The arabidopsis phytoglobin 1 (Pgb1) involvement in somatic embryogenesis is linked to changes in ethylene and the class VII ethylene transcription factor HRE2.

Abstract

Main conclusion: Phytoglobin1 promotes Arabidopsis somatic embryogenesis through the mediation of ethylene and the ERFVII HRE2. Generation of somatic embryos in Arabidopsis (Arabidopsis thaliana) is a two-step process, encompassing an induction phase where embryogenic tissue (ET) is formed followed by a developmental phase encouraging the growth of the embryos. Using previously characterized transgenic lines dysregulating the class 1 Phytoglobin (Pgb1) we show that suppression of Pgb1 decreases somatic embryogenesis (SE). Both the formation of ET (SE efficiency) and production of SE (SE productivity) are repressed in explants where Pgb1 is downregulated. The levels of Pgb1 transcripts peak in the middle phase of the induction period coinciding with the formation of the ET. Presence of Pgb1 results in a transcriptional depression of ethylene synthesis and of the class VII ethylene transcription factor (ERFVII) HRE2. Suppression of ethylene after day 3 of induction, or repression of HRE2 are needed for SE efficiency and the decline in HRE2 transcripts appears to be independent from the level of ethylene. Over-expression of HRE2 inhibits SE efficiency regardless of the expression of Pgb1. Furthermore, a functional HRE2 generates a peak in Pgb1 transcripts during the middle induction phase. The expression of another ERFVII, RAP2.12, is not altered by changes in Pgb1 levels, and disruption of RAP2.12 has no effect on SE efficiency although it enhances SE productivity in a Pgb1-independent fashion. Thus, Pgb1 is an important regulator of Arabidopsis SE, and its action is linked to changes in ethylene and the ERFVII HRE2.