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Ovary
Culture - Culture of unfertilized ovaries to obtain haploid plants from egg cell ar other haplaid cells of the embryo sac is called ovary culture, and the process is termed as gynogenesis. The first report of gynogenesis was by San Naom in 1976 in case of barley.
Subsequently, success as been obtained with many species, e.g., wheat, rice, maize, tobacco, petunia, Gerbera, sunflower, sugarbeet, rubber, etc. About 0.2-6% of the cultured ovaries show gynogenesis and one ar two, rarely up to 8, plantlets originate from each ovary.
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The rate of success varies considerably with species and is markedly influenced by explant genotype so that same cultivars do not respond at all. In rice japonica gene types are far mare responsive than indica cultivars.In most cases, the optimum stage far ovary culture is the nearly mature embryo sac, but in rice ovaries at free nuclear embryo sac stage are the mast responsive.
Generally, culture of whale flowers, ovary and avulse attached to placenta respond better, but in Gerbera and sunflower isolated avulse show better response. Clad pretreatment (24-48 hr at 4°C in sunflower and 24 hr at 7°C in rice) of the inflorescence before ovary culture enhances gynogenesis.
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GRs are crucial in gynogenesis, and at higher levels they may induce callusing of somatic tissues and even suppress gynogenesis. GR requirement seems to depend an species. Far example, in sunflower GR-free medium is the best, and even a law level of MCP A (2-methyl-4-chlaraphenaxyacetic acid) induces somatic calli and SEs.
But in rice, 0.125-0.5 mg/l MCP A is optimum far gynogenesis. Sucrose level also appears to be critical; in sunflower 12% sucrose leads to gynogenic embryo production, while at lower levels somatic calli and somatic embryos were also produced.
Ovaries/avulse are generally cultured in light, but at least in same species, e.g., sunflower and rice, dark incubation favours gynogenesis and minimizes somatic callusing; 111 rice light may lead to degeneration of gynogenic proembryos.
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Generally, gynogenesis has two or many stages and each stage may have distinct requirements. In rice, two stages, viz, induction and regeneration are recognized. During induction, ovaries are floated on a liquid medium having low auxin and kept in dark, while for regeneration they are transferred on to an agar medium with higher auxin concentration and incubated in light.
Haploid plants generally originate from egg cell in most of the species (in vitro parthenogenesis) but in some species, e.g., rice, they arise chiefly from synergids; in at least Allium tuberosum even antipodal produce haploid plants (in vitro apogamic).
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As in anther culture, gynogenesis may occur either via embryogenesis or through plantlet regeneration from callus. In rice MCP A generally leads to a small amount of protocorm like callus formation from which shoots and roots regenerate, while picloram promotes embryo regeneration.
In contrast, sugarbeet usually shows embryo development, while in sunflower embryos regenerate following a callus phase. In general, regeneration from a callus phase appears, at least for the present, to be easier than direct embryogenesis.
Ovary culture has mainly two limitations:
(i) so far it has been successful only in less than two dozen species, and
(ii) the frequency of responding ovaries (1-5%) and the number of plantlets/ovary (1-2) is quite low. Therefore, anther culture is preferred over ovary culture; only in those cases where anther culture fails, e.g., sugarbeet, and for male sterile lines, ovary culture assumes significance.
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