Abstract
In many flowering plants, asymmetric division of the zygote generates apical and basal cells with different fates. In Arabidopsis thaliana, the apical cell generates the embryo while the basal cell divides anticlinally, leading to a suspensor of six to nine cells that remain extra-embryonic and eventually senesce. In some genetic backgrounds, or upon ablation of the embryo, suspensor cells can undergo periclinal cell divisions and eventually form a second twin embryo. Likewise, embryogenesis can be induced from somatic cells by various genes, but the relationship with suspensor-derived embryos is unclear. Here, we addressed the nature of the suspensor to embryo fate transformation and its genetic triggers. We expressed most known embryogenesis-inducing genes specifically in suspensor cells. We next analyzed morphology and fate-marker expression in embryos in which suspensor division was activated by different triggers to address the developmental paths towards reprogramming. Our results show that reprogramming of Arabidopsis suspensor cells towards embryonic identity is a specific cellular response that is triggered by defined regulators, follows a conserved developmental trajectory and shares similarity to the process of somatic embryogenesis from post-embryonic tissues.
Highlights
In flowering plants, embryogenesis is initiated by fertilization of the egg cell
Excessively dividing cells in M0171>>bdl suspensors did at times resemble embryo-like structures, twin seedlings were not observed in these embryos (Radoeva et al, 2019)
We explored a candidate gene approach employing suspensor-specific expression of genes known to promote somatic embryogenesis. This revealed that three genes, RKD1, RKD4 and WUS, were able to induce twin seedlings
Summary
In flowering plants (angiosperms), embryogenesis is initiated by fertilization of the egg cell. Genes from the plant-specific RWP-RK domain-containing (RKD) family, involved in maintaining egg cell identity, are able to induce loss of cell identity (Köszegi et al, 2011) or actively promote somatic embryogenesis (Waki et al, 2011) upon ectopic expression. These genes were identified and tested in different experimental systems, ranging from Brassica microspores to Arabidopsis meristems and seedlings. This makes it challenging to infer whether these factors are part of the same genetic network or pathway, and it is unclear how these factors, or the process of somatic embryogenesis, relates to the reprogramming of suspensor cells. Our work shows that suspensor reprogramming is activated by specific triggers, and reveals a striking similarity between suspensor-derived and other somatic embryogenesis processes
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