Pollen wall ontogeny involves the specification of three main architectural features: 1. generally uniform numbers of precisely positioned colpi; 2. patterned arrays of baculae; 3. exine stratification. In the majority of species, the position of the colpi is consistent and genetically determined. For example, in the lily tetrad, which has a two-by-two arrangement common to monocotyledons, the single aperture of the spore always develops at a specific position toward the perimeter of the cell. In the majority of dicotyledons, including Brassica and tobacco, the tetrad of microspores is arranged in a tetrahedron, and each spore forms the three radially symmetrical equatorial colpi. In these species, as first pointed out by Wodehouse (1935), this arrangement is determined by the positions of maximum contact with the other three spores within the tetrad.
How then is this most obvious architectural feature of the pollen grain determined? Significantly, primexine is not deposited in areas destined to become colpi or germinal apertures, which emphasises the importance of this matrix to sporopollenin polymerisation. In several species, including lily, the plasma membrane in the primexine-less areas of the future colpi is invariably associated with an underlying plate of endoplasmic reticulum - the colpal shield. Heslop-Harrison (1963a) proposed that the apposition of the colpal shield to the plasma-membrane blocks deposition of primexine, which in turn prevents sexine growth. Subsequently, experiments conducted by Dover (1972) in wheat implicated the meiotic cytoskeleton in the positioning of the single distal aperture, since the apertures form in positions close to the poles of the first meiotic division. Furthermore, complete disruption of the spindle by centrifugation prevented both division and colpal formation, whereas partial disruption usually resulted in colpus formation.
Sheldon and Dickinson (1986) suggested that the transduction mechanism by which spindle position influences colpus position operates via the spindle microtubule organising centre (MTOC) which forces cytoplasmic components, including smooth endoplasmic reticulum (SER) against the plasma membrane, thereby forming the colpal shield. Thus, at least in plants with a single distal aperture, the specification of aperture position is associated with the mode of microspore partitioning and displays symmetry related to that of the meiotic spindle. Although experimental data is lacking for tricolpate pollen grains, it is likely that the meiotic spindle plays a very similar role in aperture positioning.
Despite the clear diversity in pollen wall patterning, a basic feature shared by many species is the reticulate arrangement of baculae. This is very obvious in mature pollen of Lilium (Fig. 4A) and Brassica (Fig. 4B, right panel), where the basic pattern unit is very often the hexagon. Note that each line of baculae is common to two pattern elements, indicating a large degree of interdependency between elements, rather than an independent origin for each element. As shown in Fig. 4B (left panel), the positioning of the baculae into the final configuration is complete at
the tetrad stage. Tobacco pollen is tectate-perforate and therefore in SEMs the baculae are obscured from view by the tectum (Fig. 4C, right panel). Nevertheless, at the tetrad stage the arrangement of the baculae is quite similar to that of Brassica (Fig. 4B and 4C, left panels). This indicates that development of a tectate exine also involves the expression of pattern information. This is to be expected since, as stated earlier, the obviously patterned semitectate and intectate exine types are probably derivative of the fully tectate condition.
A particularly striking example of the hexagonal arrangement of baculae is provided by the immature pollen of Ipomea purpurea (Fig. 4D). In this species, the reticulate pattern of the tetrad stage microspore is remarkably consistent; of the 54 complete elements that make up the spore face depicted in Fig. 4D, 48 are regular hexagons. However, it is noteworthy that the pattern is not quite perfect since six of the constituent elements are pentagonal. Another interesting feature of Ipomea is that almost every three-way intersection (triple junction) between the polygonal elements carries a spine. These examples demonstrate that pattern specification involves the precise positioning of wall elements, principally baculae, and that the unit of pattern is very often a regular hexagon.
Several lines of evidence suggest that the genes responsible for determining the species-specific exine pattern are transcribed within the diploid nucleus of the pre-meiotic sporocyte, and that the pattern information is inherited by the microspores (Heslop-Harrison, 1963a, 1968«; Rogers & Harris, 1969). Unfortunately, since there is no evidence for the participation of any subcellular organelle in this process, the colpal-shield model cannot be extended to explain how this information is utilised to
Fig. 4. Reticulation in pollen walls, bubbles and basalt. A. Scanning electron micrograph (SEM) of Lilium pollen wall showing reticulate pattern formed by the baculae. The nexine I is visible through the lacunae. B. Reticulation in the semitectate exine wall of Brassica napus. Left panel: TEM of a glancing section through the surface of a tetrad-stage microspore. Right panel: SEM of a mature grain. C. Reticulation in the tectate-perforate pollen wall of Nicotiana tabacum. Left and Right panels: technical details as in 5B. D. Light micrograph of a tetrad-stage microspore of Ipomea purpurea stained with primuline to show the position of the young baculae (small dots) and spines (larger dots at triple junctions) (photograph, courtesy of L. Waterkeyn, Université Catholique de Louvain). E. Monolayer of soap bubbles floating over water. F. Basalt columns of the Giant's Causeway, Co. Antrim, Ireland. Magnification bar: 5 nm, A, B [right], C [right]; 28 nm, B [left], C [left]; 0.6 nm, D.
direct baculae positioning. However, some progress has been made toward formulating a satisfactory model (Sheldon & Dickinson, 1983, 1986; Dickinson & Sheldon, 1986). A series of elegant experiments involving the centrifugation of microsporocytesin in situ established that the reticulate patterning of the exine was prone to disruption as early as meiotic prophase, but as meiosis proceeded was only modified within the new cross walls of the tetrad (Sheldon & Dickinson, 1983). This indicated that the agent(s) responsible for imposing pattern on the primexine must appear in the cytoplasm at the beginning of meiosis, where it is sensitive to centrifugation, and is then progressively inserted into the plasma membrane during meiosis. However, since the colpal shield subsequently modifies the reticulate pattern, the pattern inducing material must remain latent, and malleable to a certain extent, until after colpus specification. To date, the only component of the prophase cytoplasm that constitutes a good candidate for the determinant of pattern formation are so-called coated vesicles (CVs) (Sheldon & Dickinson, 1983). These small protein-coated vesicles are formed from prophase onwards from SER, and migrate to the cell surface where they fuse with the plasma membrane. The implication is that the CVs insert protein into the plasma membrane which somehow influences the position of the probaculae.
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