An evolutionary approach to optimising synthetic apomixis in cereal crops

We propose to investigate the key fundamental processes underlying seed-mediated clonal asexual reproduction (apomixis) in plants with the ultimate aim of delivering an efficient system in cereal crops. The ability to deploy, control and modulate apomixis has the potential to revolutionise the multiplication of hybrid seeds and accelerate the implementation of modern plant breeding by enabling the use of a greater range of parental plants. More generally, advancing our understanding of this trait could also help to remove the need for pollination for fruit and seed production opening endless opportunities to develop biotechnology tools.

Most flowering plants reproduce sexually by the production of male and female gametes through the processes of pollination and fertilisation generating seeds. During pollination male gametes transfer to the female ovules followed by a step of double-fertilisation which leads to the production of the embryo as well as the endosperm; in this way supporting the processes of adaption and evolution. There is, however, a large number of flowering plants that have evolved an alternative apomictic mode by which seeds are generated to produce embryos genetically identically to the mother plant. There are many forms of seed-mediated apomixis, the most common in grasses relies on the circumvention of female meiosis (apomeiosis) and the fertilization of the secondary endosperm nucleus without fertilising the unreduced egg cell (parthenogenesis). The underlying genetic factors and mechanisms governing these processes are not yet fully characterised. Developing a working understanding of either has potential value in its own right but considered together, they offer the possibility to implement seed-mediated apomixis.

We propose to deploy and optimise the proof-of-concept rice model of Khanday et al. (2019) to deliver synthetic apomixis in barley, which will in turn support further work in other commercially important complex crops such as wheat. We will seek to improve on the rice system with the aim of raising the proportion of apomictic offspring towards commercially viable levels. This will be complemented by a forward genetics approach using the apomictic forage grass Eragrostis curvula as a source of gene targets which will also be validated in barley.

This project draws on a large body of preliminary work and data, and on the strengths of a team with a particularly prominent track record in working with plant reproduction, new breeding technologies, transformation platforms, and bioinformatics applied to crop genetics. In the light of recent breakthroughs in the field and progress made by NIAB and collaborators, there is a unique opportunity to make a substantial contribution to the understanding of apomixis.

The focus of this proposal is to advance our understanding of apomeiosis and parthenogenesis with the ultimate aim of deploying apomixis in planta. We will evaluate three hypotheses: (1) apomeiosis in apomictic plants acts on female gametocytes only leading to clonal seeds with a 2:3 balanced karyotype, (2) sperm cell fate is a key factor determining embryogenesis and therefore an essential driver for efficient parthenogenesis, and (3) the mechanisms governing apomixis are generic and translatable between plant species.

We will deploy the proof-of-concept rice model of Khanday et al. (2019) in barley to deliver synthetic apomixis in a Triticeae species. We propose to combine genome editing tools and to implement the MiMe construct to disable meiosis with a PLT (BBM-like) transgene to circumvent sperm-egg fertilization as a prerequisite for zygote development. The rice model suffers from two mechanistic constraints: first, it disables meiosis whereas wild-type apomicts circumvent it and second, it does not preclude sperm-egg fertilization as occurs in parthenogenetic apomictic plants. We will address these limitations by advancing our molecular understanding of apomixis in Eragrostis curvula and in this way test the hypotheses. We propose to identify candidate genes responsible for the circumvention of female meiosis and for the failed delivery of sperm to egg nucleus. E. curvula is suitable as a model for such a targeted forward genetics strategy because we have closely related accessions that are obligate sexuals or obligate apomicts, a segregating population and also a series of facultative apomict clones with an associated stress-induction system. This resource will form the basis for a whole transcriptome differential expression analysis that we propose to implement at key stages of the apomixis pathway in E. curvula. We will again use barley to functionally characterise the candidate genes.