Evolution and development of land plant embryos

Land plants evolved from aquatic green algae around 470 million years ago. The transition from water to land was facilitated by the evolution of a number of traits that permitted growth and survival in the relatively harsh terrestrial environment. One of these acquired traits was the embryo, a feature so fundamental that all land plants are collectively known as 'embryophytes'.

Both molecular and morphological analyses reveal that the algal group that is most closely related to the land plants is the charophytes. Charophycean green algae exhibit a range of body plans, ranging from single cells to highly branched multicellular structures, but these are all found in the haploid (gametophyte) generation of the lifecycle. The diploid (sporophyte) phase of the charophyte lifecycle is invariant and unicellular - zygotes are formed after fertilization between two haploid mating types and then the first zygotic cell division is 'meiotic' - halving the chromosome number in the two daughter cells to form haploid spores that germinate to form new gametophytes. By contrast, the zygote in all land plants undergoes multiple 'mitotic' cell divisions that retain the chromosome number in each daughter cell and create a multicellular diploid embryo that develops into a sporophyte body. The sporophyte initially goes through a vegetative phase that can persist for days (e.g. in mosses) or years (e.g. in trees) before converting to reproductive mode and undergoing meiosis to produce haploid gametophytes. The evolution of the embryo thus required the adoption of a mechanism to promote mitosis and/or delay meiosis in the zygote, intercalating a vegetative phase between zygote formation and the production of new gametophytes.

This proposal aims to investigate the mechanisms that regulate embryo formation in two of the earliest divergent land plant lineages - the liverworts and the mosses. Five regulators of major transitions in the moss lifecycle have previously been identified by mutant analysis [1-5]. Through transgenic analysis, we aim to characterize the ancestral role of these regulators in charophyte algae, and determine the extent to which their role in embryo development is conserved between mosses and liverworts. In addition, we aim to identify novel regulators of embryo patterning, through the analysis of a dataset that distinguishes genes expressed in unicellular zygotes of charophyte algae from those expressed in early stage embryos of mosses and/or liverworts. The function of five genes that are expressed in embryos but not in single-celled zygotes will be tested by transgenic analysis in mosses and liverworts. In combination, the results will provide an understanding of how the role of key developmental regulators was altered as algal and land plant lineages diverged, generate a mechanistic understanding of a fundamental developmental process, and provide a comparative framework in which to test new hypotheses of how multicellular land plant sporophytes evolved.
1) Sakakibara K et al (2008) Evol Dev 10, 555 2) Sakakibara K et al (2013) Science 339, 1067 3) Tanahashi T et al (2005) Development 132, 1727 4) Okana Y et al (2009) PNAS 106, 16321 5) Mosquna A et al (2009) Development 136, 2433.

A multicellular diploid embryo is such a defining feature of plants that collectively all land plants are known as embryophytes. Evolution of the embryo occurred as plants evolved from aquatic green algae and as such, extant algae and land plants both have multicellular haploid (gametophyte) bodyplans, but only land plants develop a multicellular diploid (sporophyte) bodyplan. In the moss Physcomitrella patens, an extant member of the earliest divergent bryophyte grade of land plants, CURLY LEAF (CLF), LEAFY (LFY) and KNOTTED1-LIKE HOMEOBOX (KNOX) proteins regulate key steps in the development of the multicellular sporophyte. This proposal aims to test the hypothesis that the evolution of multicellular land plant sporophytes was facilitated by altering the roles of these three regulatory proteins, and to identify novel components of the developmental toolkit that were recruited to pattern sporophyte bodyplans during this transition. The ancestral roles of CLF, LFY and KNOX will be determined by transgenic analysis in the charophyte alga Penium maragaritaceum, and the extent to which gene function is conserved in bryophytes will be tested through transgenic analyses in the liverwort Marchantia polymorpha. To identify novel regulators of embryo patterning, we will use Boolean filters to interrogate transcriptome profiles of unicellular zygotes and early stage embryos that we have generated using two charophyte and two bryophyte species. Transgenic analyses in both P. patens and M. polymorpha will allow the role of candidate regulators in embryo patterning to be verified. In combination, the results will provide an understanding of how the role of key developmental regulators was altered as algal and land plant lineages diverged, generate a mechanistic understanding of a fundamental developmental process, and provide a comparative framework in which to test new hypotheses of how multicellular land plant sporophytes evolved.

The primary and immediate impact of this research will be enhanced knowledge and understanding of a fundamental biological process, namely how embryogenesis is regulated in the earliest divergent land plants. This knowledge will be communicated to the general public in a number of ways; for example in seminars at the University of Oxford Botanic Garden and through interactive displays at the University of Oxford Museum of Natural History. Advances in understanding will also be disseminated at secondary education level by engaging in dialogue with both students and teachers. Together these knowledge exchange activities will enrich societal understanding of the scientific method and of biosciences research in particular.

In the long-term, the outputs of this project may have commercial applications. For example, an understanding of the mechanisms that regulate embryogenesis in the earliest divergent land plant lineages may provide novel ways to induce either somatic embryogenesis in vegetative tissue, or fertilization independent embryogenesis in female gametophytes (apomixis). Apomixis is uncommon in wild plants and rare in crops, but because it effectively clones the maternal plant, the ability to induce apomixis is highly sought after by seed companies: plant breeders would be able to produce new seed varieties more quickly and more cheaply, and farmers would be able to clone hybrid seed for use in the following season.

More broadly, the training of highly skilled personnel and the publication of high impact science will contribute to the UK's position as a leading country for R & D, and will help sustain the 'Knowledge Based BioEconomy'.