Some wild plants show the particular characteristic of apomixis. This is a process adopted for producing seeds without fertilization or genetic recombination, giving rise to exact copies of the mother plant. This is an unusual method of reproduction and IRD researchers and their partners1 are seeking to apply it to agriculturally useful plant species, such as wheat or maize. Such a revolution would spare farmers of the need to buy seed every year, particularly in the countries of the South.
However, several challenges arise from the development of these kinds of varieties. Can a normally sexually reproducing plant do without paternal input? Results reported in an article in the journal Cell shed light on this fundamental question. A research team recently demonstrated that the embryo could develop without contribution from the paternal genome. During the first growth stages, the genes expressed are predominantly inherited from the maternal genome. The respective contributions then become more balanced, but the maternal genome is still in control of the relative parts played by the two parents.
The research team hopes that these results will eventually pave the way to developing crop plants that will persistently keep their selected qualities, harvest after harvest, especially beneficial for farmers in countries of the South.
Sexual reproduction effectively redistributes the genes of the two parents. However, in crop plants the process tends to cause erosion from generation to generation of their selected beneficial properties -such as productivity, plant vigour, resistance to drought- or pathogens. This makes it difficult for farmers to keep up the yields of a field of maize or wheat year in year out. They are obliged to renew the seed for each new growing cycle. Worldwide 25 billion Euros per year are given over to seed buying. But such expenditure is difficult to support for many producers in the developing countries.
An end to gene mixing
To enable farmers to produce their own seeds, IRD biologists and their research partners1 have been working for more than ten years on ways of applying a relatively unusual reproductive strategy found in some wild plants to agronomically important species. This is apomixis, meaning simply "without mixing." A form of asexual reproduction, it is a way of avoiding the usual genetic intermingling. The process involves neither fertilization nor meiosis, in other words no genetic recombination between the two parent plants; the seeds give rise to exact copies of the mother plant. In the wild, about 400 species, as diverse as dandelion, mango or hawthorn adopt apomixis.
Can a sexually reproducing plant do without paternal genes?
How then can sexually reproducing plants, as most crops are, be made apomictic? Plant hybridization has not enabled scientists to arrive at satisfactory results, so they have changed tack deciding to prompt 100% maternal development in the embryo of a sexually reproducing plant whose genome is normally made up half-and-half by paternal and maternal DNA.
This innovatory approach presents a number of technical and theoretical challenges. For instance, can the plant function completely independently of the "father's" input, without harm to its development? This is indeed the case, as the research team reports in an article in the journal Cell. Early in its development, the embryo grows essentially under the activity of the maternal chromosomes it inherits, while the paternal ones remain inactive. In total, more than 90% of the RNAs2, the molecules that induce differentiation of the embryonic tissues and organs, prove to be of maternal origin.
To shed light on this fundamental question, the team's geneticists chose the plant model mouse-ear cress, Arabidopsis thaliana. Under the microscope, they crossed two distinct varieties, then extracted the young embryos 15 hours after fertilization. These were composed of only two to four cells at this stage. Analysis of these cells' RNA using very high throughput methods and sequencing indicates the differences in sequences between the two parental genomes. For each of 3000 genes observed, the method detects whether the RNAs have been produced by maternally inherited chromosomes or from paternally inherited ones. In this way the respective contributions from the two parents was quantified.
After the 2-4 cell stage, as the embryo's development proceeds, the parental contributions progressively become more balanced. However, there again it is the maternal influence which controls the situation. In the same investigations, the research team identified the two mechanisms, set in train by the maternal genome, which hold back and keep silent the paternal genes in early development, then activate them by producing specific proteins.
These new results suggest that it would be possible to prompt a sort of clonal multiplication in sexually reproducing species. Yet, with no fertilization involved, what is the signal that triggers seed growth? The hope for the long term is that, once that barrier has been removed, farmers, especially in the countries of the South, can be provided with apomictic food plants in which selected agronomic properties would be fixed for successive generations.
1. These investigations were conducted in partnership with the University of Zurich in Switzerland, the LANGEBIO (Laboratorio Nacional de Genomica para la Biodiversidad) in Irapuato in Mexico, the University of Kiel in Germany and the CNRS Centre d'Ecologie Fonctionnelle at Montpellier.
2. RNA is a substance present in cells which transports genetic information carried by DNA or which uses it to make proteins.
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