Gametes and Fertilization: Maize as a Model System for Experimental Embryogenesis in Flowering Plants

Genetic and molecular mechanisms underlying the parthenocarpic fruit mutation in tomato

Parthenocarpy allows fruit set independently of fertilization. In parthenocarpic-prone tomato genotypes, fruit set can be achieved under pollen-limiting environmental conditions and in sterile mutants. Parthenocarpy is also regarded as a quality-related trait, when seedlessness is associated with positive fruit quality aspects. Among the different sources of genetic parthenocarpy described in tomato, the parthenocarpic fruit (pat) mutation is of particular interest because of its strong expressivity, high fruit set, and enhanced fruit quality. The complexity of the pat "syndrome" associates a strong competence for parthenocarpy with a complex floral phenotype involving stamen and ovule developmental aberrations. To understand the genetic basis of the phenotype, we mapped the pat locus within a 0.19-cM window of Chr3, comprising nine coding loci. A non-tolerated missense mutation found in the 14th exon of Solyc03g120910, the tomato ortholog of the Arabidopsis HD-Zip III transcription factor HB15 (SlHB15), cosegregated with the pat phenotype. The role of SlHB15 in tomato reproductive development was supported by its expression in developing ovules. The link between pat and SlHB15 was validated by complementation and knock out experiments by co-suppression and CRISPR/Cas9 approaches. Comparing the phenotypes of pat and those of Arabidopsis HB15 mutants, we argued that the gene plays similar functions in species with fleshy and dry fruits, supporting a conserved mechanism of fruit set regulation in plants.

Conservation Study of Imprinted Genes in Maize Triparental Heterozygotic Kernels

Genomic imprinting is a classic epigenetic phenomenon related to the uniparental expression of genes. Imprinting variability exists in seeds and can contribute to observed parent-of-origin effects on seed development. Here, we conducted allelic expression of the embryo and endosperm from four crosses at 11 days after pollination (DAP). First, the F1 progeny of B73(♀) × Mo17(♂) and the inducer line CAU5 were used as parents to obtain reciprocal crosses of BM-C/C-BM. Additionally, the F1 progeny of Mo17(♀) × B73(♂) and CAU5 were used as parents to obtain reciprocal crosses of MB-C/C-MB. In total, 192 and 181 imprinted genes were identified in the BM-C/C-BM and MB-C/C-MB crosses, respectively. Then, by comparing the allelic expression of these imprinted genes in the reciprocal crosses of B73 and CAU5 (BC/CB), fifty-one Mo17-added non-conserved genes were identified as exhibiting imprinting variability. Fifty-one B73-added non-conserved genes were also identified by comparing the allelic expression of imprinted genes identified in BM-C/C-BM, MB-C/C-MB and MC/CM crosses. Specific Gene Ontology (GO) terms were not enriched in B73-added/Mo17-added non-conserved genes. Interestingly, the imprinting status of these genes was less conserved across other species. The cis-element distribution, tissue expression and subcellular location were similar between the B73-added/Mo17-added conserved and B73-added/Mo17-added non-conserved imprinted genes. Finally, genotypic and phenotypic analysis of one non-conserved gene showed that the mutation and overexpression of this gene may affect embryo and kernel size, which indicates that these non-conserved genes may also play an important role in kernel development. The findings of this study will be helpful for elucidating the imprinting mechanism of genes involved in maize kernel development.

High Temporal-Resolution Transcriptome Landscape of Early Maize Seed Development

The early maize (Zea mays) seed undergoes several developmental stages after double fertilization to become fully differentiated within a short period of time, but the genetic control of this highly dynamic and complex developmental process remains largely unknown. Here, we report a high temporal-resolution investigation of transcriptomes using 31 samples collected at an interval of 4 or 6 h within the first six days of seed development. These time-course transcriptomes were clearly separated into four distinct groups corresponding to the stages of double fertilization, coenocyte formation, cellularization, and differentiation. A total of 22,790 expressed genes including 1415 transcription factors (TFs) were detected in early stages of maize seed development. In particular, 1093 genes including 110 TFs were specifically expressed in the seed and displayed high temporal specificity by expressing only in particular period of early seed development. There were 160, 22, 112, and 569 seed-specific genes predominantly expressed in the first 16 h after pollination, coenocyte formation, cellularization, and differentiation stage, respectively. In addition, network analysis predicted 31,256 interactions among 1317 TFs and 14,540 genes. The high temporal-resolution transcriptome atlas reported here provides an important resource for future functional study to unravel the genetic control of seed development.

Sequential gene activation and gene imprinting during early embryo development in maize

Gene imprinting is a widely observed epigenetic phenomenon in maize endosperm; however, whether it also occurs in the maize embryo remains controversial. Here, we used high-throughput RNA sequencing on laser capture microdissected and manually dissected maize embryos from reciprocal crosses between inbred lines B73 and Mo17 at six time points (3-13 days after pollination, DAP) to analyze allelic gene expression patterns. Co-expression analysis revealed sequential gene activation during maize embryo development. Gene imprinting was observed in maize embryos, and a greater number of imprinted genes were identified at early embryo stages. Sixty-four strongly imprinted genes were identified (at the threshold of 9:1) on manually dissected embryos 5-13 DAP (more imprinted genes at 5 DAP). Forty-one strongly imprinted genes were identified from laser capture microdissected embryos at 3 and 5 DAP (more imprinted genes at 3 DAP). Furthermore, of the 56 genes that were completely imprinted (at the threshold of 99:1), 36 were not previously identified as imprinted genes in endosperm or embryos. In situ hybridization demonstrated that most of the imprinted genes were expressed abundantly in maize embryonic tissue. Our results shed lights on early maize embryo development and provide evidence to support that gene imprinting occurs in maize embryos.

Germline Development and Fertilization Mechanisms in Maize

Maize is the most important agricultural crop used for food, feed, and biofuel as well as a raw material for industrial products such as packaging material. To increase yield and to overcome hybridization barriers, studies of maize gamete development, the pollen tube journey, and fertilization mechanisms were initiated more than a century ago. In this review, we summarize and discuss our current understanding of the regulatory components for germline development including sporogenesis and gametogenesis, the progamic phase of pollen germination and pollen tube growth and guidance, as well as fertilization mechanisms consisting of pollen tube arrival and reception, sperm cell release, fusion with the female gametes, and egg cell activation. Mechanisms of asexual seed development are not considered here. While only a few molecular players involved in these processes have been described to date and the underlying mechanisms are far from being understood, maize now represents a spearhead of reproductive research for all grass species. Recent development of essentially improved transformation and gene-editing systems may boost research in this area in the near future.

Two Membrane-Anchored Aspartic Proteases Contribute to Pollen and Ovule Development

Aspartic proteases are a class of proteolytic enzymes with conserved aspartate residues, which are implicated in protein processing, maturation, and degradation. Compared with yeast and animals, plants possess a larger aspartic protease family. However, little is known about most of these enzymes. Here, we characterized two Arabidopsis (Arabidopsis thaliana) putative glycosylphosphatidylinositol (GPI)-anchored aspartic protease genes, A36 and A39, which are highly expressed in pollen and pollen tubes. a36 and a36 a39 mutants display significantly reduced pollen activity. Transmission electron microscopy and terminal-deoxynucleotidyl transferase-mediated nick end labeling assays further revealed that the unviable pollen in a36 a39 may undergo unanticipated apoptosis-like programmed cell death. The degeneration of female gametes also occurred in a36 a39 Aniline Blue staining, scanning electron microscopy, and semi in vitro guidance assays indicated that the micropylar guidance of pollen tubes is significantly compromised in a36 a39 A36 and A39 that were fused with green fluorescent protein are localized to the plasma membrane and display punctate cytosolic localization and colocalize with the GPI-anchored protein COBRA-LIKE10. Furthermore, in a36 a39, the abundance of highly methylesterified homogalacturonans and xyloglucans was increased significantly in the apical pollen tube wall. These results indicate that A36 and A39, two putative GPI-anchored aspartic proteases, play important roles in plant reproduction in Arabidopsis.

Dynamic transcriptome landscape of maize embryo and endosperm development

Maize (Zea mays) is an excellent cereal model for research on seed development because of its relatively large size for both embryo and endosperm. Despite the importance of seed in agriculture, the genome-wide transcriptome pattern throughout seed development has not been well characterized. Using high-throughput RNA sequencing, we developed a spatiotemporal transcriptome atlas of B73 maize seed development based on 53 samples from fertilization to maturity for embryo, endosperm, and whole seed tissues. A total of 26,105 genes were found to be involved in programming seed development, including 1,614 transcription factors. Global comparisons of gene expression highlighted the fundamental transcriptomic reprogramming and the phases of development. Coexpression analysis provided further insight into the dynamic reprogramming of the transcriptome by revealing functional transitions during maturation. Combined with the published nonseed high-throughput RNA sequencing data, we identified 91 transcription factors and 1,167 other seed-specific genes, which should help elucidate key mechanisms and regulatory networks that underlie seed development. In addition, correlation of gene expression with the pattern of DNA methylation revealed that hypomethylation of the gene body region should be an important factor for the expressional activation of seed-specific genes, especially for extremely highly expressed genes such as zeins. This study provides a valuable resource for understanding the genetic control of seed development of monocotyledon plants.

Ribosomal RNA of Hyacinthus orientalis L. female gametophyte cells before and after fertilization

The nucleolar activity of Hyacinthus orientalis L. embryo sac cells was investigated. The distributions of nascent pre-rRNA (ITS1), 26S rRNA and of the 5S rRNA and U3 snoRNA were determined using fluorescence in situ hybridization (FISH). Our results indicated the different rRNA metabolism of the H. orientalis female gametophyte cells before and after fertilization. In the target cells for the male gamete, i.e., the egg cell and the central cell whose activity is silenced in the mature embryo sac (Pięciński et al. in Sex Plant Reprod 21:247-257, 2008; Niedojadło et al. in Planta doi: 10.1007/s00425-012-1599-9 , 2011), rRNA metabolism is directed at the accumulation of rRNPs in the cytoplasm and immature transcripts in the nucleolus. In both cells, fertilization initiates the maturation of the maternal pre-rRNA and the expression of zygotic rDNA. The resumption of rRNA transcription observed in the hyacinth zygote indicates that in plants, there is a different mechanism for the regulation of RNA Pol I activity than in animals. In synergids and antipodal cells, which have somatic functions, the nucleolar activity is correlated with the metabolic activity of these cells and changes in successive stages of embryo sac development.

Seed development and genomic imprinting in plants

Genomic imprinting refers to an epigenetic phenomenon where the activity of an allele depends on its parental origin. Imprinting at individual genes has only been described in mammals and seed plants. We will discuss the role imprinted genes play in seed development and compare the situation in plants with that in mammals. Interestingly, many imprinted genes appear to control cell proliferation and growth in both groups of organisms although imprinting in plants may also be involved in the cellular differentiation of the two pairs of gametes involved in double fertilization. DNA methylation plays some role in the control of parent-of-origin-specific expression in both mammals and plants. Thus, although imprinting evolved independently in mammals and plants, there are striking similarities at the phenotypic and possibly also mechanistic level.

Green sperm. Identification of male gamete promoters in Arabidopsis

Previously, in an effort to better understand the male contribution to fertilization, we completed a maize (Zea mays) sperm expressed sequence tag project. Here, we used this resource to identify promoters that would direct gene expression in sperm cells. We used reverse transcription-polymerase chain reaction to identify probable sperm-specific transcripts in maize and then identified their best sequence matches in the Arabidopsis (Arabidopsis thaliana) genome. We tested five different Arabidopsis promoters for cell specificity, using an enhanced green fluorescent protein reporter gene. In pollen, the AtGEX1 (At5g55490) promoter is active in the sperm cells and not in the progenitor generative cell or in the vegetative cell, but it is also active in ovules, roots, and guard cells. The AtGEX2 (At5g49150) promoter is active only in the sperm cells and in the progenitor generative cell, but not in the vegetative cell or in other tissues. A third promoter, AtVEX1 (At5g62850) [corrected] was active in the vegetative cell during the later stages of pollen development; the other promoters tested (At1g66770 and At1g73350) did not function in pollen. Comparisons among GEX1 and GEX2 homologs from maize, rice (Oryza sativa), Arabidopsis, and poplar (Populus trichocarpa) revealed a core binding site for Dof transcription factors. The AtGEX1 and AtGEX2 promoters will be useful for manipulating gene expression in sperm cells, for localization and functional analyses of sperm proteins, and for imaging of sperm dynamics as they are transported in the pollen tube to the embryo sac.

Parthenocarpic fruit development in tomato

Parthenocarpic fruit development is a very attractive trait for growers and consumers. In tomato, three main sources of facultative parthenocarpy, pat, pat-2, pat-3/pat-4, are known to have potential applications in agriculture. The parthenocarpic fruit development in these lines is triggered by a deregulation of the hormonal balance in some specific tissues. Auxins and gibberellins are considered as the key elements in parthenocarpic fruit development of those lines. An increased level of these hormones in the ovary can substitute for pollination and trigger fruit development. This has opened up genetic engineering approaches for parthenocarpy that have given promising results, both in quality and quantity of seedless fruit production.

The Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR KINASE 1 gene is expressed in developing ovules and embryos and enhances embryogenic competence in culture

We report here the isolation of the Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE 1 (AtSERK1) gene and we demonstrate its role during establishment of somatic embryogenesis in culture. The AtSERK1 gene is highly expressed during embryogenic cell formation in culture and during early embryogenesis. The AtSERK1 gene is first expressed in planta during megasporogenesis in the nucellus [corrected] of developing ovules, in the functional megaspore, and in all cells of the embryo sac up to fertilization. After fertilization, AtSERK1 expression is seen in all cells of the developing embryo until the heart stage. After this stage, AtSERK1 expression is no longer detectable in the embryo or in any part of the developing seed. Low expression is detected in adult vascular tissue. Ectopic expression of the full-length AtSERK1 cDNA under the control of the cauliflower mosaic virus 35S promoter did not result in any altered plant phenotype. However, seedlings that overexpressed the AtSERK1 mRNA exhibited a 3- to 4-fold increase in efficiency for initiation of somatic embryogenesis. Thus, an increased AtSERK1 level is sufficient to confer embryogenic competence in culture.

The Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR KINASE 1 gene is expressed in developing ovules and embryos and enhances embryogenic competence in culture

We report here the isolation of the Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE 1 (AtSERK1) gene and we demonstrate its role during establishment of somatic embryogenesis in culture. The AtSERK1 gene is highly expressed during embryogenic cell formation in culture and during early embryogenesis. The AtSERK1 gene is first expressed in planta during megasporogenesis in the nucellus [corrected] of developing ovules, in the functional megaspore, and in all cells of the embryo sac up to fertilization. After fertilization, AtSERK1 expression is seen in all cells of the developing embryo until the heart stage. After this stage, AtSERK1 expression is no longer detectable in the embryo or in any part of the developing seed. Low expression is detected in adult vascular tissue. Ectopic expression of the full-length AtSERK1 cDNA under the control of the cauliflower mosaic virus 35S promoter did not result in any altered plant phenotype. However, seedlings that overexpressed the AtSERK1 mRNA exhibited a 3- to 4-fold increase in efficiency for initiation of somatic embryogenesis. Thus, an increased AtSERK1 level is sufficient to confer embryogenic competence in culture.

The Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR KINASE 1 gene is expressed in developing ovules and embryos and enhances embryogenic competence in culture

We report here the isolation of the Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE 1 (AtSERK1) gene and we demonstrate its role during establishment of somatic embryogenesis in culture. The AtSERK1 gene is highly expressed during embryogenic cell formation in culture and during early embryogenesis. The AtSERK1 gene is first expressed in planta during megasporogenesis in the nucellus [corrected] of developing ovules, in the functional megaspore, and in all cells of the embryo sac up to fertilization. After fertilization, AtSERK1 expression is seen in all cells of the developing embryo until the heart stage. After this stage, AtSERK1 expression is no longer detectable in the embryo or in any part of the developing seed. Low expression is detected in adult vascular tissue. Ectopic expression of the full-length AtSERK1 cDNA under the control of the cauliflower mosaic virus 35S promoter did not result in any altered plant phenotype. However, seedlings that overexpressed the AtSERK1 mRNA exhibited a 3- to 4-fold increase in efficiency for initiation of somatic embryogenesis. Thus, an increased AtSERK1 level is sufficient to confer embryogenic competence in culture.

The Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR KINASE 1 gene is expressed in developing ovules and embryos and enhances embryogenic competence in culture

We report here the isolation of the Arabidopsis SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE 1 (AtSERK1) gene and we demonstrate its role during establishment of somatic embryogenesis in culture. The AtSERK1 gene is highly expressed during embryogenic cell formation in culture and during early embryogenesis. The AtSERK1 gene is first expressed in planta during megasporogenesis in the nucellus [corrected] of developing ovules, in the functional megaspore, and in all cells of the embryo sac up to fertilization. After fertilization, AtSERK1 expression is seen in all cells of the developing embryo until the heart stage. After this stage, AtSERK1 expression is no longer detectable in the embryo or in any part of the developing seed. Low expression is detected in adult vascular tissue. Ectopic expression of the full-length AtSERK1 cDNA under the control of the cauliflower mosaic virus 35S promoter did not result in any altered plant phenotype. However, seedlings that overexpressed the AtSERK1 mRNA exhibited a 3- to 4-fold increase in efficiency for initiation of somatic embryogenesis. Thus, an increased AtSERK1 level is sufficient to confer embryogenic competence in culture.

Genetic analysis of female gametophyte development and function

The female gametophyte is an absolutely essential structure for angiosperm reproduction. It produces the egg cell and central cell (which give rise to the embryo and endosperm, respectively) and mediates several reproductive processes including pollen tube guidance, fertilization, the induction of seed development, and perhaps also maternal control of embryo development. Although much has been learned about these processes at the cytological level, specific molecules mediating and controlling megagametogenesis and female gametophyte function have not been identified. A genetic approach to the identification of such molecules has been initiated in Arabidopsis and maize. Although genetic analyses are still in their infancy, mutations affecting female gametophyte function and specific steps of megagametogenesis have already been identified. Large-scale genetic screens aimed at identifying mutants affecting every step of megagametogenesis and female gametophyte function are in progress; the characterization of genes identified in these screens should go a long way toward defining the molecules that are required for female gametophyte development and function.

Sperm Identification in Maize by Fluorescence in Situ Hybridization

The two sperm cells of common origin within the pollen tube of flowering plants are each involved in a fertilization event. It has long been recognized that preferential fusion of one sperm with the egg can occur in B chromosome-containing lines of maize. If the second pollen mitosis begins with a single B chromosome, nondisjunction will result in one sperm possessing two B chromosomes and the other containing no B chromosomes. The B chromosome-containing sperm most often fertilizes the egg, whereas the sperm nucleus with no B chromosomes fuses with the polar nuclei. Despite the obvious advantages of being able to recognize and then track, separate, and analyze one sperm type from the other, it has not been possible because of the lack of sufficient detectable differences between the two types of sperms. In this study, we used a B chromosome-specific DNA sequence (pZmBs) and in situ hybridization to identify and track the B chromosome-containing sperm cell within mature pollen and pollen tubes. Our results are consistent with conclusions from previous genetic studies related to B chromosome behavior during pollen formation. Within pollen tubes, the position in which the B chromosome-containing sperm travels (leading or trailing) in relation to the sperm cell lacking B chromosomes appears to be random.

The mac1 gene: controlling the commitment to the meiotic pathway in maize

The switch from the vegetative to the reproductive pathway of development in flowering plants requires the commitment of the subepidermal cells of the ovules and anthers to enter the meiotic pathway. These cells, the hypodermal cells, either directly or indirectly form the archesporial cells that, in turn, differentiate into the megasporocytes and microsporocytes. We have isolated a recessive pleiotropic mutation that we have termed multiple archesporial cells1 (mac1) and located it to the short arm of chromosome 10. Its cytological phenotype suggests that this locus plays an important role in the switch of the hypodermal cells from the vegetative to the meiotic (sporogenous) pathway in maize ovules. During normal ovule development in maize, only a single hypodermal cell develops into an archesporial cell and this differentiates into the single megasporocyte. In mac1 mutant ovules several hypodermal cells develop into archesporial cells, and the resulting megasporocytes undergo a normal meiosis. More than one megaspore survives in the tetrad and more than one embryo sac is formed in each ovule. Ears on mutant plants show partial sterility resulting from abnormalities in megaspore differentiation and embryo sac formation. The sporophytic expression of this gene is therefore also important for normal female gametophyte development.

Embryogenesis and plant regeneration from maize zygotes by in vitro culture of fertilized embryo sacs

Fertilized embryo sacs of Zea mays were isolated and cultured In vitro. Each explant contained one zygote and 2-4 endosperm nuclei which formed, respectively, embryo and cellular endosperm during the culture. In our double-layer/two-phase culture system, NBM medium (Mòl et al. 1993) supplemented with 0.1-1.0 mg·l(-1) zeatin and 12 % sucrose showed the best results. On this medium, embryos were isolated from 37-54 % of two-week-old explants. They were similar to maize embryos developing in vivo. We have shown that development of stage-2 embryos (according to Abbe and Stein 1954) with two leaf primordia and normally differentiated provascular tissue is possible from the maize zygote in an in vitro culture system. Some embryos with enlarged and deformed scutellum or whole apical parts were also found. Up to 62 % of the embryos germinating on a simple medium regenerated into mature and fertile plants; i.e. 23 % of explants yielded plants. This unproved culture method results in better embryo differentiation and 14-fold increase of regeneration frequency than previous protocol.

Detection of the apomictic mode of reproduction in maize-Tripsacum hybrids using maize RFLP markers

Polyploid plants in the genus Tripsacum, a wild relative of maize, reproduce through gametophytic apomixis of the diplosporous type, an asexual mode of reproduction through seed. Moving gene(s) responsible for the apomictic trait into crop plants would open new areas in plant breeding and agriculture. Efforts to transfer apomixis from Tripsacum into maize at CIMMYT resulted in numerou intergeneric F1 hybrids obtained from various Tripsacum species. A bulk-segregant analysis was carried out to identify molecular markers linked to diplospory in T. dactyloides. This was possible because of numerous genome similarities among related species in the Andropogoneae. On the basis of maize RFLP probes, three restriction fragments co-segregating with diplospory were identified in one maize-Tripsacum dactyloides F1 population that segregated 1∶1 for the mode of reproduction. The markers were also found to be linked in the maize RFLP map, on the distal end of the long arm of chromosome 6. These results support a simple inheritance of diplospory in Tripsacum. Manipulation of the mode of reproduction in maize-Tripsacum backcross generations, and implications for the transfer of apomixis into maize, are discussed.

Characterisation of isolated egg cells, in vitro fusion products and zygotes of Zea mays L. using the technique of image analysis and confocal laser scanning microscopy

Changes in membrane Ca2+, calcium receptor protein calmodulin, endoplasmic reticulum (ER), mitochondria and cellulose in unfixed, living, isolated egg cells and fusion products of pairs of one egg and one sperm cell of Zea mays L. have been investigated using chlorotetracycline, fluphenazine, immunocytochemical techniques, 3,3'-dihexyloxa-carbocyanine iodide (DiOC6(3)) and calcofluor white in conjunction with computer-controlled video image analysis. In addition, confocal laser scanning microscopy has been used in conjunction with ethidium bromide to detect the nature and location of the sperm cell nuclear chromatin before and after karyogamy. Digitised video images of chlorotetracycline (CTC) fluorescence reveal that egg cells contain high levels of membrane Ca2+ in organelles present around the nucleus while the cytosolic signal is relatively low. Intense CTC fluorescence is invariably present just below the plasma membrane of egg cells and a certain degree of regionalised distribution of Ca2+ in cytoplasm is also discernible. Similarly, the fluphenazine (FPZ)-detectable calmodulin (CaM) and that localised immunocytochemically using monoclonal anti-CaM antibodies reveal high levels of CaM in the vicinity of the nucleus in egg cells. Only a few ER profiles and mitochondria could be visualised in the egg cell and no calcofluor fluorescence could be detected. Following in vitro fertilisation of single isolated eggs substantial changes in the Ca2+ levels occur which include an increase in the membrane Ca2+ of the fusion product, particularly in the cytosol and around the nucleus. Unlike in the eggs the fine CTC fluorescence signal below the plasma membrane is not detectable in the fusion products.(ABSTRACT TRUNCATED AT 250 WORDS)

Signal molecules involved in plant embryogenesis

In plant embryogenesis, inductive interactions mediated by diffusable signal molecules are most likely of great importance. Evidence has been presented that at late globular stages in plant embryogenesis, perturbation of the polar auxin transport results in abberrant embryo morphology. Rhizobium lipooligosaccharides or Nod factors are a newly discovered class of bacterial molecules that are able to trigger initial steps in root nodule development in legumes. Part of the activity of Nod factors may be directed towards alteration of endogenous plant growth regulator balance. The same bacterial Nod factors promoted the formation of globular embryos in the carrot cell line ts11. Whether there exist plant analogues of the Nod factors and whether these molecules are active as a more universal control system perhaps designed to initiate and or mediate gradients in auxin and cytokinin remains to be determined.

Regeneration of Fertile Barley Plants from Mechanically Isolated Protoplasts of the Fertilized Egg Cell

A simple procedure is described for the mechanical isolation of protoplasts of unfertilized and fertilized barley egg cells from dissected ovules. Viable protoplasts were isolated from ~75% of the dissected ovules. Unfertilized protoplasts did not divide, whereas almost all fertilized protoplasts developed into microcalli. These degenerated when grown in medium only. When cocultivated with barley microspores undergoing microspore embryogenesis, the protoplasts of the fertilized egg cells developed into embryo-like structures that gave rise to fully fertile plants. On average, 75% of cocultivated protoplasts of fertilized egg cells developed into embryo-like structures. Fully fertile plants were regenerated from ~50% of the embryo-like structures. The isolation-regeneration techniques may be largely genotype independent, because similar frequencies were obtained in two different barley varieties with very different performance in anther and microspore culture. Protoplasts of unfertilized and fertilized eggs of wheat were isolated by the same procedure, and a fully fertile wheat plant was regenerated by cocultivation with barley microspores.

Isolation and microinjection of active sperm nuclei into egg cells and central cells of isolated maize embryo sacs

Artificial fertilisation was attempted in maize by microinjecting sperm nuclei into the egg cell or central cell of isolated embryo sacs. A protocol for isolation of nuclei from pollen grains was developed and a pure fraction of sperm nuclei was obtained after centrifugation on a Percoll gradient. The in vitro transcriptional activity of the nuclei was tested by incorporation of radioactive UTP into RNA. The level of labelled nucleotide incorporation increased and reached a maximum after between 30 and 40 min in the incubation medium. The embryo sacs were enzymatically isolated and their viability determined by observation of cytoplasmic streaming in the female cells. The embryo sacs were immobilised by embedding in low-melting-point agarose and a single male nucleus was injected with a bevelled microcapillary. The presence of the injected nucleus in the egg or central cell was demonstrated using a cytological approach. This paper presents an alternative method for studying the intimate processes of fertilisation in plants.