The BABY BOOM Transcription Factor Activates the LEC1-ABI3-FUS3-LEC2 Network to Induce Somatic Embryogenesis

Abstract

Somatic embryogenesis is an example of induced cellular totipotency, where embryos develop from vegetative cells rather than from gamete fusion. Somatic embryogenesis can be induced in vitro by exposing explants to growth regulators and/or stress treatments. The BABY BOOM (BBM) and LEAFY COTYLEDON1 (LEC1) and LEC2 transcription factors are key regulators of plant cell totipotency, as ectopic overexpression of either transcription factor induces somatic embryo formation from Arabidopsis (Arabidopsis thaliana) seedlings without exogenous growth regulators or stress treatments. Although LEC and BBM proteins regulate the same developmental process, it is not known whether they function in the same molecular pathway. We show that BBM transcriptionally regulates LEC1 and LEC2, as well as the two other LAFL genes, FUSCA3 (FUS3) and ABSCISIC ACID INSENSITIVE3 (ABI3). LEC2 and ABI3 quantitatively regulate BBM-mediated somatic embryogenesis, while FUS3 and LEC1 are essential for this process. BBM-mediated somatic embryogenesis is dose and context dependent, and the contextdependent phenotypes are associated with differential LAFL expression. We also uncover functional redundancy for somatic embryogenesis among other Arabidopsis BBM-like proteins and show that one of these proteins, PLETHORA2, also regulates LAFL gene expression. Our data place BBM upstream of other major regulators of plant embryo identity and totipotency.

A transcriptional view on somatic embryogenesis

“…Overexpression of AIL5 also triggers somatic embryo and adventitious organ formation (Tsuwamoto, Yokoi, & Takahata, 2010). Recently, it became clear that overexpression of all AIL proteins, except the phylogenetically distinct AIL1 and ANT, induces SE (Horstman et al., 2017). This shows that the embryo‐inducing capacity of AIL proteins is not limited to embryo‐expressed AILs, and suggests that AIL proteins can regulate similar target genes.…”

“…Using chemical activation of BBM at different time points, we showed that BBM induces direct SE when activated in a narrow time window before seed germination and indirect SE from callus when activated shortly after germination (Horstman et al., 2017). Direct SE proceeds quickly and the initial steps involve direct activation of LAFL gene expression (see below).…”

“…ABA levels decrease as embryos convert to seedlings, suggesting that endogenous ABA levels are sufficient for LEC1‐induced SE during seed germination, but not at later time points. BBM does not require ABA to induce SE after germination and does not appear to target genes in the ABA pathway (Horstman et al., 2017), suggesting that BBM regulates target genes in a different pathway to switch cells of older seedlings to a competent state.…”

“…It is not known whether the callus that precedes TF‐induced indirect SE originates from pericycle cells or whether this callus has lateral root identity. The embryogenic callus formed during BBM‐induced indirect SE is derived from the outer layers of the cotyledon in the regions above the vasculature, rather than from pericycle‐like cells (Horstman et al., 2017). Both the ground and vascular tissue also form callus in the same explant, but somatic embryos do not seem to develop from this callus.…”

“…By modulating the amount of nuclear‐localized BBM we showed that a relatively low BBM dose inhibits cell differentiation, a relatively medium dose induces root and shoot organogenesis and a relatively high dose induces SE (Horstman et al., 2017). Conceivably, this dose‐dependent regeneration could be achieved by transcriptional regulation of specific BBM target genes at different BBM doses or by different expression levels of the same target genes.…”