Clonal reproduction through seeds, known as apomixis, occurs in nature but not in cultivated species. This alternative mode of reproduction has enormous potential for sustainable agriculture because it would allow direct fixation and conservation of genotypes of interest in crops. A recent study published by a multidisciplinary consortium of plant biologists and modelers sheds light on the biological mechanisms underlying apomixis.
Avoiding the mixing of genes that results from sexual reproduction is one of the dreams of plant breeders ... the ability to induce clonal reproduction of seeds at will, producing genetically identical plants from one generation to another, has been the object of relentless research in the plant biology community.
Visualisation 3D et coupe optique d'un ovule montrant les cellules digitalisées et la cellule sexuée (en jaune) / 3D visualization and optical section of an ovule showing digitalized cells and the germ cell (in yellow)
© IRD - D.Autran et UZH - C. Baroux & E. Mendocilla-Sato
Studying the architecture of ovules to understand apomixis
The first event that differentiates sexual reproduction from apomixis?a form of asexual multiplication, without fertilization, which involves the formation of maternal seeds without male gametes occurs in the female reproductive organ, the ovule. In sexual species (animal or plant), a single cell transits from a somatic cellular identity?non-sexual to a germinal identity?sexual. This cell will undergo meiosis?process of double cell division allowing the formation of gametes or sexual cells, then form an egg cell which, after fertilization, will produce an embryo and subsequently a new individual. « In apomictic plant species, explains Daphné Autran, biologist at UMR DIADE, additional cells of the ovule can form an egg without going through meiosis, resulting in an embryo cloned from the mother plant ». Thus, developmental plasticity of ovule cells determines whether sexual or apomictic reproduction occurs. Understanding the mechanisms of this plasticity is an important goal for a large international community of academic and private laboratories. Even though many candidate genes have been identified, how these multiple factors are coordinated remains a mystery. « We hypothesized that the growth and division patterns of ovule cells determine the formation and plasticity of female germ cells », adds the biologist.
Simulation biomécanique de la croissance des ovules / Biomechanical simulation of ovule growth.
© UZH - G. Mosca
A multidisciplinary approach to elucidate the biomechanics of plant ovules
Ovules of angiosperms?flowering plants are difficult to access because they are enclosed in floral parts. In order to follow ovule development, the scientists used three-dimensional, deep cell-resolution microscopy, allowing a quantitative analysis of the growth. « The ovule is a relatively simple biological object, made up of few cells, and therefore particularly suitable for modeling, explains Daphné Autran. Our modeling partners in the multidisciplinary IMAGO project have developed virtual ovules, integrating many growth parameters, which can be sequentially modulated to test their respective importance ». This predictive approach defined the most plausible scenarios explaining the growth of the ovule and the particular shape of the germ cells. Finally, the analysis of molecular markers of cellular behavior as well as mutants affected in these processes has enabled functional testing of the hypotheses.
Architecture cellulaire d’un ovule apomictique de Paspalum rufum (microscopie confocale sur tissus transparisés) / Cellular architecture of the Paspalum rufum ovule (confocal microscopy on cleared tissues)
© IRD - D. Autran et UNR/CONICET, Argentine - L. Delgado
The early plasticity of germ cells depends on ovule architecture
Scientists have constructed a new 3D framework for the rules governing early ovule development in Arabidopsis, the secrets of which are gradually being revealed. It was shown that germ cell formation takes place earlier than previously described, and an early plasticity exists even in the ovule of sexual plants, which is then restricted to give a single germ cell in each ovule. The geometry of the ovule appears to be a crucial parameter in controlling this plasticity, and potentially the transition to the apomictic mode of reproduction, which would be a dream for breeders. These hypotheses - functionally validated in a model species - remain to be tested on cultivated plants. This is the subject of new national, European, and international collaborations that tackle the challenge in the sexually cultivated cereal maize, and in the apomictic model grass Paspalum, an important forage plant in South America.
Publication : Hernandez-Lagana E, Mosca G, Mendocilla-Sato E, Pires N, Frey A, Giraldo-Fonseca A, Michaud C, Grossniklaus U, Hamant O, Godin C, Boudaoud A, Grimanelli D, Autran D, Baroux C. 2021. Organ geometry channels reproductive cell fate in the Arabidopsis ovule primordium, Elife, https://doi.org/10.7554/eLife.66031
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Contact science : Daphne Autran, IRD, UMR DIADE daphne.autran@ird.fr
Contacts communication : Fabienne Doumenge, Julie Sansoulet communication.occitanie@ird.fr