Embryo devlopment in peas:

General development. In almost every plant, the first division of the zygote is transverse to the long axis of the cell and is often asymmetrical, producing a large basal cell and a smaller apical cell. The embryo proper is derived mainly from the apical cell. There is much variation in the extent of the contribution of the basal cell to the formation of the organogenic part of the embryo and usually it forms all or part of a suspensor. On the basis of this variation and also on the plane of the first division of the apical cell, plant embryo ontogenesis has been classified into six different types - Onograd, Asterad, Chenopodiad, Solanad, Caryophyllad and Piperad - based on the contribution that the basal cell makes towards the organogenic part of the embryo. The first five types follow Maheshwari's (1950) classification and the sixth was added by Johansen (1950) to account for those zygotes that divide longitudinally (Johri, 1984). In nearly all plants, the first division of the zygote is transverse to the long axis of the cell which produces the basal cell (often the larger) and an apical cell.

The Leguminosae (Papilionaceae) exhibit a wide range of developmental types in their embryos. Pea belongs to the sub-family, Faboideae (Papilionoideae). This family displays four of the six possible types - Onograd, Caryophyllad, Solanad and Asterad (Prakash, 1987) - pea (Pisum sativum L.) being of the Solanad type.

The suspensor of the pea is the most uniform and distinctive of all members of the tribe Vicieae (Johri et al., 1992; Lersten, 1983) having 4 multinucleate cells two of which extend considerably during early development to move the embryo away from the micropyle and into the bulk of the endosperm. The endosperm is of the nuclear type although Marinos (1970a) proposed that cellulosic walls or strands are present to act as an anchor for a time during the early development of the embryo.

The legume suspensor has provided the best evidence for a function of this organ in embryo nutrition. Structural specializations, in the form of wall ingrowths, that increase the surface area for absorption have been noted in the suspensor of pea for many years (see review by Gunning and Pate, 1974; Marinos, 1970a. These have been used as evidence that the cells of the suspensor help to move materials between embryo and seed coat and thus act as transfer cells (Gunning and Pate, 1974).

Pea cotyledons show no clear differentiation of tissue type into pallisade and mesophyll tissues (Smith and Flinn, 1967) which are often seen in those cotyledons that serve a photosynthetic function after germination. As such pea cotyledons serve only as storage organs, as in all members of the Vicieae. Pea cotyledons consist of three tissues: epidermis, storage parenchyma and provascular tissues. The epidermis is a relatively thin layer of cells derived from the protoderm, that are much smaller than those of the underlying parenchyma. Endoreduplication (endopolyploidy) occurs in the cotyledons of legumes and was present in those of all the Vicieae and about 50% of Papilionoideae examined by Smith (1981). In Pisum sativum there was a close correlation between cell and nuclear size, and DNA content (Smith, 1973; 1974).

Embryo development in pea. The process of embryo development in dicots is often divided into four stages: globular, heart-shaped, torpedo-shaped and cotyledonary. The embryology of pea has been studied by a number of workers (Cooper, G.O 1938; Cooper, D.C. 1938; Reeve, 1948; Souèges, 1948; Marinos, 1970a; 1970b). In pea, a torpedo-shaped embryo cannot be observed during embryo development because of the rapid cell division and expansion that occurs in the cotyledons after the heart-shaped stage and the delayed growth of the root axis. An alternative description system was proposed by Marinos (1970a) who divided pea embryo development into 25 stages using a number of criteria.

The first 13 stages covered flower development and fertilization, and the remainder, seed development. Within the last two stages the embryo matures, accumulates storage products, amplifies its DNA, and desiccates to produce a dry seed. Hence, embryo development in pea could simply be divided into two phases, organ formation and organ maturation (Wang and Hedley, 1993), most of cotyledon growth being confined to the latter phase.

As described by D.C. Cooper (1938), the basal cell of the pea embryo divides longitudinally to produce two suspensor cells, whereas the transversely-divided apical cell produces an apical embryo mother cell and a middle cell. The latter, in turn, goes through a longitudinal division to form the two-celled bulbous middle piece of the suspensor. Several cycles of nuclear division without cytokinesis occur in all the four cells of the suspensor. As a result, the two elongated basal cells contain 64 nuclei each and every middle cell possesses 32 nuclei (Cooper, D.C., 1938). The suspensor is a short-lived organ which is fully developed at the proembryo stage and subsequently degenerates. As mentioned above, it is believed the suspensor channels nutrients to the embryo until the heart-shape stage. Thereafter, transport from the mother plant to the embryo is via the endosperm and transfer cells (Marinos, 1970a).

Division of the apical embryonic mother cell produces an axially symmetrical globular embryo, but the protoderm in pea is not formed until the late globular stage, relatively late in comparison with cruciferous plants. In contrast, the shoot apex is initiated much earlier than in crucifers, and, as Reeve (1948) indicated, it occurs at about the same time as a distinct protoderm becomes established and well before the formation of the cotyledons.

Two tunica layers are present in the shoot apex, and within them occasional periclinal divisions can be observed (Reeve, 1948), by the time the cotyledonary lobes are evident. In contrast, the formation of the shoot apical dome can be delayed until the time of seed germination in the crucifer, Arabidopsis thaliana.

Establishment of bilateral symmetry, by the initiation of two cotyledons to form a heart-shape embryo, is often considered to be a landmark for the end of the proembryo stage.

One feature of pea that has not really been noted by previous workers is that it is bilaterally symmetrical symmetry with one plane of symmetry in the embryo. Furthermore, the cotyledons enlarge significantly prior to any appreciable elongation of the embryo axis; extension of the radicle below the bases of the cotyledons occurs only when the shoot apex has enlarged into a high and rounded dome.

Cotyledon cellular development in relation to storage product accumulation. The cotyledons of pea are storage organs that accumulate large amounts of starch and protein and a little lipid, for use in seed germination. As mentioned, the cotyledons comprise a storage parenchyma, epidermis and provascular traces. These traces do not develop into a functional vasculature until germination has been initiated and then the procambium tissue gives rise to mature xylem and phloem elements within 2 days (Smith and Flinn, 1967). As mentioned earlier, no specialization occurs in the organs with respect to photosynthesis. The cells of the parenchyma differ greatly in size throughout development and genotype differences can be detected (Wang and Hedley, 1993). There is a shift towards a population of large cells during development (Ambrose et al., 1987). Nuclear endoreduplication occurs in the cotyledonary parenchyma at the later stages of embryo development. The cells of developing cotyledons continue to duplicate DNA after the cell number has reached a plateau, and the cells of fully-grown cotyledons have DNA levels averaging between 32C and 64C (Scharpé and van Parijs, 1973; Corke et al., 1987). DNA endoreduplication can be detected in a few cells of very young cotyledons, if one looks at individual cells, and the number of cells with ploidy levels greater than the diploid level increases as the embryo grows (Corke et al., 1987).

The patterns of cell division and endoreduplication within the pea cotyledons can be related to the accumulation of storage products (Corke et al., 1987; 1990a; 1990b; Hauxwell et al., 1990; 1993; Yang et al, 1990; Liu et al., 1996). Storage protein accumulation appears to correlate with a cessation in cell division with the 'oldest' cells of the cotyledon (those that ceased proliferating first), being the first to show the presence of messenger RNAs for storage protein.

For additional information on embryo development in other plants click here.

 For the list of references used, click here

| Dr Kay Denyer | Dr Claire Domoney | Dr Stanislav Kopriva | Professor Anne Osbourn| Professor Alison Smith | Dr Trevor Wang