Pollen: Structure and Development

Michael Borg, University of Leicester, Leicester, UK
David Twell, University of Leicester, Leicester, UK

Published online: October 2011

DOI: 10.1002/9780470015902.a0002039.pub2

Pollen grains contain the highly reduced haploid microgametophyte generation in seed plants that establishes the male germline during sexual reproduction. Pollen protects the germline from environmental injury, promotes gamete dispersal and ensures delivery of a pair of sperm cells to the female gametes via the pollen tube. Microgametogenesis is initiated from haploid microspores that undergo one or more asymmetric mitotic divisions followed by differentiation to form a tube cell and a pair of sperm cells. Knowledge of the molecular mechanisms governing microgametogenesis has advanced rapidly through genetic analysis of mutants that disturb cellular patterning and gamete specification in flowering plants and through the application of genome-wide transcriptomic studies. Recent evidence also supports a diverse and significant role for functional small RNA pathways and epigenetic regulation during microgametogenesis. Together these advances are helping to establish models of the gene regulatory networks governing gametophyte development and gamete specification.

Key Concepts:

  • Pollen grains harbour the haploid microgametophytes of seed plants (spermatophytes) that deliver twin sperm cells to the female gametophyte via the pollen tube.
  • Pollen contains high levels of stored transcripts and protein for use during germination and pollen tube growth.
  • The complexity of haploid gene expression is reduced during pollen development and is accompanied by a relative increase in the expression of pollen-specific transcripts.
  • Sperm cells possess a reduced but extensive transcriptome that has a relatively high number of sperm cell-specific transcripts.
  • Formation of the male germline results from microtubule-dependent asymmetric division of a progenitor microspore.
  • Ubiquitin-mediated proteolysis is essential for germline cell cycle progression.
  • Male germline specification involves the coordination of cell cycle and differentiation by the germline-specific transcription factor DUO1.
  • Multiple small RNA pathways function in the microgametophyte to control gene expression, transposon activity and fertilisation.

Keywords: pollen; male gametophyte; microgametophyte; cell differentiation; vegetative cell; generative cell; sperm cells; asymmetric division; germline

Figure 1. (a) Schematic diagram of the structure of bicellular (left) and tricellular (right) pollen. (b) Nuclear morphology in bicellular tobacco pollen (left) and tricellular Arabidopsis pollen (right) treated with a DNA-specific stain and visualised by fluorescence microscopy. Generative and sperm cell nuclei are often elongated and possess highly compact chromatin with intense fluorescence. Vegetative nuclei are larger and irregular in shape with a more dispersed chromatin showing weaker fluorescence. Bar, 20 m.
Figure 2. Pollen wall structure. Surface structure of Arabidopsis thaliana pollen visualised by scanning electron microscopy. At low magnification (a) apertural furrows are visible as longitudinal folds in the exine. At high magnification (b) details of the reticulate exine patterning is visible. Schematic diagram of a cross-section through the complex wall layers in pollen with tectate and pilate exine patterning (c).
Figure 3. Schematic diagram of the morphological stages that occur during the development of tricellular pollen in Arabidopsis. During microsporogenesis the microsporocyte (or pollen mother cell), which is surrounded by a thick callose wall, undergoes the two nuclear divisions of meiosis to produce four haploid nuclei within a common cytoplasm. Cytokinesis is simultaneous, producing a tetrad of four haploid microspores in a tetrahedral arrangement. During microgametogenesis the microspores are released into the anther locule by dissolution of the callose wall, where they undergo two stereotypical mitotic divisions, pollen mitosis I and pollen mitosis II, to produce mature tricellular pollen grains.
Figure 4. Microtubule dynamics observed in live tobacco pollen from transgenic lines expressing a GFP-tubulin marker (Oh et al., 2010). Following pollen mitosis I, the engulfed generative cell elongates to form a spindle-like shape that is maintained by a cortical cage of bundled microtubules (a). As pollen matures, the generative cell forms a prominent cytoplasmic ‘tail-like’ projection and comes in direct contact with the vegetative cell nucleus to form the male germ unit (b). White arrowheads indicate the microtubule-rich tail that connects the generative cell cytoplasmic tail to the vegetative cell nucleus. GN, generative cell nucleus; VN, vegetative cell nucleus.
Figure 5. (a) Transcriptome profile of the number of expressed genes during pollen development overlayed with a line graph showing the proportion of pollen-specific genes. Although the overall number of genes expressed is reduced during development, there is a clear increase in the expression of genes belonging to the pollen-specific subset. (b) A line graph of representative gene clusters expressed in pollen highlighting the distinct early and late transcriptional programmes operating during development.
Figure 6. Division symmetry and pollen cell fate. Tobacco microspores normally divide asymmetrically at pollen mitosis I to produce a large vegetative cell and smaller generative cell (a). Vegetative cell fate can be monitored by the activity of a vegetative cell-specific marker (blue staining nucleus marked with ProLAT52:GUS/NIa; Eady et al., 1995) that is not active in the generative cell. When symmetrical division of the microspore is chemically induced by mitotic spindle displacement, both daughter cells adopt a vegetative cell fate and express vegetative-cell-specific genes (b). Thus, distinctive germ cell fate is strictly dependent on division asymmetry normally established at pollen mitosis I.
Figure 7. Model of regulatory events in male germ cell and vegetative cell fate. Three regulatory pathways are indicated. (1) Following asymmetric division of the microspore, the cyclin-dependent kinase (CDK) inhibitors KIP-related proteins KRP6 and KRP7 are present in both the germ cell and vegetative cell. Transient expression of F-box like 17 (FBL17) in the germ cell leads to KRP6 and KRP7 degradation allowing CDKA/CYCD to phosphorylate retinoblastoma protein (RBR). RBR-mediated repression of the E2F/DP pathway is relieved and allows progression of the germ cell through S-phase. (2) FBL17 is not expressed in the vegetative cell; thus CDKA inhibition by high levels of KRP6 and KRP7 may result in continued repression of the E2F/DP pathway by unphosphorylated RBR. Together, the RBR and FBL17 pathways prevent entry of the vegetative cell into the cell cycle. (3) Gamete specification begins shortly after germ cell formation, where the expression of DUO1 and DUO3 in the germ cell leads to the activation of an overlapping set of germline differentiation genes. Following S-phase, the DUO1-dependent activation of CYCB1;1 promotes entry into mitosis. In parallel, DUO3 also controls G2/M transition in the germ cell but involves a CYCB1;1-independent pathway. DUO1 and DUO3 therefore integrate germline differentiation with cell division, and the cooperation of their regulatory networks results in a pair of fully differentiated sperm cells. Black dashed arrows show promotion of the indicated process or component of progression to an activated state. Solid black arrows indicate progression of the cell cycle from S phase to G2/M and exit of the vegetative cell from the cell cycle to G0. Red dashed arrows indicate FBL17-mediated degradation of KRP6/7 by the proteasome. Reproduced with permission from Berger and Twell (2011) from Annual Reviews Inc.
Figure 8. Monitoring sperm cell differentiation in Arabidopsis pollen. Pictured is a group of wild-type and mutant duo1 pollen grains harbouring the sperm cell-specific GFP marker ProMGH3:H2B-GFP (Brownfield et al., 2009a), with corresponding images of nuclear DNA (a) and GFP expression (b). Arrows indicate the twin sperm cell nuclei of wild-type pollen (a), which show GFP expression (b). Arrowheads indicate the single germ cell nucleus of duo1 mutant pollen (a), which fail to express the GFP marker (b). This marker expression approach illustrates that the transcription factor DUO1 is required to coordinate cell division and differentiation in the male germline.
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    Twell D (2011) Male gametogenesis and germline specification in flowering plants. Sexual Plant Reproduction 24(2): 149–160.
    Yang WC, Shi DQ and Chen YH (2010) Female gametophyte development in flowering plants. Annual Review of Plant Physiology 61: 89–108.

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Article Title: Pollen: Structure and Development
 
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Borg, Michael, and Twell, David(Oct 2011) Pollen: Structure and Development. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002039.pub2]