Abstract
Somatic embryogenesis (SE) is a type of induced cell totipotency where embryos develop from vegetative tissues of the plant instead of from gamete fusion after fertilization. SE can be induced in vitro by exposing explants to growth regulators, such as the auxinic herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). The plant hormone abscisic acid (ABA) has been proposed to be a downstream signalling component at the intersection between 2,4-D- and stress-induced SE, but it is not known how these pathways interact to induce cell totipotency. Here we show that 2,4-D-induced SE from the shoot apex of germinating Arabidopsis thaliana seeds is characterized by transcriptional maintenance of an ABA-dependent seed maturation pathway. Molecular-genetic analysis of Arabidopsis mutants revealed a role for ABA in promoting SE at three different levels: ABA biosynthesis, ABA receptor complex signalling, and ABA-mediated transcription, with essential roles for the ABSCISIC ACID INSENSITIVE 3 (ABI3) and ABI4 transcription factors. Our data suggest that the ability of mature Arabidopsis embryos to maintain the ABA seed maturation environment is an important first step in establishing competence for auxin-induced cell totipotency. This finding provides further support for the role of ABA in directing processes other than abiotic stress response.
Highlights
Plant embryogenesis begins at fertilization with the formation proceeds through defined developmental stages that are characof a totipotent zygote that develops into an embryo within the teristic for each plant species
We show that 2,4-D-induced Somatic embryogenesis (SE) from the shoot apex of germinating Arabidopsis thaliana seeds is characterized by transcriptional maintenance of an abscisic acid (ABA)-dependent seed maturation pathway
We show that 2,4-D-induced SE from the shoot apex of germinating after-ripened Arabidopsis embryos is characterized by the maintenance of an ABA-dependent seed maturation environment
Summary
In Arabidopsis, ABA enhances 2,4-D-induced SE from otherwise non-embryogenic seedling root explants of the POLYCOMB REPRESSIVE COMPLEX 2 CURLY LEAF/SWINGER mutant (clf swn) through an unknown mechanism (Mozgová et al, 2017), and modulates auxin response and transport during 2,4-D-induced secondary SE from embryogenic callus (Su et al, 2013) It is not clear whether ABA is required during SE in its role as a developmental regulator or as a stress response modulator. We show that the AUXIN RESPONSE FACTORS (ARFs) ARF10 and ARF16, which act upstream and downstream of ABI3 expression, are required for efficient SE These data provide a mechanistic link between 2,4-D and ABA signalling in somatic embryo induction, and suggest a developmental role for ABA in promoting plant cell totipotency.
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