Integrating membrane transport with male gametophyte development and function through transcriptomics

Genomic inventory and transcriptional analysis of Medicago truncatula transporters

Transporters move hydrophilic substrates across hydrophobic biological membranes and play key roles in plant nutrition, metabolism, and signaling and, consequently, in plant growth, development, and responses to the environment. To initiate and support systematic characterization of transporters in the model legume Medicago truncatula, we identified 3,830 transporters and classified 2,673 of these into 113 families and 146 subfamilies. Analysis of gene expression data for 2,611 of these transporters identified 129 that are expressed in an organ-specific manner, including 50 that are nodule specific and 36 specific to mycorrhizal roots. Further analysis uncovered 196 transporters that are induced at least 5-fold during nodule development and 44 in roots during arbuscular mycorrhizal symbiosis. Among the nodule- and mycorrhiza-induced transporter genes are many candidates for known transport activities in these beneficial symbioses. The data presented here are a unique resource for the selection and functional characterization of legume transporters.

The nucleotide sugar transporters AtUTr1 and AtUTr3 are required for the incorporation of UDP-glucose into the endoplasmic reticulum, are essential for pollen development and are needed for embryo sac progress in Arabidopsis thaliana

Uridine 5'-diphosphate (UDP)-glucose is transported into the lumen of the endoplasmic reticulum (ER), and the Arabidopsis nucleotide sugar transporter AtUTr1 has been proposed to play a role in this process; however, different lines of evidence suggest that another transporter(s) may also be involved. Here we show that AtUTr3 is involved in the transport of UDP-glucose and is located at the ER but also at the Golgi. Insertional mutants in AtUTr3 showed no obvious phenotype. Biochemical analysis in both AtUTr1 and AtUTr3 mutants indicates that uptake of UDP-glucose into the ER is mostly driven by these two transporters. Interestingly, the expression of AtUTr3 is induced by stimuli that trigger the unfolded protein response (UPR), a phenomenon also observed for AtUTr1, suggesting that both AtUTr1 and AtUTr3 are involved in supplying UDP-glucose into the ER lumen when misfolded proteins are accumulated. Disruption of both AtUTr1 and AtUTr3 causes lethality. Genetic analysis showed that the atutr1 atutr3 combination was not transmitted by pollen and was poorly transmitted by the ovules. Cell biology analysis indicates that knocking out both genes leads to abnormalities in both male and female germ line development. These results show that the nucleotide sugar transporters AtUTr1 and AtUTr3 are required for the incorporation of UDP-glucose into the ER, are essential for pollen development and are needed for embryo sac progress in Arabidopsis thaliana.

Ion transporters involved in pollen germination and pollen tube tip-growth

Pollen germination (PG) and pollen tube growth (PTG) play crucial roles in sexual reproduction of flowering plants by sending sperm cells to the ovule. These two processes are regarded as ideal model system for the study of cell signaling and cell polarized growth. It has been considered for a long time that ion transports across the pollen tube membranes are essential for pollen tube navigation and growth. Previous transcriptome analyses for Arabidopsis have shown that the transcripts related to cellular transport are correspondingly overrepresented during the process of pollen tube growth. Here, we showed that 459 transporter genes expressed during PG and PTG in Arabidopsis. In addition, the gene expression profiles of ion (including Ca(2+), H(+), K(+), Cl(-)) channels and transporters were further analyzed. This analysis provides novel information for the potential candidate genes involving in ion fluxes across the pollen tube membranes and in regulation of pollen tube tip growth.

Penetration of the stigma and style elicits a novel transcriptome in pollen tubes, pointing to genes critical for growth in a pistil

Pollen tubes extend through pistil tissues and are guided to ovules where they release sperm for fertilization. Although pollen tubes can germinate and elongate in a synthetic medium, their trajectory is random and their growth rates are slower compared to growth in pistil tissues. Furthermore, interaction with the pistil renders pollen tubes competent to respond to guidance cues secreted by specialized cells within the ovule. The molecular basis for this potentiation of the pollen tube by the pistil remains uncharacterized. Using microarray analysis in Arabidopsis, we show that pollen tubes that have grown through stigma and style tissues of a pistil have a distinct gene expression profile and express a substantially larger fraction of the Arabidopsis genome than pollen grains or pollen tubes grown in vitro. Genes involved in signal transduction, transcription, and pollen tube growth are overrepresented in the subset of the Arabidopsis genome that is enriched in pistil-interacted pollen tubes, suggesting the possibility of a regulatory network that orchestrates gene expression as pollen tubes migrate through the pistil. Reverse genetic analysis of genes induced during pollen tube growth identified seven that had not previously been implicated in pollen tube growth. Two genes are required for pollen tube navigation through the pistil, and five genes are required for optimal pollen tube elongation in vitro. Our studies form the foundation for functional genomic analysis of the interactions between the pollen tube and the pistil, which is an excellent system for elucidation of novel modes of cell–cell interaction.

For successful reproduction in flowering plants, a single-celled pollen tube must rapidly extend through female pistil tissue, locate female gametes, and deliver sperm. Pollen tubes undergo a dramatic transformation while growing in the pistil; they grow faster compared to tubes grown in vitro and become competent to perceive and respond to navigation cues secreted by the pistil. The genes expressed by pollen tubes in response to growth in the pistil have not been characterized. We used a surgical procedure to obtain large quantities of uncontaminated pollen tubes that grew through the pistil and defined their transcriptome by microarray analysis. Importantly, we identify a set of genes that are specifically expressed in pollen tubes in response to their growth in the pistil and are not expressed during other stages of pollen or plant development. We analyzed mutants in 33 pollen tube–expressed genes using a sensitive series of pollen function assays and demonstrate that seven of these genes are critical for pollen tube growth; two specifically disrupt growth in the pistil. By identifying pollen tube genes induced by the pistil and describing a mutant analysis scheme to understand their function, we lay the foundation for functional genomic analysis of pollen–pistil interactions.

AtbZIP34 is required for Arabidopsis pollen wall patterning and the control of several metabolic pathways in developing pollen

Sexual plant reproduction depends on the production and differentiation of functional gametes by the haploid gametophyte generation. Currently, we have a limited understanding of the regulatory mechanisms that have evolved to specify the gametophytic developmental programs. To unravel such mechanisms, it is necessary to identify transcription factors (TF) that are part of such haploid regulatory networks. Here we focus on bZIP TFs that have critical roles in plants, animals and other kingdoms. We report the functional characterization of Arabidopsis thaliana AtbZIP34 that is expressed in both gametophytic and surrounding sporophytic tissues during flower development. T-DNA insertion mutants in AtbZIP34 show pollen morphological defects that result in reduced pollen germination efficiency and slower pollen tube growth both in vitro and in vivo. Light and fluorescence microscopy revealed misshapen and misplaced nuclei with large lipid inclusions in the cytoplasm of atbzip34 pollen. Scanning and transmission electron microscopy revealed defects in exine shape and micropatterning and a reduced endomembrane system. Several lines of evidence, including the AtbZIP34 expression pattern and the phenotypic defects observed, suggest a complex role in male reproductive development that involves a sporophytic role in exine patterning, and a sporophytic and/or gametophytic mode of action of AtbZIP34 in several metabolic pathways, namely regulation of lipid metabolism and/or cellular transport.

Placing metal micronutrients in context: transport and distribution in plants

Plants have developed finely tuned mechanisms to efficiently acquire and balance the concentrations of essential metal micronutrients including iron, zinc, copper, and manganese, both at the cellular and systemic levels. The application of new emerging technologies to the study of Arabidopsis thaliana is providing a novel spatiotemporal view of plant metal homeostasis. These advances are uncovering unexpected links of metal homeostasis to central cellular processes, such as compartmentalization, daily redox oscillations, or transcriptional regulation. The intracellular compartmentalization of metals seems essential for optimizing the use of micronutrients during development and in response to deficiencies. Furthermore, recent discoveries indicate that protein metallation is highly sensitive to surrounding conditions, including metal redox state and concentration. Thus, some steps in metal delivery occur during protein folding at specific intracellular compartments. Finally, the daily nature in redox oscillations should be taken into account for a comprehensive understanding of global plant metal homeostasis.

Comparative proteomic analysis of Arabidopsis mature pollen and germinated pollen

Proteomic analysis was applied to generating the map of Arabidopsis mature pollen proteins and analyzing the differentially expressed proteins that are potentially involved in the regulation of Arabidopsis pollen germination. By applying 2-D electrophoresis and silver staining, we resolved 499 and 494 protein spots from protein samples extracted from pollen grains and pollen tubes, respectively. Using the matrix-assisted laser desorption ionization time-of-flight mass spectrometry method, we identified 189 distinct proteins from 213 protein spots expressed in mature pollen or pollen tubes, and 75 new identified proteins that had not been reported before in research into the Arabidopsis pollen proteome. Comparative analysis revealed that 40 protein spots exhibit reproducible significant changes between mature pollen and pollen tubes. And 21 proteins from 17 downregulated and six upregulated protein spots were identified. Functional category analysis indicated that these differentially expressed proteins mainly involved in signaling, cellular structure, transport, defense/stress responses, transcription, metabolism, and energy production. The patterns of changes at protein level suggested the important roles for energy metabolism-related proteins in pollen tube growth, accompanied by the activation of the stress response pathway and modifications to the cell wall.

Genomic expression profiling of mature soybean (Glycine max) pollen

BACKGROUND

Pollen, the male partner in the reproduction of flowering plants, comprises either two or three cells at maturity. The current knowledge of the pollen transcriptome is limited to the model plant systems Arabidopsis thaliana and Oryza sativa which have tri-cellular pollen grains at maturity. Comparative studies on pollen of other genera, particularly crop plants, are needed to understand the pollen gene networks that are subject to functional and evolutionary conservation. In this study, we used the Affymetrix Soybean GeneChip to perform transcriptional profiling on mature bi-cellular soybean pollen.

RESULTS

Compared to the sporophyte transcriptome, the soybean pollen transcriptome revealed a restricted and unique repertoire of genes, with a significantly greater proportion of specifically expressed genes than is found in the sporophyte tissue. Comparative analysis shows that, among the 37,500 soybean transcripts addressed in this study, 10,299 transcripts (27.46%) are expressed in pollen. Of the pollen-expressed sequences, about 9,489 (92.13%) are also expressed in sporophytic tissues, and 810 (7.87%) are selectively expressed in pollen. Overall, the soybean pollen transcriptome shows an enrichment of transcription factors (mostly zinc finger family proteins), signal recognition receptors, transporters, heat shock-related proteins and members of the ubiquitin proteasome proteolytic pathway.

CONCLUSION

This is the first report of a soybean pollen transcriptional profile. These data extend our current knowledge regarding regulatory pathways that govern the gene regulation and development of pollen. A comparison between transcription factors up-regulated in soybean and those in Arabidopsis revealed some divergence in the numbers and kinds of regulatory proteins expressed in both species.

A cyclic nucleotide-gated channel is necessary for optimum fertility in high-calcium environments

* Arabidopsis cngc2 plants are hypersensitive to external calcium and exhibit reduced plant size and fertility, especially when they are treated with elevated but physiologically relevant levels of calcium. This report focuses on the role of cyclic nucleotide-gated channel 2 (CNGC2) in plant fertility. * To determine the cause of the reduced fertility, we investigated the flower structure and growth potential of both male and female reproductive organs in cngc2 plants grown in high-calcium conditions. * cngc2 mutants had short stamens that may limit pollen deposition and pistils that were not conducive to pollen tube growth. * Our data indicate that sporophytic, but not gametophytic, defects are the main cause of the observed reduction in seed yield in cngc2 plants, and suggest that correct cyclic nucleotide and calcium signaling are important for cell elongation and pollen tube guidance.

AtAMT1;4, a pollen-specific high-affinity ammonium transporter of the plasma membrane in Arabidopsis

Pollen represents an important nitrogen sink in flowers to ensure pollen viability. Since pollen cells are symplasmically isolated during maturation and germination, membrane transporters are required for nitrogen import across the pollen plasma membrane. This study describes the characterization of the ammonium transporter AtAMT1;4, a so far uncharacterized member of the Arabidopsis AMT1 family, which is suggested to be involved in transporting ammonium into pollen. The AtAMT1;4 gene encodes a functional ammonium transporter when heterologously expressed in yeast or when overexpressed in Arabidopsis roots. Concentration-dependent analysis of (15)N-labeled ammonium influx into roots of AtAMT1;4-transformed plants allowed characterization of AtAMT1;4 as a high-affinity transporter with a K(m) of 17 microM. RNA and protein gel blot analysis showed expression of AtAMT1;4 in flowers, and promoter-gene fusions to the green fluorescent protein (GFP) further defined its exclusive expression in pollen grains and pollen tubes. The AtAMT1;4 protein appeared to be localized to the plasma membrane as indicated by protein gel blot analysis of plasma membrane-enriched membrane fractions and by visualization of GFP-tagged AtAMT1;4 protein in pollen grains and pollen tubes. However, no phenotype related to pollen function could be observed in a transposon-tagged line, in which AtAMT1;4 expression is disrupted. These results suggest that AtAMT1;4 mediates ammonium uptake across the plasma membrane of pollen to contribute to nitrogen nutrition of pollen via ammonium uptake or retrieval.

AtTIP1;3 and AtTIP5;1, the only highly expressed Arabidopsis pollen-specific aquaporins, transport water and urea

Pollination includes processes where water and/or solute movements must be finely regulated, suggesting participation of aquaporins. Using information available from different transcriptional profilings of Arabidopsis thaliana mature pollen, we showed that the only aquaporins that are selectively and highly expressed in mature pollen are two TIPs: AtTIP1;3 and AtTIP5;1. Pollen exhibited a lower number and more exclusive type of aquaporin expressed genes when compared to other single cell transcriptional profilings. When characterized using Xenopus oocyte swelling assays, AtTIP1;3 and AtTIP5;1 showed intermediate water permeabilities. Although they displayed neither glycerol nor boric acid permeability they both transported urea. In conclusion, these results suggest a function for AtTIP1;3 and AtTIP5;1 as specific water and urea channels in Arabidopsis pollen.

Transcriptome analyses show changes in gene expression to accompany pollen germination and tube growth in Arabidopsis

Pollen germination, along with pollen tube growth, is an essential process for the reproduction of flowering plants. The germinating pollen with tip-growth characteristics provides an ideal model system for the study of cell growth and morphogenesis. As an essential step toward a detailed understanding of this important process, the objective of this study was to comprehensively analyze the transcriptome changes during pollen germination and pollen tube growth. Using Affymetrix Arabidopsis (Arabidopsis thaliana) ATH1 Genome Arrays, this study is, to our knowledge, the first to show the changes in the transcriptome from desiccated mature pollen grains to hydrated pollen grains and then to pollen tubes of Arabidopsis. The number of expressed genes, either for total expressed genes or for specifically expressed genes, increased significantly from desiccated mature pollen to hydrated pollen and again to growing pollen tubes, which is consistent with the finding that pollen germination and tube growth were significantly inhibited in vitro by a transcriptional inhibitor. The results of Gene Ontology analyses showed that expression of genes related to cell rescue, transcription, signal transduction, and cellular transport was significantly changed, especially for up-regulation, during pollen germination and tube growth. In particular, genes of the calmodulin/calmodulin-like protein, cation/hydrogen exchanger, and heat shock protein families showed the most significant changes during pollen germination and tube growth. These results demonstrate that the overall transcription of genes, both in the number of expressed genes and in the levels of transcription, was increased. Furthermore, the appearance of many novel transcripts during pollen germination as well as tube growth indicates that these newly expressed genes may function in this complex process.

AtCCX3 is an Arabidopsis endomembrane H+ -dependent K+ transporter

The Arabidopsis (Arabidopsis thaliana) cation calcium exchangers (CCXs) were recently identified as a subfamily of cation transporters; however, no plant CCXs have been functionally characterized. Here, we show that Arabidopsis AtCCX3 (At3g14070) and AtCCX4 (At1g54115) can suppress yeast mutants defective in Na(+), K(+), and Mn(2+) transport. We also report high-capacity uptake of (86)Rb(+) in tonoplast-enriched vesicles from yeast expressing AtCCX3. Cation competition studies showed inhibition of (86)Rb(+) uptake in AtCCX3 cells by excess Na(+), K(+), and Mn(2+). Functional epitope-tagged AtCCX3 fusion proteins were localized to endomembranes in plants and yeast. In Arabidopsis, AtCCX3 is primarily expressed in flowers, while AtCCX4 is expressed throughout the plant. Quantitative polymerase chain reaction showed that expression of AtCCX3 increased in plants treated with NaCl, KCl, and MnCl(2). Insertional mutant lines of AtCCX3 and AtCCX4 displayed no apparent growth defects; however, overexpression of AtCCX3 caused increased Na(+) accumulation and increased (86)Rb(+) transport. Uptake of (86)Rb(+) increased in tonoplast-enriched membranes isolated from Arabidopsis lines expressing CCX3 driven by the cauliflower mosaic virus 35S promoter. Overexpression of AtCCX3 in tobacco (Nicotiana tabacum) produced lesions in the leaves, stunted growth, and resulted in the accumulation of higher levels of numerous cations. In summary, these findings suggest that AtCCX3 is an endomembrane-localized H(+)-dependent K(+) transporter with apparent Na(+) and Mn(2+) transport properties distinct from those of previously characterized plant transporters.

AtPTR1 and AtPTR5 transport dipeptides in planta

Transporters for di- and tripeptides belong to the large and poorly characterized PTR/NRT1 (peptide transporter/nitrate transporter 1) family. A new member of this gene family, AtPTR5, was isolated from Arabidopsis (Arabidopsis thaliana). Expression of AtPTR5 was analyzed and compared with tissue specificity of the closely related AtPTR1 to discern their roles in planta. Both transporters facilitate transport of dipeptides with high affinity and are localized at the plasma membrane. Mutants, double mutants, and overexpressing lines were exposed to several dipeptides, including toxic peptides, to analyze how the modified transporter expression affects pollen germination, growth of pollen tubes, root, and shoot. Analysis of atptr5 mutants and AtPTR5-overexpressing lines showed that AtPTR5 facilitates peptide transport into germinating pollen and possibly into maturating pollen, ovules, and seeds. In contrast, AtPTR1 plays a role in uptake of peptides by roots indicated by reduced nitrogen (N) levels and reduced growth of atptr1 mutants on medium with dipeptides as the sole N source. Furthermore, overexpression of AtPTR5 resulted in enhanced shoot growth and increased N content. The function in peptide uptake was further confirmed with toxic peptides, which inhibited growth. The results show that closely related members of the PTR/NRT1 family have different functions in planta. This study also provides evidence that the use of organic N is not restricted to amino acids, but that dipeptides should be considered as a N source and transport form in plants.

AtCHX13 is a plasma membrane K+ transporter

Potassium (K+) homeostasis is essential for diverse cellular processes, although how various cation transporters collaborate to maintain a suitable K+ required for growth and development is poorly understood. The Arabidopsis (Arabidopsis thaliana) genome contains numerous cation:proton antiporters (CHX), which may mediate K+ transport; however, the vast majority of these transporters remain uncharacterized. Here, we show that AtCHX13 (At2g30240) has a role in K+ acquisition. AtCHX13 suppressed the sensitivity of yeast (Saccharomyces cerevisiae) mutant cells defective in K+ uptake. Uptake experiments using (86)Rb+ as a tracer for K+ demonstrated that AtCHX13 mediated high-affinity K+ uptake in yeast and in plant cells with a K(m) of 136 and 196 microm, respectively. Functional green fluorescent protein-tagged versions localized to the plasma membrane of both yeast and plant. Seedlings of null chx13 mutants were sensitive to K+ deficiency conditions, whereas overexpression of AtCHX13 reduced the sensitivity to K+ deficiency. Collectively, these results suggest that AtCHX13 mediates relatively high-affinity K+ uptake, although the mode of transport is unclear at present. AtCHX13 expression is induced in roots during K+-deficient conditions. These results indicate that one role of AtCHX13 is to promote K+ uptake into plants when K+ is limiting in the environment.

Ectopic expression of Arabidopsis thaliana plasma membrane intrinsic protein 2 aquaporins in lily pollen increases the plasma membrane water permeability of grain but not of tube protoplasts

To investigate the role of aquaporin-mediated water transport during pollen grain germination and tube growth, Arabidopsis thaliana plasma membrane intrinsic proteins (PIPs) were expressed in pollen of Lilium longiflorum (lily). Successful expression of AtPIPs in particle-bombarded lily pollen grains was monitored by co-expression with fluorescent proteins and single-cell RT-PCR, and by measuring the water permeability coefficient (P(os)) in swelling assays using protoplasts prepared from transformed pollen grains and tubes. Expression of AtPIP1;1 and AtPIP1;2 in pollen grains resulted in P(os) values similar to those measured in nontransformed pollen grain protoplasts (6.65 +/- 2.41 microm s(-1)), whereas expression of AtPIP2 significantly increased P(os) (AtPIP2;1, 13.79 +/- 6.38; AtPIP2;2, 10.16 +/- 3.30 microm s(-1)). Transformation with combinations of AtPIP1 and AtPIP2 did not further enhance P(os). Native pollen tube protoplasts showed higher P(os) values (13.23 +/- 4.14 microm s(-1)) than pollen grain protoplasts but expression of AtPIP2;1 (18.85 +/- 7.60 microm s(-1)) did not significantly increase their P(os) values. Expression of none of the tested PIPs had any effect on pollen tube growth rates. The ectopic expression of AtPIP2s in lily pollen increased the water permeability of the plasma membrane in pollen grains, but not in pollen tubes. The measured endogenous water permeability does not limit water uptake during tube growth, but has to be regulated to prevent tube bursting.

Arabidopsis sucrose transporter AtSUC1 is important for pollen germination and sucrose-induced anthocyanin accumulation

The Arabidopsis (Arabidopsis thaliana) sucrose transporter AtSUC1 (At1g71880) is highly expressed in pollen; however, its function has remained unknown. Here, we show that suc1 mutant pollen is defective in vivo, as evidenced by segregation distortion, and also has low rates of germination in vitro. AtSUC1-green fluorescent protein was localized to the plasma membrane in pollen tubes. AtSUC1 is also expressed in roots and external application of sucrose increased AtSUC1 expression in roots. AtSUC1 is important for sucrose-dependent signaling leading to anthocyanin accumulation in seedlings. suc1 mutants accumulated less anthocyanins in response to exogenous sucrose or maltose and microarray analysis revealed reduced expression of many genes important for anthocyanin biosynthesis. The results indicate that AtSUC1 is important for sugar signaling in vegetative tissue and for normal male gametophyte function.

The origin and function of calmodulin regulated Ca2+ pumps in plants

While Ca2+ signaling plays an important role in both plants and animals, the machinery that codes and decodes these signals have evolved to show interesting differences and similarities. For example, typical plant and animal cells both utilize calmodulin (CaM)-regulated Ca2+ pumps at the plasma membrane to help control cytoplasmic Ca2+ levels. However, in flowering plants this family of pumps has evolved with a unique structural arrangement in which the regulatory domain is located at the N-terminal instead of C-terminal end. In addition, some of the plant isoforms have evolved to function at endomembrane locations. For the 14 Ca2+ pumps present in the model plant Arabidopsis, molecular genetic analyses are providing exciting insights into their function in diverse aspects of plant growth and development.

Structural and signaling networks for the polar cell growth machinery in pollen tubes

Pollen tubes elongate within the pistil to transport sperms to the female gametophytes for fertilization. Pollen tubes grow at their tips through a rapid and polarized cell growth process. This tip growth process is supported by an elaborate and dynamic actin cytoskeleton and a highly active membrane trafficking system that together provide the driving force and secretory activities needed for growth. A polarized cytoplasm with an abundance of vesicles and tip-focused Ca(2+) and H(+) concentration gradients are important for the polar cell growth process. Apical membrane-located Rho GTPases regulate Ca(2+) concentration and actin dynamics in the cytoplasm and are crucial for maintaining pollen tube polarity. Pollen tube growth is marked by periods of rapid and slow growth phases. Activities that regulate and support this tip growth process also show oscillatory fluctuations. How these activities correlate with the rapid, polar, and oscillatory pollen tube growth process is discussed.

Global analysis of gene expression in flower buds of Ms-cd1 Brassica oleracea conferring male sterility by using an Arabidopsis microarray

The dominant male sterility gene Ms-cd1 is identified in Brassica oleracea. Electron microscopical observations revealed that abortion of pollen development starts after tetrad formation. This important male sterility phenotype is characterized by lack of degradation of the primary pollen mother cell (PMC) wall and delayed degradation of callose surrounding the tetrads and thus arrest of microspore release. Gene expression of the male sterile and fertile buds was analyzed by heterologous hybridization of Brassica oleracea cRNA onto an Arabidopsis whole genome oligonucleotide microarray. A total of 277 suppressed genes including 40 kinase-, 32 cell wall modification and 29 transport related genes were found to be significantly down regulated >3-fold in the male sterile mutant. The vast majority of the differentially expressed transcripts are found to present late pollen stage specific genes. Kinase genes, cell wall modification genes and ion transport genes were greatly over-represented when compared to their percentage of all flower bud expressed genes and represent 36.5% of the genes suppressed by Ms-cd1. Our results also suggest that Ms-cd1 may blocks an anther developmental pathway with a small number of genes suppressed in tapetum cells which prevent the degradation of callose and PMC wall, which further leads to the suppression of a large number of genes involved in signaling pathways, cell wall modification and ion transport in pollen grains.

How many genes are needed to make a pollen tube? Lessons from transcriptomics

BACKGROUND

Pollen is the male gametophyte of higher plants. Upon pollination, it germinates and develops into a fast-growing cytoplasmic extension, the pollen tube, which ultimately delivers the sperm into the ovary. The biological relevance of its role, and the uniqueness of this kind of cellular organization, have made pollen the focus of many approaches, and it stands today as one of the best-known models in plant cell biology. In contrast, the genetic background of its development has been until recently largely unknown. Some genes involved have been described and a few functional mutants have been characterized, but only to a limited extent and allowing only a limited understanding of the regulatory mechanisms. Yet, being a relatively simple organ (2 or 3 cells), pollen stands as an excellent target for molecular-biology-based approaches.

RECENT PROGRESS

Recent studies on Arabidopsis thaliana have characterized the transcriptional profile of pollen grains and microgametogenesis in comparison to sporophytic tissues. They underline the unique characteristics of pollen, not only in terms of a strongly reduced set of genes being expressed, but also in terms of the functions of the proteins encoded and the pathways they are involved in. These approaches have expanded the number of genes with known expression in pollen from a few hundred to nearly eight thousand. While for the first time allowing systems and/or gene-family approaches, this information also expands dramatically the possibility of hypothesis-driven experimentation based on specific gene function predictions. Recent studies reveal this to be the case in, for example, transcriptional regulation, cell-cycle progression and gene-silencing mechanisms in mature pollen.

A cyclic nucleotide-gated channel is essential for polarized tip growth of pollen

Ion signals are critical to regulating polarized growth in many cell types, including pollen in plants and neurons in animals. Genetic evidence presented here indicates that pollen tube growth requires cyclic nucleotide-gated channel (CNGC) 18. CNGCs are nonspecific cation channels found in plants and animals and have well established functions in excitatory signal transduction events in animals. In Arabidopsis, male sterility was observed for two cngc18 null mutations. CNGC18 is expressed primarily in pollen, as indicated from a promoter::GUS (beta-glucuronidase) reporter analysis and expression profiling. The underlying cause of sterility was identified as a defect in pollen tube growth, resulting in tubes that were kinky, short, often thin, and unable to grow into the transmitting tract. Expression of a GFP-tagged CNGC18 in mutant pollen provided complementation and evidence for asymmetric localization of CNGC18 to the plasma membrane at the growing tip, starting at the time of pollen grain germination. Heterologous expression of CNGC18 in Escherichia coli resulted in a time- and concentration-dependent accumulation of more Ca2+. Thus, CNGC18 provides a mechanism to directly transduce a cyclic nucleotide (cNMP) signal into an ion flux that can produce a localized signal capable of regulating the pollen tip-growth machinery. These results identify a CNGC that is essential to an organism's life cycle and raise the possibility that CNGCs have a widespread role in regulating cell-growth dynamics in both plant and animals.

The COMATOSE ATP-binding cassette transporter is required for full fertility in Arabidopsis

COMATOSE (CTS) encodes a peroxisomal ATP-binding cassette transporter required not only for beta-oxidation of storage lipids during germination and establishment, but also for biosynthesis of jasmonic acid and conversion of indole butyric acid to indole acetic acid. cts mutants exhibited reduced fertilization, which was rescued by genetic complementation, but not by exogenous application of jasmonic acid or indole acetic acid. Reduced fertilization was also observed in thiolase (kat2-1) and peroxisomal acyl-Coenzyme A synthetase mutants (lacs6-1,lacs7-1), indicating a general role for beta-oxidation in fertility. Genetic analysis revealed reduced male transmission of cts alleles and both cts pollen germination and tube growth in vitro were impaired in the absence of an exogenous carbon source. Aniline blue staining of pollinated pistils demonstrated that pollen tube growth was affected only when both parents bore the cts mutation, indicating that expression of CTS in either male or female tissues was sufficient to support pollen tube growth in vivo. Accordingly, abundant peroxisomes were detected in a range of maternal tissues. Although gamma-aminobutyric acid levels were reduced in flowers of cts mutants, they were unchanged in kat2-1, suggesting that alterations in gamma-aminobutyric acid catabolism do not contribute to the reduced fertility phenotype through altered pollen tube targeting. Taken together, our data support an important role for beta-oxidation in fertility in Arabidopsis (Arabidopsis thaliana) and suggest that this pathway could play a role in the mobilization of lipids in both pollen and female tissues.

Plant proton pumps

Chemiosmotic circuits of plant cells are driven by proton (H(+)) gradients that mediate secondary active transport of compounds across plasma and endosomal membranes. Furthermore, regulation of endosomal acidification is critical for endocytic and secretory pathways. For plants to react to their constantly changing environments and at the same time maintain optimal metabolic conditions, the expression, activity and interplay of the pumps generating these H(+) gradients have to be tightly regulated. In this review, we will highlight results on the regulation, localization and physiological roles of these H(+)- pumps, namely the plasma membrane H(+)-ATPase, the vacuolar H(+)-ATPase and the vacuolar H(+)-PPase.

Cyclic nucleotide-gated channels in plants

Until recently the role of cyclic nucleotide monophosphates (cNMPs) in plants had been controversial, with equivocal data about their concentrations, biosynthetic and degrading enzymes, and cellular targets. This review discusses the current knowledge in this field, with focus on the largest class of cNMP targets in plant cells, the cyclic nucleotide-gated channels (CNGCs). Aspects of structure and function are addressed, with reference to studies in heterologous systems and in planta. The picture emerging, albeit still fragmented, is of proteins with diverse functions in the control of ion homeostasis, development, and defense against biotic and abiotic threats.

Identification of microspore-active promoters that allow targeted manipulation of gene expression at early stages of microgametogenesis in Arabidopsis

BACKGROUND

The effective functional analysis of male gametophyte development requires new tools enabling the spatially and temporally controlled expression of both marker genes and modified genes of interest. In particular, promoters driving expression at earlier developmental stages including microspores are required.

RESULTS

Transcriptomic datasets covering four progressive stages of male gametophyte development in Arabidopsis were used to select candidate genes showing early expression profiles that were male gametophyte-specific. Promoter-GUS reporter analysis of candidate genes identified three promoters (MSP1, MSP2, and MSP3) that are active in microspores and are otherwise specific to the male gametophyte and tapetum. The MSP1 and MSP2 promoters were used to successfully complement and restore the male transmission of the gametophytic two-in-one (tio) mutant that is cytokinesis-defective at first microspore division.

CONCLUSION

We demonstrate the effective application of MSP promoters as tools that can be used to elucidate gametophytic gene functions in microspores in a male-specific manner.