An evolutionary approach to optimising synthetic apomixis in cereal crops

SURBANOVSKI N; SELVA JP; WALLINGTON E; CARBALLO J; MILLNER M; LAWRENCE P; ZAPPACOSTA D; BELLIDO A; ECHENIQUE V; CACCAMO M

Conferencia; The 26th International Conference on Sexual Plant Reproduction; 2022

Czech Academy of Sciences

Seed-mediated apomixis evolved as an alternative to the reproductivepathway whereby unreduced cells within the ovule acquire a reproductivefate. A recent breakthrough study showed that male-derived expressionof the transcription factor BBM1 in rice, which triggers the embryonicprogramme upon fertilisation, can be used to deliver parthenogenesiswhen ectopically expressed in the oocyte. Feasibility of apomixis in ricewas shown by combining the BBM1 expression in the egg with amitosis/meiosis substitution construction known as MiMe. However, asMiMe disables both male and female meiosis, it affects bothgametophytes, creating polyploids and this ‘disarming meiosis’ approachalso leads to inevitable change to the expected 2:3 zygote:endospermgenome ratio. Thus, whilst we aim to introduce the proof-of-concept ricesystem into barley, we also seek to contend with its shortcomings: highfrequencies of polyploids and sexual offspring. Our goal is to addressthese weaknesses through targeted dissection of the natural apomicticsystem in Eragrostis curvula. Apomixis in E. curvula starts with theformation of the embryo sac from the MMC itself, avoiding meiosis andfollowing directly into two rounds of mitosis, generating ultimately twosynergid cells (2n), the egg cell (2n) and the polar nucleus (2n). Only thepolar nucleus is fertilised creating the endosperm (3n) which results in the2:3 embryo-endosperm ratio. Our study aims to gain sufficient molecularunderstanding of the E. curvula system through forward-geneticsapproaches including single-cell transcriptomics, to replace the meiosis-disabling method with one that circumvents female meiosis in the MMCthereby avoiding problems associated with unreduced male gametes.Reverse genetics is being used to replicate the rice approach and validateEragrostis candidates in barley, whilst further optimisation of the systemsis to be achieved through inducing male sterility and using haploid-inducer lines as the male parent.