Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Apomixis Introductory article Article Contents Abed Chaudhury, Commonwealth Scientific and Research Organization (CSIRO) Plant Industry, Canberra, Australia . Introduction Anna Koltunow, CSIRO Horticulture Unit, Glen Osmond, Australia ES Dennis, CSIRO Plant Industry, Canberra, Australia WJ Peacock, CSIRO Plant Industry, Canberra, Australia . Types of Apomixis . Importance of Apomixis to Agriculture Apomixis is the formation of seeds by asexual processes. Apomixis comprises an ensemble of developmental processes that together alter the reproductive functions in the ovule of flowering plants, converting the sexual programme to an asexual one. The result of these alterations is that the apomictically derived embryo is not a product of male and female gamete fusion as occurs in sexual reproduction but is derived solely from cells in maternal ovule tissues. Introduction Apomixis can best be described in reference to the sexual process. During sexual reproduction, developmental steps occurring inside the ovule produce the female gametophyte (embryo sac) and following a double fertilization event give rise to embryo and endosperm structures. Steps in the formation of the most common type of embryo sac (polygonum-type) include the differentiation of the megaspore mother cell (mmc) from a vegetative nucellar cell, meiosis to convert the mmc into four spores, retention of one of these spores and the degradation of the other three. Finally, the elaboration of an embryo sac occurs after three mitotic divisions, nuclear migration and cellularization events to produce a seven-celled, embryo sac, containing eight nuclei. When the two sperm cells enter the embryo sac through the micropylar end, they selectively fuse with the egg and central cell nuclei to produce the embryo and endosperm, respectively. The embryo and endosperm compartments differ in the ratio of their maternal and paternal genomic content. In the embryo the ratio is one paternal genome to one maternal genome, while in the endosperm the ratio is one paternal to two maternal. Apomixis modifies the processes described above and produces a functional female gametophytic structure that precludes the sexual assortment of genes and recombination of genes associated with meiosis. In apomixis the double fertilization event does not occur and the embryo develops autonomously from the unreduced female gamete. In some apomicts the endosperm develops autonomously while in others (pseudogamous apomicts) fertilization of the central cell by a sperm cell may be required to produce a functional endosperm. Types of Apomixis Three types of apomixis are generally recognized – diplospory, apospory and adventitious embryony. These apomictic processes are depicted compared to sexual processes in the formation of a common polygonum-type embryo sac in Figure 1. Diplospory In diplospory, the unreduced embryo sac is derived from the megaspore mother cell either directly by mitotic division or by aborted meiotic events. Three major types of diplospory have been reported, named after the plants in which they occur, and they are the Taraxacum, Ixeris and Antennaria types. In the Taraxacum type, meiotic prophase is initiated but then the process is aborted resulting in two unreduced dyads one of which gives rise to the embryo sac by mitotic division. In the Ixeris type, two further mitotic divisions of the nuclei to give rise to an eight-nucleate embryo sac follow equational division following meiotic prophase. The Taraxacum and Ixeris types are known as meiotic diplospory because they involve modifications of meiosis. By contrast, in the Antennaria type, referred to as mitotic diplospory, the mmc does not initiate meiosis and directly divides three times to produce the unreduced embryo sac. Apospory In apospory, the nucellar cells that give rise to the apomictic embryo sac, termed aposporous initials, are distinct from the mmc. They are similar in appearance to the mmc and may differentiate close to the mmc and develop into an apomictic embryo sac. Once the apospor- ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 1 Apomixis the adjoining sexual embryo sac and subsequent endosperm formation is necessary to form viable seeds. The developing embryos closest to the embryo sac grow towards it, presumably to obtain nutrient and other developmental signals from the embryo sac. Adventitious embryo formation is rapid; often multiple embryo form and these can hinder development of the zygotic embryo. Importance of Apomixis to Agriculture Figure 1 Apomictic reproduction compared to sexual reproduction in flowering plants. In sexual reproduction the megaspore mother cell (mmc) undergoes meiosis, spore selection (ss), and mitosis to produce the common eight-nucleate polygonum-type embryo sac (esc). The embryo and endosperm compartments of the seed initiate development after double fertilization (df) and give rise to mature seed (ms). In the panel showing diplospory, the Antennaria type is depicted where the mmc undergoes mitosis to form the embryo sac and endosperm and embryo production are autonomous. In other diplosporous plants fertilization of the polar nuclei (pseudogamy) is necessary for endosperm production. In apospory, the initials (ai) differentiate near the mmc, which may or may not have undergone meiosis. Sexual and aposporous embryo sacs may coexist or the sexual embryo sac may degenerate during formation. The aposporous embryo sac may degenerate during formation. The aposporous embryo sac can differ in structure to that in sexual species, as shown by the four-nucleate Panicum-type (p) as illustrated. Seed formation may be autonomous or pseudogamous. Polyembryony (pe) may be evident if multiple embryo sacs form. In adventitious embryony, numerous adventitious initial cells (avi) differentiate next to an embryo sac derived by sexual processes. After double fertilization of the sexual embryo sac the numerous developing adventitious embryos gain access to the nutritive endosperm and seed contains multiple embryos. ous initial cells differentiate they immediately enter mitosis to produce an embryo sac. Some ovules can contain multiple embryo sacs and, depending on the species, the structure of the embryo sac may be quite different from that seen in the sexual process. The initiation of the aposporous embryo sac can occur together with a sexual one or it can displace or inhibit sexual embryo sac formation. Termination of the sexual process is most complete if the aposporous initial cells differentiate early, around the time of mmc meiosis, producing an unreduced apomictic embryo sac. Adventitious embryony In this process embryos initiate parthenogenetically outside of an embryo sac structure. Adventitious embryony is most commonly initiated later in ovule development from nucellar and integument tissues. In general, fertilization in 2 Apomixis is of great importance to agriculture primarily because of its potential to facilitate multiplication of F1 hybrid seeds. Hybrid seeds are preferred for use in modern agriculture mainly because they provide a yield advantage. Hybrid seed is usually produced from two inbred parental lines and outperforms the yield and vigour of either of the parental lines. Currently production of hybrid seed is expensive because the parental lines need to be maintained as pure lines and during normal sexual development the hybrid vigour present in the F1 seed breaks down in subsequent generations because of assortment and recombination, thus necessitating purchase of hybrid seed each season. If apomixis were to be available as a controlled tool where it could be switched on and off as required in plant breeding, hybrid vigour could be fixed, enabling indefinite multiplication of the hybrid of uniform quality without decrease in the yield advantage. Controlled apomixis would enable the use of a greater range of parent plants in breeding plants for specialized agricultural habitats. The current world use of hybrids such as maize (corn) and rice is based on and relatively restricted by a rather small base of inbred parental lines. Furthermore, these hybrids do not perform well under all conditions. Use of apomixis following crosses between two candidate parental lines would generate progeny that would be immediately fixed in a hybrid genotype and could be trialled for productivity. In this way apomixis could eliminate dependency on current inbred lines and enable us to expand our current agricultural genetic base. Apomixis would also remove the need for pollination for fruit and seed production. Adverse environmental conditions such as drought and cold inhibit pollen formation and result in crop losses in sexually reproducing systems; in apomictically grown plants these losses could be avoided. A significant proportion of the assimilate of the growing plant is channelled into the production of the male reproductive apparatus and gametes; these could be redirected towards increasing yield. Apomixis will also facilitate uniform clonal seed propagation of many plants that are currently vegetatively propagated, including fruit trees, plantation timbers, and potatoes. Due to these and other potential benefits of apomixis in agriculture, apomixis is likely to be an important breeding tool for the future. Little is known about the genes ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net Apomixis controlling apomixis. Research is currently being pursued to isolate these genes and to understand how the process of apomixis can be regulated to result in high yields of clonal seed. The value of apomixis will only be realized if it can be efficiently controlled under conditions of commercial production. Further Reading Asker SE and Jerling L (1992) Apomixis in Plants. Boca Raton, FL: CRC Press. Chaudhury AM, Craig S, Dennis ES and Peacock WJ (1998) Ovule and embryo development, apomixis and fertilization. Current Opinions in Plant Biology 1: 26–31. Koltunow AM (1993) Apomixis: embryo sacs and embryos formed without meiosis or fertilization in ovules. Plant Cell 5: 1425–1437. Koltunow AM, Bicknell RA and Chaudhury AM (1995) Apomixis: molecular strategies for the generation of genetically identical seeds without fertilization. Plant Physiology 108: 1345–1352. Koltunow AM, Soltys K, Nito M and McClure S (1995) Anther, ovule, seed and nucellar embryo development in Citrus sinensis cv Valencia. Canadian Journal of Botany 73: 1567–1582. Nogler GA (1984) Gametophytic apomixis. In: Johri BM (ed.) Embryology of Angiosperms, pp. 475–518. Berlin: Springer Verlag. ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Macmillan Publishers Ltd, Nature Publishing Group / www.els.net 3