Douglas-fir LEAFY COTYLEDON1 (PmLEC1) is an active transcription factor during zygotic and somatic embryogenesis

Abstract

Douglas-fir (Pseudotsuga menziesii) is one of the world's premier lumber species and somatic embryogenesis (SE) is the most promising method for rapid propagation of superior tree genotypes. The development and optimization of SE protocols in conifers is hindered by a lack of knowledge of the molecular basis of embryogenesis and limited sequence data. In Arabidopsis, the LEAFY COTYLEDON1 (AtLEC1) gene is a master regulator of embryogenesis that induces SE when expressed ectopically. We isolated the LEC1 homologue from Douglas-fir, designated as PmLEC1. PmLEC1 expression in somatic embryos and developing seeds demonstrated a unique, alternating pattern of expression with the highest levels during early stages of embryogenesis. PmLEC1 protein accumulation during seed development correlated with its transcriptional levels during early embryogenesis; however, substantial protein levels persisted until 2 weeks on germination medium. Treatment of mature, stratified seeds with 2,4-epibrassinolide, sorbitol, mannitol, or NaCl upregulated PmLEC1 expression, which may provide strategies to induce SE from mature tissues. Sequence analysis of the PmLEC1 gene revealed a 5′ UTR intron containing binding sites for transcription factors (TFs), such as ABI3, LEC2, FUS3, and AGL15, which are critical regulators of embryogenesis in angiosperms. Regulatory elements for these and other seed-specific TFs and biotic and abiotic signals were identified within the PmLEC1 locus. Most importantly, functional analysis of PmLEC1 showed that it rescued the Arabidopsis lec1-1 null mutant and, in the T2 generation, led to the development of embryo-like structures, indicating a key role of PmLEC1 in the regulation of embryogenesis.