongation, and thus plant fertility.

Cover Caption: The Role of TMEM18 in Arabidopsis

TMEM18 proteins are conserved in most eukaryotes, and may associate with obesity in human. In this issue, Dou et al. (pp.679–692) showed that the Arabidopsis TMEM18 (AtTMEM18) is localized around nuclei and plays important roles in plant growth. Mutation in AtTMEM18 affected callose deposition, and caused defects in pollen tube growth.

Class XI Myosins Move Specific Organelles in Pollen Tubes and Are Required for Normal Fertility and Pollen Tube Growth in Arabidopsis.

Pollen tube growth is an essential aspect of plant reproduction because it is the mechanism through which nonmotile sperm cells are delivered to ovules, thus allowing fertilization to occur. A pollen tube is a single cell that only grows at the tip, and this tip growth has been shown to depend on actin filaments. It is generally assumed that myosin-driven movements along these actin filaments are required to sustain the high growth rates of pollen tubes. We tested this conjecture by examining seed set, pollen fitness, and pollen tube growth for knockout mutants of five of the six myosin XI genes expressed in pollen of Arabidopsis (Arabidopsis thaliana). Single mutants had little or no reduction in overall fertility, whereas double mutants of highly similar pollen myosins had greater defects in pollen tube growth. In particular, myo11c1 myo11c2 pollen tubes grew more slowly than wild-type pollen tubes, which resulted in reduced fitness compared with the wild type and a drastic reduction in seed set. Golgi stack and peroxisome movements were also significantly reduced, and actin filaments were less organized in myo11c1 myo11c2 pollen tubes. Interestingly, the movement of yellow fluorescent protein-RabA4d-labeled vesicles and their accumulation at pollen tube tips were not affected in the myo11c1 myo11c2 double mutant, demonstrating functional specialization among myosin isoforms. We conclude that class XI myosins are required for organelle motility, actin organization, and optimal growth of pollen tubes.

Loss of the Arabidopsis thaliana P4-ATPase ALA3 Reduces Adaptability to Temperature Stresses and Impairs Vegetative, Pollen, and Ovule Development

Members of the P4 subfamily of P-type ATPases are thought to help create asymmetry in lipid bilayers by flipping specific lipids between the leaflets of a membrane. This asymmetry is believed to be central to the formation of vesicles in the secretory and endocytic pathways. In Arabidopsis thaliana, a P4-ATPase associated with the trans-Golgi network (ALA3) was previously reported to be important for vegetative growth and reproductive success. Here we show that multiple phenotypes for ala3 knockouts are sensitive to growth conditions. For example, ala3 rosette size was observed to be dependent upon both temperature and soil, and varied between 40% and 80% that of wild-type under different conditions. We also demonstrate that ala3 mutants have reduced fecundity resulting from a combination of decreased ovule production and pollen tube growth defects. In-vitro pollen tube growth assays showed that ala3 pollen germinated ∼2 h slower than wild-type and had approximately 2-fold reductions in both maximal growth rate and overall length. In genetic crosses under conditions of hot days and cold nights, pollen fitness was reduced by at least 90-fold; from ∼18% transmission efficiency (unstressed) to less than 0.2% (stressed). Together, these results support a model in which ALA3 functions to modify endomembranes in multiple cell types, enabling structural changes, or signaling functions that are critical in plants for normal development and adaptation to varied growth environments.

Mutations in the Arabidopsis Nuclear-Encoded Mitochondrial Phage-Type RNA Polymerase Gene RPOTm Led to Defects in Pollen Tube Growth, Female Gametogenesis and Embryogenesis

The mitochondrial genes in Arabidopsis thaliana are transcribed by a small family of nuclear-encoded T3/T7 phage-type RNA polymerases (RPOTs). At least two nuclear-encoded RPOTs (RPOTm and RPOTmp) are located in mitochondria in A. thaliana. Their genetic roles are largely unknown. Here we report the characterization of novel mutations in the A. thaliana RPOTm gene. The mutations did not affect pollen formation, but significantly retarded the growth of the rpoTm mutant pollen tubes and had an impact on the fusion of the polar nuclei in the rpoTm mutant embryo sacs. Moreover, development of the rpoTm/- mutant embryo was arrested at the globular stage. The rpoTm rpoTmp double mutation could enhance the rpoTm mutant phenotype. Expression of RPOTmp under control of the RPOTm promoter could not complement the phenotype of the rpoTm mutations. All these data indicate that RPOTm is important for normal pollen tube growth, female gametogenesis and embryo development, and has distinct genetic and molecular roles in plant development, which cannot be replaced by RPOTmp.

In Planta Analysis of the Cell Cycle-Dependent Localization of AtCDC48A and Its Critical Roles in Cell Division, Expansion, and Differentiation

CDC48/p97 is a conserved homohexameric AAA-ATPase chaperone required for a variety of cellular processes but whose role in the development of a multicellular model system has not been examined. Here, we have used reverse genetics, visualization of a functional Arabidopsis (Arabidopsis thaliana) CDC48 fluorescent fusion protein, and morphological analysis to examine the subcellular distribution and requirements for AtCDC48A in planta. Homozygous Atcdc48A T-DNA insertion mutants arrest during seedling development, exhibiting decreased cell expansion and displaying pleiotropic defects in pollen and embryo development. Atcdc48A insertion alleles show significantly reduced male transmission efficiency due to defects in pollen tube growth. Yellow fluorescent protein-AtCDC48A, a fusion protein that functionally complements the insertion mutant defects, localizes in the nucleus and cytoplasm and is recruited to the division mid-zone during cytokinesis. The pattern of nuclear localization differs according to the stage of the cell cycle and differentiation state. Inducible expression of an Atcdc48A Walker A ATPase mutant in planta results in cytokinesis abnormalities, aberrant cell divisions, and root trichoblast differentiation defects apparent in excessive root hair emergence. At the biochemical level, our data suggest that the endogenous steady-state protein level of AtCDC48A is dependent upon the presence of ATPase-active AtCDC48A. These results demonstrate that CDC48A/p97 is critical for cytokinesis, cell expansion, and differentiation in plants.