SARATOV, RUSSIA
Saratov State University In vivo and in vitro endospermogenesis in parthenogenetical maize lines --Alatortseva, TA, Tyrnov, VS Morphogenesis in vitro is known to depend significantly on genotypical, epigenetical, karyotypical and other peculiarities of explant. The specificity of the female gametophyte structure, consisting of cells with different functions and ploidy, has to be taken into account for in vitro culture of ovaries, both fertilized and unfertilized. The ploidy of embryo sac elements can change significantly. For example, two haploid polar nuclei form a central nucleus with ploidy 2n after fusion; after its fertilisation the initial endosperm nucleus has 3n ploidy.

Endosperm development is of interest for: a) its trophic, morphogenetical and regulatory functions in relation to the embryo; b) its role in parthenogenesis; c) the possibility of production from apomictic endosperm haploid cells and tissues or homozygous diploid ones; d) its positive or negative influence on embryogenesis and regeneration in vitro.

We investigated parthenogenetical line AT-1, described earlier (Tyrnov, MNL 71:73-74, 1997). Unfertilized ovaries, 1 to 17 days after appearance of silks, were used as explants.

MS medium modified by sucrose, vitamins and 2,4-D was added. Culture was in the dark at a temperature of about 25 C. A portion of ovaries was used for cytoembryological analysis before plating onto medium and during the culture period (after 3, 7, 14, 21 days). This allowed us to follow the course of embryological and morphogenetical processes. In vivo as well as in vitro, we regularly observed the following events: 1) Egg's division and development of embryo from bicellular to globular. A number of multicellular embryos appear with increasing ovary age. Different ovaries of the same age can include proembryos of different developmental stages. Embryo development to the globular stage can proceed without endosperm formation. 2) Division only of the central cell. 3) Division of both the central cell and egg.

In the last two cases the first division of the central cell is preceded by fusion of polar nuclei. This situation was observed in 7-8 days ovaries. The in vivo and in vitro ovary ages are determined as days after silk appearance.

17-18 days after silk appearance, endosperm is multinuclear. Usually karyokinesis is not followed by cytokinesis (cellularization), and the endosperm appears cenocyte. In some cases, fragments of nuclear or cellular endosperm were observed. If endospermogenesis and embryogenesis proceed together in embryo sac, the last one begins earlier and passes ahead of embryo development. At the moment of fusion of polar nuclei, the embryo can contain from 2 to 100 cells.

The analysis of stages of ovaries, cultured in vitro, demonstrated that the presence of both processes ( endospermogenesis and embryogenesis) leads to their mutual inhibition. Embryos stop development at the globular stage. In addition, at the same stage, many embryo-like structures begin to form on the surface. They do not reach the stage of mature embryoid and are not able to form plantlets when plated on regeneration medium.

The development of embryos continues in the absence of endospermal tissue. They can form plantlets or produce by gemmagenesis a great number of embryoids, able to produce plantlets.

Thus, it can be suggested, that in unfertilized ovaries in vitro there is a special type of interrelationship between haploid embryo and endosperm.

A negative influence of endosperm on embryo can be conditioned by some circumstances. We suggest the following possibilities: 1) Endosperm, by enveloping the embryo, is either a barrier, or competitor for nutrient substances. 2) Anomalous characters of endosperm, by its ploidy and variable developmental timing lead to anomalies of embryogenesis. 3) If the endosperm realizes a regulatory role by means of physiologically active substances, its interrelation with endogenic factors from its surroundings can give undesirable effects for embryogenesis.
 
 

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