The pollen tube cytoskeleton

A quantitative proteomic analysis of the molecular mechanism underlying fertility conversion in thermo-sensitive genetic male sterility line AnnongS-1

BACKGROUND

Thermo-sensitive genetic male sterile (TGMS) lines have been widely used in two-line hybrid rice breeding. The two-line hybrids have increased rice yields substantially. However, the effect of environmental temperatures on the fertility conversion is still not fully clear. In this study, we performed a tandem mass tag (TMT)-based proteomic analysis on the anthers of the TGMS line AnnongS-1 grown under permissive (low) temperature (21 °C) and restrictive (high) temperature (> 26 °C) conditions in an attempt to explore the effect of temperature on the fertility of the male sterile line.

RESULTS

After the AnnongS-1 plants were induced under either permissive or restrictive conditions, morphological observations and I₂-KI staining confirmed that the pollen grains formed under high temperature conditions were abortive while those formed under low temperature developed normally. In comparison to the plants grown under permissive conditions, the restrictive high-temperature conditions led to the differential accumulation of 89 proteins in the anthers, of which 46 were increased in abundance and 43 were decreased in abundance. Most of the subcellular compartments of the anther cells had one or more proteins that had been differentially accumulated, with the cytoplasm and chloroplast having the greatest accumulations. More than 40% of the differentially abundant proteins (DAPs) were enzymes involved in photosynthesis, energy metabolism, biosynthesis and catabolism of cellular components, metabolic regulation, defense and stress, etc. The DAPs related to protein metabolism accounted for the largest proportion (21.35%), followed by those related to defense and stress (12.36%), metabolic regulation (10.11%) and carbohydrate metabolism (8.99%), indicating that such biological processes in anther cells were more susceptible to high temperature stress.

CONCLUSIONS

The restrictive temperature induction caused fertility-sterility conversion in the TGMS line AnnongS-1 mainly by adversely affecting the metabolism of protein, carbohydrate and energy, and decreasing the abundances of important proteins closely related to defense and stress, thereby impeding the growth and development of the pollen and weakening the overall defense and ability to endure stress of AnnongS-1. These data are helpful for deepening our understanding of the molecular mechanism underlying fertility conversion in TGMS lines.

The cytoskeleton in the pollen tube

The cytoskeleton in pollen tubes has been intensively studied, because of its abundance and prominent roles and because the pollen tube is an excellent experimental system for cell biological studies. Pollen actin microfilaments (MFs) exist as multiple distinct populations, each participating in a specific cellular trafficking or organization process. Consequently, MFs are essential for pollen tube growth and are tightly regulated in response to various signals. Pollen microtubules (MTs) are non-essential and less characterized, but recent studies have implicated MTs in vesicle trafficking and cell wall construction in pollen tubes. This review summarizes recent advances in understanding the organization and regulation of both MFs and MTs and discusses their roles in cellular trafficking and the modulation of pollen-tube tip growth.

Identification and characterization of plasma membrane proteins that bind to microtubules in pollen tubes and generative cells of tobacco

The organization and function of microtubules in plant cells are important in many developmental stages. Connections between microtubules and the endomembrane system of plant cells have been discovered by microscopy, but the molecular characteristics of these relationships are mostly unknown except for a few cases. Using two antibodies raised against microtubule-associated proteins (MAPs) from maize, we have identified two polypeptides that share properties of the MAP family in the pollen tube of Nicotiana tabacum. The two polypeptides (with an apparent Mr of 161 and 90 kDa) bind efficiently to animal and plant microtubules and are found in association with the cellular membranes of the pollen tube, from which they can be solubilized with a zwitterionic detergent. One of these proteins has been purified and shown to promote the assembly of tubulin and, to a lesser extent, the bundling of microtubules. Subcellular fractionation indicated that the two proteins are associated with the plasma membrane compartment. The two proteins are found to co-localize in situ with cortical microtubules in the vegetative cytoplasm of tobacco pollen tubes; co-localization is also evident in the generative cell. According to these data, both the 161 and 90 kDa polypeptides are likely to mediate the interactions between the plasma membrane and microtubules in pollen tubes. In addition, functional data indicate that these MAP-like proteins take part in the process of microtubule assembly and reorganization occurring during cell growth. The evidence that both proteins associate with different cellular compartments also suggests a broad-spectrum role in mediating the dynamic relationships between microtubules and plant cell membranes.

Morphological anomalies in pollen tubes ofActinidia deliciosa (kiwi) exposed to 50 Hz magnetic field

The role of the pollen grain, with respect to the reproductive process of higher plants, is to deliver the spermatic cells to the embryo sac for egg fertilisation. Delivery occurs through the pollen tube, a self produced organ that is generated when the pollen grain reaches the stigma surface. The effect of magnetic fields on pollen tube growth was reported in a recent publication by Germanà et al. Pollen tube growth is an interesting candidate for the detailed study of the effects of electromagnetic fields on cytoplasmic structures and organelles. In this research Actinidia deliciosa (kiwifruit) pollen grains were germinated in the presence of an alternating magnetic field (50 Hz). Our results, although of preliminary nature, show that pollen tube growth is affected by magnetic fields. The analysis of the observed anomalies in the pollen tube appear to be the result of changes in the ionic charges within the pollen tube cytoplasm.

Herbicides and the microtubular apparatus of Nicotiana tabacum pollen tube: immunofluorescence and immunogold labelling studies

Herbicides are chemical compounds widely used in agriculture. As their intensive application is becoming a cause of environmental pollution, detailed and more sophisticated investigations are needed to understand better their consequences at the biological level. After herbicides are dispersed in the fields, they establish chemical interactions with both target and non-target plants. In both cases, herbicides can interact with the plant reproductive apparatus; consequently they could play a role during the fertilisation process in higher plants. Using an antibody to the alpha-tubulin subunit in immunofluorescence and immunoelectron microscopy techniques, we investigated the distribution of microtubules in Nicotiana tabacum pollen tubes grown under in vitro conditions in the presence of five different herbicides selected among those used frequently in central Italy. Herbicides have a specific effect on the microtubular apparatus of both pollen tube and generative cell. In addition to other tests and assays, these results suggest that the microtubule cytoskeleton of pollen tubes can be used as a bioindicator for studying the toxicity effects induced by herbicides.

Immunocytochemical localisation of actin and profilin in the generative cell of angiosperm pollen: TEM studies on high-pressure frozen and freeze-substituted Ledebouria socialis Roth (Hyacinthaceae)

Actin was demonstrated for the first time at the EM level in the generative cell of mature angiosperm pollen by using immuno-gold labelling of high-pressure frozen and freeze-substituted Ledebouria socialis Roth anthers. In addition, profilin, an actin-monomer binding protein, is shown to coexist in the generative cell. We attribute the detection of actin and profilin to the applied cryomethods which yield a much better preservation of ultrastructure and antigenicity of delicate cytoskeletal constituents than conventional fixation techniques. Actin labelling was observed within the cytoplasm of the generative cell and became especially clear in close vicinity to microtubular bundles. Filamentous structures congruent with the actin labelling patterns do occur, but are not a frequent feature. Profilin was localised throughout the cytoplasm.

Identification and localization of three classes of myosins in pollen tubes of Lilium longiflorum and Nicotiana alata

The presence and localization of actin and myosin have been examined in pollen tubes of Lilium longiflorum and Nicotiana alata. Immunoblot analysis of pollen tube extracts with antibodies to actin, myosins IA and IB, myosin II, and myosin V reveals the presence of these contractile proteins. Immunofluorescence microscopy using various methods to preserve the pollen tubes; chemical fixation, rapid freeze fixation and freeze substitution (RF-FS) followed by rehydration or by embeddment in a methacrylate mixture, was performed to optimize preservation. Immunocytochemistry reaffirmed that actin is localized longitudinally in the active streaming lanes and near the cortical surface of the pollen tube. Myosin I was localized to the plasma membrane, larger organelles, the surface of the generative cell and the vegetative nucleus, whereas, myosin V was found in the vegetative cytoplasm in a punctate fashion representing smaller organelles. Myosin II subfragment 1 and light meromyosin were localized in a punctate fashion on the larger organelles throughout the vegetative cytoplasm. In addition, isolated generative cells and vegetative nuclei labeled only with the myosin I antibody. Competition studies indicated the specificity of the heterologous antibodies utilized in this study suggesting the presence of three classes of myosins in pollen. These results lead to the following hypothesis: Myosin I may move the generative cell and vegetative nucleus unidirectionally through the pollen tube to the tip, while myosin V moves the smaller organelles and myosins I and II move the larger organelles (bidirectionally) that are involved in growth.

Confocal image analysis of spatial variations in immunocytochemically identified calmodulin during pollen hydration, germination and pollen tube tip growth in Nicotiana tabacum L

Using monoclonal anti-calmodulin antibodies in conjunction with confocal scanning laser microscopy we have analysed the spatial variations in the distribution pattern of calmodulin (CaM) during the sequential events of pollen hydration, germination and tube growth in Nicotiana tabacum. These immunocytochemical observations have been complemented by immunochemical studies wherein the anti-calmodulin antibody raised against pea CaM recognises a polypeptide of c. 18 kDa in the pollen extracts. Digitisation of confocally acquired optical sections of immunofluorescence images reveals that in hydrated pollen a high level of CaM is consistently present in the region of the germinal apertures. Subsequently, with the onset of germination a high CaM concentration was found associated with the plasma membrane of the germination bubble and in the cytoplasm in its vicinity, while in the vegetative cytoplasm a weak diffuse and intense punctate signal was registered. CaM immunostain was also detected in association with the plasma membrane of the tube tips in both short and long pollen tubes. Furthermore, the cytosol of the tubes invariably manifested an apically focused CaM gradient. We were, however, unable to detect any vacuolar association of CaM in the older regions of the pollen tubes. Although punctate immunostain was obvious across the pollen tube numerous punctate structures were invariably present in the extreme tip. The possible implications of these findings in development of cell polarity, polarised growth, maintenance of calcium homeostasis and CaM interactions with other mechanochemical motor proteins in effecting propulsion of organelles during pollen hydration, germination and pollen tube growth are discussed.

Kinesin-related polypeptide is associated with vesicles from Corylus avellana pollen

A 100-kDa polypeptide with microtubule-interacting properties was identified in a Golgi vesicle-enriched fraction from Corylus avellana pollen. The k71s23 antibody (directed to the kinesin heavy chain from bovine brain) [Tiezzi et al., 1992: Cell Motil. Cytoskeleton 21:132-137] localized the polypeptide on the external surface of membrane-bounded organelles. Some 100-kDa-containing vesicles copelleted with microtubules (polymerized from purified bovine brain tubulin) either in presence or absence of 5 mM AMPPNP, but they could be released by 10 mM ATP or 0.5 M KCl. The pollen microtubule-interacting protein, salt-extracted from membranes and partially purified by gel filtration, exhibited an ATPase activity (16.2 nmolPi/mg/min) which could be stimulated about 2-fold (32.5 nmolPi/mg/min) by addition of bovine brain microtubules. We suppose that the 100-kDa polypeptide is part of a molecular complex showing properties of the kinesin class.