A transcriptional view on somatic embryogenesis

Abstract

Somatic embryogenesis is a form of induced plant cell totipotency where embryos develop from somatic or vegetative cells in the absence of fertilization. Somatic embryogenesis can be induced in vitro by exposing explants to stress or growth regulator treatments. Molecular genetics studies have also shown that ectopic expression of specific embryo- and meristem-expressed transcription factors or loss of certain chromatin-modifying proteins induces spontaneous somatic embryogenesis. We begin this review with a general description of the major developmental events that define plant somatic embryogenesis and then focus on the transcriptional regulation of this process in the model plant Arabidopsis thaliana (arabidopsis). We describe the different somatic embryogenesis systems developed for arabidopsis and discuss the roles of transcription factors and chromatin modifications in this process. We describe how these somatic embryogenesis factors are interconnected and how their pathways converge at the level of hormones. Furthermore, the similarities between the developmental pathways in hormone- and transcription-factor-induced tissue culture systems are reviewed in the light of our recent findings on the somatic embryo-inducing transcription factor BABY BOOM.

Keywords: chromatin modifications; regeneration; somatic embryogenesis; totipotency; transcription factors.

Figure 1

Somatic embryogenesis in arabidopsis. (A) Schematic overview of somatic embryogenesis (SE) systems in arabidopsis. SE can be induced from a range of tissues throughout the arabidopsis life cycle, and proceeds either directly or indirectly via a callus phase. SE can be induced using the synthetic auxin 2,4‐D or by overexpression of specific transcription factors, including BABY BOOM (BBM), LEAFY COTYLEDON (LEC), WUSCHEL (WUS), RWP‐RK DOMAIN‐CONTAINING 4 (RKD4) and WOUND INDUCED DEDIFFERENTIATION 1 (WIND1). (B)−(F) SE from different arabidopsis explants. (B) Direct SE from immature zygotic embryos treated with 2,4‐D on solid medium. Embryos develop from the edge of the original explant (dotted line), while the underlying tissue forms callus. (C) Indirect SE from immature zygotic embryos treated with 2,4‐D in liquid medium. (D) Secondary SE. Callus from primary somatic embryos cultured in liquid medium produces secondary somatic embryos after removal of 2,4‐D (Su et al., 2009). (E) 35S:BBM induces direct SE on the cotyledons and shoot meristem of germinated seedlings. (F) Scanning electron micrograph showing somatic embryo development on a 35S:BBM cotyledon. The embryos develop directly from the explant (>), are bipolar and undergo direct secondary embryogenesis (*). (G) Indirect SE on a 35S:BBM‐GR seedling. Embryos (arrow) develop from callus produced on the cotyledons. c, callus; ct, cotyledon; le, leaf

Figure 2

Schematic overview of the molecular regulation of arabidopsis somatic embryogenesis. Chromatin‐modifying proteins (green hexagons) repress or restrict expression of transcription factors (TFs) (blue ovals) during arabidopsis development. The TF VAL, which can recruit PRC1 and HDACs, is indicated on the chromatin level (blue‐green oval). These TFs regulate each other's expression, as well as expression of common target genes involved in the auxin and cytokinin pathways (purple boxes). Evidence for these interactions was not always obtained in somatic embryo tissue, and only a few of the TF−gene interactions have been shown to be direct (see main text for more details). Note that the importance of the TF−hormone interactions for TF‐induced SE was only shown for AGL15‐IAA30