Abstract
Formation of somatic embryos in plants is known to require high concentrations of auxin or 2,4-dichlorophenoxyacetic acid (2,4-D), which presumably acts to trigger a signalling cascade. However, very little is known about the molecular mechanism that mediates the vegetative-to-embryogenic transition. We have employed a genetic approach to dissect the signal transduction pathway during somatic embryogenesis. In a functional screen using a chemical-inducible activation-tagging system, we identified two alleles of Arabidopsis gene PGA6 whose induced overexpression caused high-frequency somatic embryo formation in all tissues and organs tested, without any external plant hormones. Upon inducer withdrawal, all these somatic embryos were able to germinate directly, without any further treatment, and to develop into fertile adult plants. PGA6 was found to be identical to WUSCHEL (WUS), a homeodomain protein previously shown to be involved in specifying stem cell fate in shoot and floral meristems. Transgenic plants carrying an estradiol-inducible XVE-WUS transgene can phenocopy pga6-1 and pga6-2. Our results suggest that WUS/PGA6 also plays a key role during embryogenesis, presumably by promoting the vegetative-to-embryogenic transition and/or maintaining the identity of the embryonic stem cells.
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