An evolutionary approach to optimising synthetic apomixis in cereal crops

URBANOVSKI, NADA ; SELVA, JUAN PABLO; WALLINGTON, EMMA ; CARBALLO, JOSÉ; MILNER, MATTHEW ; PERCIVAL-ALWYN, LAWRENCE; ZAPPACOSTA, DIEGO; BELLIDO, ANDRÉS MARTÍN; ECHENIQUE, VIVIANA; CACCAMO, MARIO

Prague

Congreso; ICSPR 2022; 2022

Seed-mediated apomixis evolved as an alternative to the reproductive pathway whereby unreduced cells within the ovule acquire a reproductive fate. A recent breakthrough study showed that male-derived expression of the transcription factor BBM1 in rice, which triggers the embryonic programme upon fertilisation, can be used to deliver parthenogenesis when ectopically expressed in the oocyte. Feasibility of apomixis in rice was shown by combining the BBM1 expression in the egg with a mitosis/meiosis substitution construction known as MiMe. However, as MiMe disables both male and female meiosis, it affects both gametophytes, creating polyploids and this ‘disarming meiosis’ approach also leads to inevitable change to the expected 2:3 zygote:endosperm genome ratio. Thus, whilst we aim to introduce the proof-of-concept rice system into barley, we also seek to contend with its shortcomings: high frequencies of polyploids and sexual offspring. Our goal is to address these weaknesses through targeted dissection of the natural apomictic system in Eragrostis curvula. Apomixis in E. curvula starts with the formation of the embryo sac from the MMC itself, avoiding meiosis and following directly into two rounds of mitosis, generating ultimately two synergid cells (2n), the egg cell (2n) and the polar nucleus (2n). Only the polar nucleus is fertilised creating the endosperm (3n) which results in the 2:3 embryo-endosperm ratio. Our study aims to gain sufficient molecular understanding of the E. curvula system through forward-genetics approaches including single-cell transcriptomics, to replace the meiosis-disabling method with one that circumvents female meiosis in the MMC thereby avoiding problems associated with unreduced male gametes. Reverse genetics is being used to replicate the rice approach and validate Eragrostis candidates in barley, whilst further optimisation of the systems is to be achieved through inducing male sterility and using haploid-inducer lines as the male parent.