Dynamics of vegetative cytoplasm during generative cell formation and pollen maturation in Arabidopsis thaliana

How pollen tubes fight for food: the impact of sucrose carriers and invertases of Arabidopsis thaliana on pollen development and pollen tube growth

Pollen tubes of higher plants grow very rapidly until they reach the ovules to fertilize the female gametes. This growth process is energy demanding, however, the nutrition strategies of pollen are largely unexplored. Here, we studied the function of sucrose transporters and invertases during pollen germination and pollen tube growth. RT-PCR analyses, reporter lines and knockout mutants were used to study gene expression and protein function in pollen. The genome of Arabidopsis thaliana contains eight genes that encode functional sucrose/H⁺ symporters. Apart from AtSUC2, which is companion cell specific, all other AtSUC genes are expressed in pollen tubes. AtSUC1 is present in developing pollen and seems to be the most important sucrose transporter during the fertilization process. Pollen of an Atsuc1 knockout plant contain less sucrose and have defects in pollen germination and pollen tube growth. The loss of other sucrose carriers affects neither pollen germination nor pollen tube growth. A multiple knockout line Atsuc1Atsuc3Atsuc8Atsuc9 shows a phenotype that is comparable to the Atsuc1 mutant line. Loss of AtSUC1 can`t be complemented by AtSUC9, suggesting a special function of AtSUC1. Besides sucrose carriers, pollen tubes also synthesize monosaccharide carriers of the AtSTP family as well as invertases. We could show that AtcwINV2 and AtcwINV4 are expressed in pollen, AtcwINV1 in the transmitting tissue and AtcwINV5 in the funiculi of the ovary. The vacuolar invertase AtVI2 is also expressed in pollen, and a knockout of AtVI2 leads to a severe reduction in pollen germination. Our data indicate that AtSUC1 mediated sucrose accumulation during late stages of pollen development and cleavage of vacuolar sucrose into monosaccharides is important for the process of pollen germination.

Genetic control of generative cell shape by DUO1 in Arabidopsis

The main features of generative cell morphogenesis, formation of a cytoplasmic projection and elongation of the GC body, operate through independent genetic pathways. Male gametogenesis in developing angiosperm pollen involves distinctive changes in cell morphogenesis. Re-shaping and elongation of the generative cell (GC) are linked to the formation of a GC cytoplasmic projection connected to the vegetative cell nucleus. Although genetic control of GC morphogenesis is unknown, we suspected the involvement of the germline-specific MYB transcription factor DUO POLLEN1 (DUO1). We used light and fluorescence microscopy to examine male germline development in pollen of wild-type Arabidopsis and in four allelic duo1 mutants expressing introduced cell markers. Our analysis shows that the undivided GC in duo1 pollen forms a cytoplasmic projection, but the cell body fails to elongate. In contrast GCs of cyclin-dependent kinase function mutants, which fail to divide like duo1 mutants, achieve normal morphogenesis. We conclude that DUO1 has an essential role in the elongation of the GC, but DUO1-independent pathways control the development of the GC cytoplasmic projection. The two main features of GC morphogenesis therefore operate through independently regulated genetic pathways.

Comparative Metabolic Analysis Reveals a Metabolic Switch in Mature, Hydrated, and Germinated Pollen in Arabidopsis thaliana

Pollen germination is an essential process for pollen tube growth, pollination, and therefore seed production in flowering plants, and it requires energy either from remobilization of stored carbon sources, such as lipids and starches, or from secreted exudates from the stigma. Transcriptome analysis from in vitro pollen germination previously showed that 14 GO terms, including metabolism and energy, were overrepresented in Arabidopsis. However, little is understood about global changes in carbohydrate and energy-related metabolites during the transition from mature pollen grain to hydrated pollen, a prerequisite to pollen germination, in most plants, including Arabidopsis. In this study, we investigated differential metabolic pathway enrichment among mature, hydrated, and germinated pollen using an untargeted metabolomic approach. Integration of publicly available transcriptome data with metabolomic data generated as a part of this study revealed starch and sucrose metabolism increased significantly during pollen hydration and germination. We analyzed in detail alterations in central metabolism, focusing on soluble carbohydrates, non-esterified fatty acids, glycerophospholipids, and glycerolipids. We found that several metabolites, including palmitic acid, oleic acid, linolenic acid, quercetin, luteolin/kaempferol, and γ-aminobutyric acid (GABA), were elevated in hydrated pollen, suggesting a potential role in activating pollen tube emergence. The metabolite levels of mature, hydrated, and germinated pollen, presented in this work provide insights on the molecular basis of pollen germination.

Auxin boosts energy generation pathways to fuel pollen maturation in barley

Pollen grains become increasingly independent of the mother plant as they reach maturity through poorly understood developmental programs. We report that the hormone auxin is essential during barley pollen maturation to boost the expression of genes encoding almost every step of heterotrophic energy production pathways. Accordingly, auxin is necessary for the flux of sucrose and hexoses into glycolysis and to increase the levels of pyruvate and two tricarboxylic (TCA) cycle metabolites (citrate and succinate). Moreover, bioactive auxin is synthesized by the pollen-localized enzyme HvYUCCA4, supporting that pollen grains autonomously produce auxin to stimulate a specific cellular output, energy generation, that fuels maturation processes such as starch accumulation. Our results demonstrate that auxin can shift central carbon metabolism to drive plant cell development, which suggests a direct mechanism for auxin's ability to promote growth and differentiation.

The potential of resilient carbon dynamics for stabilizing crop reproductive development and productivity during heat stress

Impaired carbon metabolism and reproductive development constrain crop productivity during heat stress. Reproductive development is energy intensive, and its requirement for respiratory substrates rises as associated metabolism increases with temperature. Understanding how these processes are integrated and the extent to which they contribute to the maintenance of yield during and following periods of elevated temperatures is important for developing climate-resilient crops. Recent studies are beginning to demonstrate links between processes underlying carbon dynamics and reproduction during heat stress, consequently a summation of research that has been reported thus far and an evaluation of purported associations are needed to guide and stimulate future research. To this end, we review recent studies relating to source-sink dynamics, non-foliar photosynthesis and net carbon gain as pivotal in understanding how to improve reproductive development and crop productivity during heat stress. Rapid and precise phenotyping during narrow phenological windows will be important for understanding mechanisms underlying these processes, thus we discuss the development of relevant high-throughput phenotyping approaches that will allow for more informed decision-making regarding future crop improvement.

Proteomic Analysis of Generative and Vegetative Nuclei Reveals Molecular Characteristics of Pollen Cell Differentiation in Lily

In plants, the cell fates of a vegetative cell (VC) and generative cell (GC) are determined after the asymmetric division of the haploid microspore. The VC exits the cell cycle and grows a pollen tube, while the GC undergoes further mitosis to produce two sperm cells for double fertilization. However, our understanding of the mechanisms underlying their fate differentiation remains limited. One major advantage of the nuclear proteome analysis is that it is the only method currently able to uncover the systemic differences between VC and GC due to GC being engulfed within the cytoplasm of VC, limiting the use of transcriptome. Here, we obtained pure preparations of the vegetative cell nuclei (VNs) and generative cell nuclei (GNs) from germinating lily pollens. Utilizing these high-purity VNs and GNs, we compared the differential nucleoproteins between them using state-of-the-art quantitative proteomic techniques. We identified 720 different amount proteins (DAPs) and grouped the results in 11 fate differentiation categories. Among them, we identified 29 transcription factors (TFs) and 10 cell fate determinants. Significant differences were found in the molecular activities of vegetative and reproductive nuclei. The TFs in VN mainly participate in pollen tube development. In comparison, the TFs in GN are mainly involved in cell differentiation and male gametogenesis. The identified novel TFs may play an important role in cell fate differentiation. Our data also indicate differences in nuclear pore complexes and epigenetic modifications: more nucleoporins synthesized in VN; more histone variants and chaperones; and structural maintenance of chromosome (SMC) proteins, chromatin remodelers, and DNA methylation-related proteins expressed in GN. The VC has active macromolecular metabolism and mRNA processing, while GC has active nucleic acid metabolism and translation. Moreover, the members of unfolded protein response (UPR) and programmed cell death accumulate in VN, and DNA damage repair is active in GN. Differences in the stress response of DAPs in VN vs. GN were also found. This study provides a further understanding of pollen cell differentiation mechanisms and also a sound basis for future studies of the molecular mechanisms behind cell fate differentiation.

Behavior of filamentous temperature-sensitive Z2 (FtsZ2) in the male gametophyte during sexual reproduction processes of flowering plants

Filamentous temperature-sensitive Z (FtsZ) is a critical division protein in bacteria that functions in forming a Z-ring structure to constrict the cell. Since the establishment of the plastid by endosymbiosis of a cyanobacterium into a eukaryotic cell, division via Z-ring formation has been conserved in the plastids of flowering plants. The FtsZ gene was transferred from the cyanobacterial ancestor of plastids to the eukaryotic nuclear genome during evolution, and flowering plants evolved two FtsZ homologs, FtsZ1 and FtsZ2, which are involved in chloroplast division through distinct molecular functions. Regarding the behaviors of FtsZ in nonphotosynthetic cells, the plastid localization of FtsZ1 proteins in the cytoplasm of microspores and pollen vegetative cells but not in generative cells or sperm cells has been reported. On the other hand, the significant accumulation of FtsZ2 transcripts in generative cells has been reported. However, the synthesis of FtsZ2 in the male gamete has not been investigated. Additionally, FtsZ2 behavior has not been analyzed in pollen, a nonphotosynthetic male tissue. Here, we report FtsZ2 protein behaviors in the male gamete by analyzing the localization patterns of GFP-fused protein at various pollen developmental stages and in gametes during the fertilization process. Our results showed that FtsZ2 localization coincided with that of plastids. FtsZ2 protein in male gametes was almost absent, despite the presence of the transcripts. Moreover, transmission of paternal FtsZ2 transcripts to the zygote and endosperm was not observed.

Dynamic changes in the starch-sugar interconversion within plant source and sink tissues promote a better abiotic stress response

Starch is a significant store of sugars, and the starch-sugar interconversion in source and sink tissues plays a profound physiological role in all plants. In this review, we discuss how changes in starch metabolism can facilitate adaptive changes in source-sink carbon allocation for protection against environmental stresses. The stress-related roles of starch are described, and published mechanisms by which starch metabolism responds to short- or long-term water deficit, salinity, or extreme temperatures are discussed. Numerous examples of starch metabolism as a stress response are also provided, focusing on studies where carbohydrates and cognate enzymes were assayed in source, sink, or both. We develop a model that integrates these findings with the theoretical and known roles of sugars and starch in various species, tissues, and developmental stages. In this model, localized starch degradation into sugars is vital to the plant cold stress response, with the sugars produced providing osmoprotection. In contrast, high starch accumulation is prominent under salinity stress, and is associated with higher assimilate allocation from source to sink. Our model explains how starch-sugar interconversion can be a convergent point for regulating carbon use in stress tolerance at the whole-plant level.

Regulation of pollen lipid body biogenesis by MAP kinases and downstream WRKY transcription factors in Arabidopsis

Signaling pathways that control the activities in non-photosynthetic plastids, important sites of plant metabolism, are largely unknown. Previously, we demonstrated that WRKY2 and WRKY34 transcription factors play an essential role in pollen development downstream of mitogen-activated protein kinase 3 (MPK3) and MPK6 in Arabidopsis. Here, we report that GLUCOSE-6-PHOSPHATE/PHOSPHATE TRANSLOCATOR 1 (GPT1) is a key target gene of WRKY2/WRKY34. GPT1 transports glucose-6-phosphate (Glc6P) into plastids for starch and/or fatty acid biosynthesis depending on the plant species. Loss of function of WRKY2/WRKY34 results in reduced GPT1 expression, and concomitantly, reduced accumulation of lipid bodies in mature pollen, which leads to compromised pollen viability, germination, pollen tube growth, and male transmission in Arabidopsis. Pollen-specific overexpression of GPT1 rescues the pollen defects of wrky2 wrky34 double mutant. Furthermore, gain-of-function activation of MPK3/MPK6 enhances GPT1 expression; whereas GPT1 expression is reduced in mkk4 mkk5 double mutant. Together, this study revealed a cytoplasmic/nuclear signaling pathway capable of coordinating the metabolic activities in plastids. High-level expression of GPT1 at late stages of pollen development drives Glc6P from cytosol into plastids, where Glc6P is used for fatty acid biosynthesis, an important step of lipid body biogenesis. The accumulation of lipid bodies during pollen maturation is essential to pollen fitness and successful reproduction.

Plastids are important sites of plant metabolism including fatty acid and starch biosynthesis. At present, how the activities in the plastids are coordinated with those in the cytoplasm and the signaling pathway(s) involved are largely unknown. Previously, we demonstrated that WRKY2 and WRKY34 transcription factors play an essential role in pollen development downstream of mitogen-activated protein kinase 3 (MPK3) and MPK6 in Arabidopsis. Here, we report that GLUCOSE-6-PHOSPHATE/PHOSPHATE TRANSLOCATOR 1 (GPT1) is a key target gene of WRKY2/WRKY34. GPT1 is localized on the membrane of plastids and transports glucose-6-phosphate (Glc6P) into plastids for starch and/or fatty acid biosynthesis depending on the plant species. Genetic analyses demonstrated that WRKY2/WRKY34 and their upstream MPK3/MPK6 are involved in regulating GPT1 expression, therefore, the accumulation of lipid bodies in mature pollen, which is critical to pollen viability, pollen germination, pollen tube growth, and male transmission in Arabidopsis. This study revealed a cytoplasmic/nuclear signaling pathway capable of coordinating the metabolic activities in plastids. High-level expression of GPT1 at late stages of pollen development drives Glc6P from cytosol into plastids, where Glc6P is used for fatty acid biosynthesis, an important step of lipid body biogenesis. The accumulation of lipid bodies during pollen maturation is essential to pollen fitness and successful reproduction.

Nucleus- and plastid-targeted annexin 5 promotes reproductive development in Arabidopsis and is essential for pollen and embryo formation

BACKGROUND

Pollen development is a strictly controlled post-meiotic process during which microspores differentiate into microgametophytes and profound structural and functional changes occur in organelles. Annexin 5 is a calcium- and lipid-binding protein that is highly expressed in pollen grains and regulates pollen development and physiology. To gain further insights into the role of ANN5 in Arabidopsis development, we performed detailed phenotypic characterization of Arabidopsis plants with modified ANN5 levels. In addition, interaction partners and subcellular localization of ANN5 were analyzed to investigate potential functions of ANN5 at cellular level.

RESULTS

Here, we report that RNAi-mediated suppression of ANN5 results in formation of smaller pollen grains, enhanced pollen lethality, and delayed pollen tube growth. ANN5 RNAi knockdown plants also displayed aberrant development during the transition from the vegetative to generative phase and during embryogenesis, reflected by delayed bolting time and reduced embryo size, respectively. At the subcellular level, ANN5 was delivered to the nucleus, nucleolus, and cytoplasm, and was frequently localized in plastid nucleoids, suggesting a likely role in interorganellar communication. Furthermore, ANN5-YFP co-immunoprecipitated with RABE1b, a putative GTPase, and interaction in planta was confirmed in plastidial nucleoids using FLIM-FRET analysis.

CONCLUSIONS

Our findings let us to propose that ANN5 influences basal cell homeostasis via modulation of plastid activity during pollen maturation. We hypothesize that the role of ANN5 is to orchestrate the plastidial and nuclear genome activities via protein-protein interactions however not only in maturing pollen but also during the transition from the vegetative to the generative growth and seed development.

Genome-wide analyses of late pollen-preferred genes conserved in various rice cultivars and functional identification of a gene involved in the key processes of late pollen development

BACKGROUND

Understanding late pollen development, including the maturation and pollination process, is a key component in maintaining crop yields. Transcriptome data obtained through microarray or RNA-seq technologies can provide useful insight into those developmental processes. Six series of microarray data from a public transcriptome database, the Gene Expression Omnibus of the National Center for Biotechnology Information, are related to anther and pollen development.

RESULTS

We performed a systematic and functional study across the rice genome of genes that are preferentially expressed in the late stages of pollen development, including maturation and germination. By comparing the transcriptomes of sporophytes and male gametes over time, we identified 627 late pollen-preferred genes that are conserved among japonica and indica rice cultivars. Functional classification analysis with a MapMan tool kit revealed a significant association between cell wall organization/metabolism and mature pollen grains. Comparative analysis of rice and Arabidopsis demonstrated that genes involved in cell wall modifications and the metabolism of major carbohydrates are unique to rice. We used the GUS reporter system to monitor the expression of eight of those genes. In addition, we evaluated the significance of our candidate genes, using T-DNA insertional mutant population and the CRISPR/Cas9 system. Mutants from T-DNA insertion and CRISPR/Cas9 systems of a rice gene encoding glycerophosphoryl diester phosphodiesterase are defective in their male gamete transfer.

CONCLUSION

Through the global analyses of the late pollen-preferred genes from rice, we found several biological features of these genes. First, biological process related to cell wall organization and modification is over-represented in these genes to support rapid tube growth. Second, comparative analysis of late pollen preferred genes between rice and Arabidopsis provide a significant insight on the evolutional disparateness in cell wall biogenesis and storage reserves of pollen. In addition, these candidates might be useful targets for future examinations of late pollen development, and will be a valuable resource for accelerating the understanding of molecular mechanisms for pollen maturation and germination processes in rice.

T-6b allocates more assimilation product for oil synthesis and less for polysaccharide synthesis during the seed development of Arabidopsis thaliana

BACKGROUND

As an Agrobacterium tumefaciens T-DNA oncogene, T-6b induces the development of tumors and the enation syndrome in vegetative tissues of transgenic plants. Most of these effects are related to increases in soluble sugar contents. To verify the potential roles of T-6b in the distribution of carbon in developing seeds, not in vegetative tissues, we fused an endosperm-specific promoter to the T-6b gene for expression in transgenic Arabidopsis thaliana plants.

RESULTS

The expression of T-6b in reproductive organs did not induce the development of the enation syndrome, and moreover, promoted endosperm expansion, which increased the total seed biomass by more than 10%. Additionally, T-6b also increased oil content in mature seeds by more than 10% accompanied with the decrease of starch and mucilage content at the same time.

CONCLUSIONS

T-6b enhances seed biomass and helps oil biosynthesis but not polysaccharides in reproductive organs without disturbing vegetative growth and development. Our findings suggest T-6b may be very useful for increasing oil production in biodiesel plants.

Importance of organellar proteins, protein translocation and vesicle transport routes for pollen development and function

Protein translocation. Cellular homeostasis strongly depends on proper distribution of proteins within cells and insertion of membrane proteins into the destined membranes. The latter is mediated by organellar protein translocation and the complex vesicle transport system. Considering the importance of protein transport machineries in general it is foreseen that these processes are essential for pollen function and development. However, the information available in this context is very scarce because of the current focus on deciphering the fundamental principles of protein transport at the molecular level. Here we review the significance of protein transport machineries for pollen development on the basis of pollen-specific organellar proteins as well as of genetic studies utilizing mutants of known organellar proteins. In many cases these mutants exhibit morphological alterations highlighting the requirement of efficient protein transport and translocation in pollen. Furthermore, expression patterns of genes coding for translocon subunits and vesicle transport factors in Arabidopsis thaliana are summarized. We conclude that with the exception of the translocation systems in plastids-the composition and significance of the individual transport systems are equally important in pollen as in other cell types. Apparently for plastids only a minimal translocon, composed of only few subunits, exists in the envelope membranes during maturation of pollen. However, only one of the various transport systems known from thylakoids seems to be required for the function of the "simple thylakoid system" existing in pollen plastids. In turn, the vesicle transport system is as complex as seen for other cell types as it is essential, e.g., for pollen tube formation.

Ultramicroscopic examination of mature massulae of Habenaria arinaria (Orchidaceae)

The mature massula of H. arinaria was examined by means of transmission electron microscopy, with the aim to understand the nature of cohesion between grains, the accumulation of pollen storage reserves, and the behavior of the nucleus of the vegetative cell in this composite type of pollen. The massula was a union of a large number of polygonal pollen grains that were tightly linked together. The exine within the massula were highly simplified, consisting of a single layer of nexine-2, lacking tectum, bacula, and nexine-1, while all the four layers comprised the exine on the massula surface. The two layers of nexine-2 of adjacent grains fused into a seamless whole. Undoubtedly the fusion of the nexine-2 was the mechanism by which the grains of the massula were linked together. No starch grains, lipid bodies, or storage proteins were present in the mature massula, and so the composite pollen of this species belonged to a novel type with regard to storage reserves. The vegetative nucleus was not lobed and revealed a huge amount of highly condensed chromatin, indicating a quiescent status. The condensed status of the vegetative nuclei in this composite type of pollen system is in striking contrast to the highly decondensed status reported in the free type of pollen grains.

An endoplasmic reticulum magnesium transporter is essential for pollen development in Arabidopsis

Magnesium is one of the essential macro-elements for plant growth and development, participated in photosynthesis and various metabolic processes. The Mg-transport abilities of the AtMGT (Magnesium Transporter) genes were identified in bacteria or yeast mutant system. In our previous studies, both the AtMGT5 and AtMGT9 were found essential for pollen development in Arabidopsis. Here we report another AtMGT member, AtMGT4, which was localized to the endoplasmic reticulum, was essential for pollen development as well. AtMGT4 expressed notably in pollen grains from bicellular pollen stage to mature pollen stage. A T-DNA insertional mutant of the gene, named mgt4-1, showed pollen abortive phenotype, thus we could not get any homozygous mutant from progenies of self-crossed +/mgt4-1 plants. Meanwhile, nearly half of pollens in AtMGT4-RNAi transgenic lines were sterile, consistent with the phenotype of +/mgt4-1 mutant. Transgenic plants expressing AtMGT4 in the mgt4-1 background could recover the pollen fertility to the wild type. Together, our findings demonstrated that the disruption of AtMGT4 in Arabidopsis could cause a defect of pollen development. The visible pollen abortion appeared at bicellular pollen stage in +/mgt4-1.

Pollen tube growth: where does the energy come from?

This review focuses on the energy metabolism during pollen maturation and tube growth and updates current knowledge. Pollen tube growth is essential for male reproductive success and extremely fast. Therefore, pollen development and tube growth are high energy-demanding processes. During the last years, various publications (including research papers and reviews) emphasize the importance of mitochondrial respiration and fermentation during male gametogenesis and pollen tube elongation. These pathways obviously contribute to satisfy the high energy demand, and there are many studies which suggest that respiration and fermentation are the only pathways to generate the needed energy. Here, we review data which show for the first time that in addition plastidial glycolysis and the balancing of the ATP/NAD(P)H ratio (by malate valves and NAD(+) biosynthesis) contribute to satisfy the energy demand during pollen development. Although the importance of energy generation by plastids was discounted during the last years (possibly due to the controversial opinion about their existence in pollen grains and pollen tubes), the available data underline their prime role during pollen maturation and tube growth.

Redundant function of two Arabidopsis COPII components, AtSec24B and AtSec24C, is essential for male and female gametogenesis

Anterograde vesicle transport from the endoplasmic reticulum to the Golgi apparatus is the start of protein transport through the secretory pathway, in which the transport is mediated by coat protein complex II (COPII)-coated vesicles. Therefore, most proteins synthesized on the endoplasmic reticulum are loaded as cargo into COPII vesicles. The COPII is composed of the small GTPase Sar1 and two types of protein complexes (Sec23/24 and Sec13/31). Of these five COPII components, Sec24 is thought to recognize cargo that is incorporated into COPII vesicles by directly interacting with the cargo. The Arabidopsis genome encodes three types of Sec24 homologs (AtSec24A, AtSec24B, and AtSec24C). The subcellular dynamics and function of AtSec24A have been characterized. The intracellular distributions and functions of other AtSec24 proteins are not known, and the functional differences among the three AtSec24s remain unclear. Here, we found that all three AtSec24s were expressed in similar parts of the plant body and showed the same subcellular localization pattern. AtSec24B knockout plant, but not AtSec24C knockdown plant, showed mild male sterility with reduction of pollen germination. Significant decrease of AtSec24B and AtSec24C expression affected male and female gametogenesis in Arabidopsis thaliana. Our results suggested that the redundant function of AtSec24B and AtSec24C is crucial for the development of plant reproductive cells. We propose that the COPII transport is involved in male and female gametogenesis in planta.

Conservation of two lineages of peroxisomal (Type I) 3-ketoacyl-CoA thiolases in land plants, specialization of the genes in Brassicaceae, and characterization of their expression in Arabidopsis thaliana

Arabidopsis thaliana has three genes encoding type I 3-ketoacyl-CoA thiolases (KAT1, KAT2, and KAT5), one of which (KAT5) is alternatively transcribed to produce both peroxisomal and cytosolic proteins. To evaluate the potential importance of these four gene products, their evolutionary history in plants and their expression patterns in Arabidopsis were investigated. Land plants as a whole have gene lineages corresponding to KAT2 and KAT5, implying conservation of distinct functions for these two genes. By contrast, analysis of synteny shows that KAT1 arose by duplication of the KAT2 locus. KAT1 is found in the Brassicaceae family, including in the genera Arabidopsis, Capsella, Thellungiella (=Eutrema) and Brassica, but not in the more distantly related Caricaceae (order Brassicales), or other plants. Gene expression analysis using qRT-PCR and β-glucuronidase reporter genes showed strong expression of KAT2 during germination and in many plant tissues throughout the life cycle, consistent with its observed dominant function in fatty acid β-oxidation. KAT1 was expressed very weakly while KAT5 was most strongly expressed during flower development and in seedlings after germination. Isoform-specific qRT-PCR analysis and promoter β-glucuronidase reporters revealed that the two splicing variants of KAT5 have similar expression profiles. Alternative splicing of KAT5 to produce cytosolic and peroxisomal proteins is specific to and ubiquitous in the Brassicaceae, and possibly had an earlier origin in the order Brassicales. This implies that an additional function for KAT5 arose between 43 and 115 mybp. We speculate that this KAT5 mutation was recruited for a cytosolic function in secondary metabolism.

Starch metabolism in Arabidopsis

Starch is the major non-structural carbohydrate in plants. It serves as an important store of carbon that fuels plant metabolism and growth when they are unable to photosynthesise. This storage can be in leaves and other green tissues, where it is degraded during the night, or in heterotrophic tissues such as roots, seeds and tubers, where it is stored over longer time periods. Arabidopsis accumulates starch in many of its tissues, but mostly in its leaves during the day. It has proven to be a powerful genetic system for discovering how starch is synthesised and degraded, and new proteins and processes have been discovered. Such work has major significance for our starch crops, whose yield and quality could be improved by the application of this knowledge. Research into Arabidopsis starch metabolism has begun to reveal how its daily turnover is integrated into the rest of metabolism and adapted to the environmental conditions. Furthermore, Arabidopsis mutant lines deficient in starch metabolism have been employed as tools to study other biological processes ranging from sugar sensing to gravitropism and flowering time control. This review gives a detailed account of the use of Arabidopsis to study starch metabolism. It describes the major discoveries made and presents an overview of our understanding today, together with some as-yet unresolved questions.

Mutations defective in ribonucleotide reductase activity interfere with pollen plastid DNA degradation mediated by DPD1 exonuclease

Organellar DNAs in mitochondria and plastids are present in multiple copies and make up a substantial proportion of total cellular DNA despite their limited genetic capacity. We recently demonstrated that organellar DNA degradation occurs during pollen maturation, mediated by the Mg(2+) -dependent organelle exonuclease DPD1. To further understand organellar DNA degradation, we characterized a distinct mutant (dpd2). In contrast to the dpd1 mutant, which retains both plastid and mitochondrial DNAs, dpd2 showed specific accumulation of plastid DNAs. Multiple abnormalities in vegetative and reproductive tissues of dpd2 were also detected. DPD2 encodes the large subunit of ribonucleotide reductase, an enzyme that functions at the rate-limiting step of de novo nucleotide biosynthesis. We demonstrated that the defects in ribonucleotide reductase indirectly compromise the activity of DPD1 nuclease in plastids, thus supporting a different regulation of organellar DNA degradation in pollen. Several lines of evidence provided here reinforce our previous conclusion that the DPD1 exonuclease plays a central role in organellar DNA degradation, functioning in DNA salvage rather than maternal inheritance during pollen development.

Visualization of plastid movement in the pollen tube of Arabidopsis thaliana

Organelle dynamics in the plant male gametophyte has received attention for its importance in pollen tube growth and cytoplasmic inheritance. We recently revealed the dynamic behaviors of plastids in living Arabidopsis pollen grains and tubes, using an inherent promoter-driven FtsZ1-green fluorescent protein (GFP) fusion. Here, we further monitored the movement of pollen tube plastids with an actin1 promoter-driven, stroma-targeted yellow fluorescent protein (YFP). In elongating pollen tubes, most plastids localized to the tube shank, where they displayed either retarded and unsteady motion, or fast, directional, and long-distance movement along the tube polarity. Efficient plastid tracking further revealed a population of tip-forwarding plastids that undergo a fluctuating motion(s) before traveling backwards. The behavior of YFP-labeled plastids in pollen basically resembled that of FtsZ1-GFP-labeled plastids, thus validating the use of FtsZ1-GFP for simultaneous visualization of the stroma and the plastid-dividing FtsZ ring.

Pollen vacuoles and their significance

Vacuoles of several types can be observed in pollen throughout its development. Their physiological significance reflects the complexity of the biological process leading to functional pollen grains. Vacuolisation always occurs during pollen development but when ripe pollen is shed the extensive translucent vacuoles present in the vegetative parts in previous stages are absent. Vacuole functions vary according to developmental stage but in ripe pollen they are mainly storage sites for reserves. Vacuoles cause pollen to increase in size by water accumulation and therefore confer some degree of resistance to water stress. Modalities of vacuolisation occur in pollen in the same manner as in other tissues. In most cases, autophagic vacuoles degrade organelles, as in the microspore after meiosis, and can be regarded as cytoplasm clean-up following the transition from the diploid sporophytic to the haploid gametophytic state. This also occurs in the generative cell but not in sperm cells. Finally, vacuoles have a function when microspores are used for pollen embryogenesis in biotechnology being targets for stress induction and afterwards contributing to cytoplasmic rearrangement in competent microspores.

Functional characterization of Arabidopsis thaliana isopropylmalate dehydrogenases reveals their important roles in gametophyte development

• Isopropylmalate dehydrogenases (IPMDHs) catalyze the oxidative decarboxylation of 3-isopropylmalate (3-IPM) in leucine biosynthesis in microorganisms. The Arabidopsis thaliana genome contains three putative IPMDH genes. • IPMDH2 and IPMDH3 proteins exhibited significantly higher activity toward 3-IPM than IPMDH1, which is indicative of a pivotal role in leucine biosynthesis. Single mutants of IPMDH2 or IPMDH3 lacked a discernible phenotype. Genetic analysis showed that ipmdh2 ipmdh3 was lethal in male gametophytes and had reduced transmission through female gametophytes. The aborted pollen grains were small, abnormal in cellular structure, and arrested in germination. In addition, half of the double mutant embryo sacs exhibited slowed development. • The IPMDH2/ipmdh2 ipmdh3/ipmdh3 genotype exhibited abnormal vegetative phenotypes, suggesting haplo-insufficiency of IPMDH2 in the ipmdh3 background. This mutant and a triple mutant containing one allele of IPMDH2 or IPMDH3 had decreased leucine biosynthetic enzyme activities and lower free leucine concentrations. The latter mutant showed changes in glucosinolate profiles different from those in the ipmdh1 mutant. • The results demonstrate that IPMDH2 and IPMDH3 primarily function in leucine biosynthesis, are essential for pollen development and are needed for proper embryo sac development.

Loss of cytosolic phosphoglucomutase compromises gametophyte development in Arabidopsis

Cytosolic phosphoglucomutase (cPGM) interconverts glucose-6-phosphate and glucose-1-phosphate and is a key enzyme of central metabolism. In this study, we show that Arabidopsis (Arabidopsis thaliana) has two cPGM genes (PGM2 and PGM3) encoding proteins with high sequence similarity and redundant functions. Whereas pgm2 and pgm3 single mutants were undistinguishable from the wild type, loss of both PGM2 and PGM3 severely impaired male and female gametophyte function. Double mutant pollen completed development but failed to germinate. Double mutant ovules also developed normally, but approximately half remained unfertilized 2 d after pollination. We attribute these phenotypes to an inability to effectively distribute carbohydrate from imported or stored substrates (e.g. sucrose) into the major biosynthetic (e.g. cell wall biosynthesis) and respiratory pathways (e.g. glycolysis and the oxidative pentose phosphate pathway). Disturbing these pathways is expected to have dramatic consequences for germinating pollen grains, which have high metabolic and biosynthetic activities. We propose that residual cPGM mRNA or protein derived from the diploid mother plant is sufficient to enable double mutant female gametophytes to attain maturity and for some to be fertilized. Mature plants possessing a single cPGM allele had a major reduction in cPGM activity. However, photosynthetic metabolism and growth were normal, suggesting that under standard laboratory conditions cPGM activity provided from one wild-type allele is sufficient to mediate the photosynthetic and respiratory fluxes in leaves.

Dynamic morphologies of pollen plastids visualised by vegetative-specific FtsZ1-GFP in Arabidopsis thaliana

The behaviour and multiplication of pollen plastids have remained elusive despite their crucial involvement in cytoplasmic inheritance. Here, we present live images of plastids in pollen grains and growing tubes from transgenic Arabidopsis thaliana lines expressing stroma-localised FtsZ1-green-fluorescent protein fusion in a vegetative cell-specific manner. Vegetative cells in mature pollen contained a morphologically heterogeneous population of round to ellipsoidal plastids, whilst those in late-developing (maturing) pollen included plastids that could have one or two constriction sites. Furthermore, plastids in pollen tubes exhibited remarkable tubulation, stromule (stroma-filled tubule) extension, and back-and-forth movement along the direction of tube growth. Plastid division, which involves the FtsZ1 ring, was rarely observed in mature pollen grains.

DGAT1 and PDAT1 acyltransferases have overlapping functions in Arabidopsis triacylglycerol biosynthesis and are essential for normal pollen and seed development

Triacylglycerol (TAG) biosynthesis is a principal metabolic pathway in most organisms, and TAG is the major form of carbon storage in many plant seeds. Acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) is the only acyltransferase enzyme that has been confirmed to contribute to TAG biosynthesis in Arabidopsis thaliana seeds. However, dgat1 null mutants display only a 20 to 40% decrease in seed oil content. To determine whether other enzymes contribute to TAG synthesis, candidate genes were expressed in TAG-deficient yeast, candidate mutants were crossed with the dgat1-1 mutant, and target genes were suppressed by RNA interference (RNAi). An in vivo role for phospholipid:diacylglycerol acyltransferase 1 (PDAT1; At5g13640) in TAG synthesis was revealed in this study. After failing to obtain double homozygous plants from crossing dgat1-1 and pdat1-2, further investigation showed that the dgat1-1 pdat1-2 double mutation resulted in sterile pollen that lacked visible oil bodies. RNAi silencing of PDAT1 in a dgat1-1 background or DGAT1 in pdat1-1 background resulted in 70 to 80% decreases in oil content per seed and in disruptions of embryo development. These results establish in vivo involvement of PDAT1 in TAG biosynthesis, rule out major contributions by other candidate enzymes, and indicate that PDAT1 and DGAT1 have overlapping functions that are essential for normal pollen and seed development of Arabidopsis.

Interaction of lipid bodies with other cell organelles in the maturing pollen of Magnolia x soulangeana (Magnoliaceae)

The pollen grain maturation in Magnolia x soulangeana was studied ultrastructurally and cytochemically using both the light and transmission electron microscope. Emphasis was given on the storage lipid bodies of the vegetative cell (VC) and their interaction with other cell organelles. Stereological analysis of electron micrographs was performed to evaluate the variation in volume density (V(V)), surface density, and surface-to-volume ratio (S/V) of various cell organelles during pollen maturation. The size and numerical density of the lipid bodies, and their frequency of association with other cell organelles, were also determined. It was noted that during pollen ontogeny and maturation, the lipid bodies changed their pattern of distribution in the VC cytoplasm, which may be a good marker for the succeeding stages of pollen development. Also, the size, osmiophily, and V(V) of the lipid bodies were progressively reduced during pollen maturation whereas the S/V was significantly increased. This seems to indicate that the lipid bodies are mobilized in part during this period of pollen maturation. In particular, the intermediate and mature pollen showed a high percentage of lipid bodies establishing a physical contact with either glyoxysomes, either protein storage vacuoles, or small vesicles presumably originated from dictyosomes. This physical contact was found in both the chemically fixed and rapid freeze-fixed pollen indicating that it is neither artifactual nor casual. On the basis of this intimate association with other cell organelles and the morphometric analysis performed, we suggest that the mobilization of lipid bodies is likely mediated not only by glyoxysomes but also by other catabolic pathways involving the interaction of lipid bodies with either protein storage vacuoles or small Golgi vesicles.

Visualization of plastids in pollen grains: involvement of FtsZ1 in pollen plastid division

Visualizing organelles in living cells is a powerful method to analyze their intrinsic mechanisms. Easy observation of chlorophyll facilitates the study of the underlying mechanisms in chloroplasts, but not in other plastid types. Here, we constructed a transgenic plant enabling visualization of plastids in pollen grains. Combination of a plastid-targeted fluorescent protein with a pollen-specific promoter allowed us to observe the precise number, size and morphology of plastids in pollen grains of the wild type and the ftsZ1 mutant, whose responsible gene plays a central role in chloroplast division. The transgenic material presented in this work is useful for studying the division mechanism of pollen plastids.

The legwd mutant uncovers the role of starch phosphorylation in pollen development and germination in tomato

Starches extracted from most plant species are phosphorylated. alpha-Glucan water dikinase (GWD) is a key enzyme that controls the phosphate content of starch. In the absence of its activity starch degradation is impaired, leading to a starch excess phenotype in Arabidopsis and in potato leaves, and to reduced cold sweetening in potato tubers. Here, we characterized a transposon insertion (legwd::Ds) in the tomato GWD (LeGWD) gene that caused male gametophytic lethality. The mutant pollen had a starch excess phenotype that was associated with a reduction in pollen germination. SEM and TEM analyses indicated mild shrinking of the pollen grains and the accumulation of large starch granules inside the plastids. The level of soluble sugars was reduced by 1.8-fold in mutant pollen grains. Overall, the transmission of the mutant allele was only 0.4% in the male, whereas it was normal in the female. Additional mutant alleles, obtained through transposon excision, showed the same phenotypes as legwd::Ds. Moreover, pollen germination could be restored, and the starch excess phenotype could be abolished in lines expressing the potato GWD homolog (StGWD) under a pollen-specific promoter. In these lines, where fertility was restored, homozygous plants for legwd::Ds were isolated, and showed the starch excess phenotype in the leaves. Overall, our results demonstrate the importance of starch phosphorylation and breakdown for pollen germination, and open up the prospect for analyzing the role of starch metabolism in leaves and fruits.

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.

Development and organization of the central vacuole of Acetabularia acetabulum

* Here we analyzed the shape of the central vacuole of Acetabularia acetabulum by visualizing its development during diplophase (from juvenility through reproduction) and haplophase (from meiosis through mating). * Light microscopy and whole-organism applications of a pH-sensitive dye, neutral red, were used to visualize the anatomy of the central vacuole. We studied connectivity within the thallus by locally applying dye to morphologically distinct regions (rhizoid, stalk, apex, hairs) and observing dye movements. * In vegetative thalli most of the rhizoid, stalk and young hairs stained with dye. In reproductive structures (caps, gametangia) dye also stained the majority of the interiors. When applied to small areas, dye moved at different rates through each region of the thallus (e.g. within the stalk). Dye moved from younger hairs, but not from older hairs, into the stalk. Errors in incorporation of central vacuole into gametangia occurred at <10(-5). * These data indicate that the central vacuole of A. acetabulum is a ramified polar organelle with, potentially, a gel-like sap that actively remodels its morphology during development.

Geminating pollen has tubular vacuoles, displays highly dynamic vacuole biogenesis, and requires VACUOLESS1 for proper function

Vacuoles perform multiple functions in plants, and VCL1 (VACUOLESS1) is essential for biogenesis with loss of expression in the vcl1 mutant leading to lethality. Vacuole biogenesis plays a prominent role in gametophytes, yet is poorly understood. Given the importance of VCL1, we asked if it contributes to vacuole biogenesis during pollen germination. To address this question, it was essential to first understand the dynamics of vacuoles. A tonoplast marker, delta-TIP::GFP, under a pollen-specific promoter permitted the examination of vacuole morphology in germinating pollen of Arabidopsis. Our results demonstrate that germination involves a complex, yet definable, progression of vacuole biogenesis. Pollen vacuoles are extremely dynamic with remarkable features such as elongated (tubular) vacuoles and highly mobile cytoplasmic invaginations. Surprisingly, vcl1 did not adversely impact vacuole morphology in pollen germinated in vitro. To focus further on VCL1 in pollen, reciprocal backcrosses demonstrated reduced transmission of vcl1 through male gametophytes, indicating that vcl1 was expressive after germination. Interestingly, vcl1 affected the fertility of female gametophytes that undergo similarly complex vacuole biogenesis. Our results indicate that vcl1 is lethal in the sporophyte but is not fully expressive in the gametophytes. They also point to the complexity of pollen vacuoles and suggest that the mechanism of vacuole biogenesis in pollen may differ from that in other plant tissues.

Arabidopsis type B monogalactosyldiacylglycerol synthase genes are expressed during pollen tube growth and induced by phosphate starvation

The galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) constitute the major glycolipids of the thylakoid membranes in chloroplasts. In Arabidopsis, the formation of MGDG is catalyzed by a family of three MGDG synthases, which are encoded by two types of genes, namely type A (atMGD1) and type B (atMGD2 and atMGD3). Although the roles of the type A enzyme have been intensively investigated in several plants, little is known about the contribution of type B enzymes to MGDG synthesis in planta. From our previous analyses, unique expression profiles of the three MGDG synthase genes were revealed in various organs and developmental stages. To characterize the expression profiles in more detail, we performed histochemical analysis of these genes using beta-glucuronidase (GUS) assays in Arabidopsis. The expression of atMGD1::GUS was detected highly in all green tissues, whereas the expression of atMGD2::GUS and atMGD3::GUS was observed only in restricted parts, such as leaf tips. In addition, intense staining was detected in pollen grains of all transformants. We also detected GUS activity in the pollen tubes of atMGD2::GUS and atMGD3::GUS transformants grown in wild-type stigmas but not in atMGD1::GUS, suggesting that type B MGDG synthases may have roles during pollen germination and pollen tube growth. GUS analysis also revealed that expression of atMGD2 and atMGD3, but not atMGD1, are strongly induced during phosphate starvation, particularly in roots. Because only DGDG accumulates in roots during phosphate deprivation, type B MGDG synthases may be acting primarily to supply MGDG as a precursor for DGDG synthesis.

Behavior of Vacuoles during Microspore and Pollen Development in Arabidopsis thaliana

Using the cryo-fixation/freeze-substitution method, we studied the ultrastructural changes and behavior of vacuoles and related organelles (rER and Golgi bodies) during microspore and pollen development, and pollen maturation of Arabidopsis thaliana. In young microspores forming exine (pollen outer cell wall), vacuoles looked like those of somatic cells. In microspores during the formation of intine (inner cell wall), a large vacuole appeared which was made by fusion of pre-existing vacuoles and probably absorption of solutions. In the young pollen grain after the first mitosis, a large vacuole was divided into small vacuoles. The manner of division was not by binary fission and centripetally, but by the invagination of tonoplasts from one side to the opposite side of a vacuole. After the second mitosis, somatic type vacuoles disappeared. In mature pollen grains just before germination, membrane-bound structures containing fine fibrillar substances (MBFs) appeared. The MBFs were considered to be storage vacuoles. In pollen grains from flowers in bloom, MBFs changed to lysosomal structures with acid phosphatases (lytic vacuole). They gradually increased in number and volume, and decomposed the cytoplasm. The autolysis of pollen grains is the first finding in this study, which may contribute to the loss of ability of pollen germination after anthesis.

The dynamin-like protein ADL1C is essential for plasma membrane maintenance during pollen maturation

Dynamin-related GTPases regulate a wide variety of dynamic membrane processes in eukaryotes. Here, we investigated the function of ADL1C, a member of the Arabidopsis 68 kDa dynamin-like protein family. Analysis of heterozygous adl1C-1 indicates that the mutation specifically affects post-meiotic male gametogenesis. Fifty percent of the mature pollen from heterozygous adl1C-1 androecia are shriveled and fail to germinate in vitro. During microspore maturation, adl1C-1 pollen grains display defects in the plasma membrane and intine morphology, suggesting that ADL1C is essential for the formation and maintenance of the pollen cell surface and viability during desiccation. Consistent with a role in cell-surface dynamics, immunofluorescence microscopy indicates that ADL1C is localized to the cell plate of dividing somatic cells and to the tip of expanding root hairs. We propose that ADL1C functions in plasma membrane dynamics, and we discuss the role of the ADL1 family in plant growth and development.

Novel patterns of ectopic cell plate growth and lipid body distribution in the Arabidopsis gemini pollen1 mutant

The nature of aberrant gametophytic cell divisions and altered pollen cell fate in the gemini pollen1 (gem1) mutant was investigated through ultrastructural analysis. The earliest noticeable defect in gem1 was the appearance of extended membrane profiles at the early bicellular stage. These were replaced by ectopic internal walls, which divided the cytoplasm into twin or multiple cell compartments. Complete or partial internal walls were callosic with highly complex profiles, indicating failed guidance or deregulated cell plate growth. Extended membrane profiles and delayed callose synthesis at division sites further suggested a novel pattern of cell plate assembly in gem1. Multiple cell compartments in gem1 adopted vegetative cell fate with regard to lipid body distribution. In the wild type, lipid bodies appear specifically in the vegetative cell, whereas in gem1, lipid bodies accumulated in all cytoplasmic compartments. Our results support the hypothesis that altered pollen cell fate in gem1 results from abnormal inheritance of cell fate determinants as a result of disturbed cytokinesis.