Phalaenopsis LEAFY COTYLEDON1-Induced Somatic Embryonic Structures Are Morphologically Distinct From Protocorm-Like Bodies

Abstract

Somatic embryogenesis is commonly used for clonal propagation of a wide variety of plant species. Induction of protocorm-like-bodies (PLBs), which are capable of developing into individual plants, is a routine tissue culture-based practice for micropropagation of orchid plants. Even though PLBs are often regarded as somatic embryos, our recent study provides molecular evidence to argue that PLBs are not derived from somatic embryogenesis. Here, we report and characterize the somatic embryonic tissues induced by Phalaenopsis aphrodite LEAFY COTYLEDON1 (PaLEC1) in Phalaenopsis equestris. We found that PaLEC1-induced somatic tissues are morphologically different from PLBs, supporting our molecular study that PLBs are not of somatic embryonic origin. The embryonic identity of PaLEC1-induced embryonic tissues was confirmed by expression of the embryonic-specific transcription factors FUSCA3 (FUS3) and ABSCISIC ACID INSENSITIVE3 (ABI3), and seed storage proteins 7S GLOBULIN and OLEOSIN. Moreover, PaLEC1-GFP protein was found to be associated with the Pa7S-1 and PaFUS3 promoters containing the CCAAT element, supporting that PaLEC1 directly regulates embryo-specific processes to activate the somatic embryonic program in P. equestris. Despite diverse embryonic structures, PaLEC1-GFP-induced embryonic structures are pluripotent and capable of generating new shoots. Our study resolves the long-term debate on the developmental identity of PLB and suggests that somatic embryogenesis may be a useful approach to clonally propagate orchid seedlings.

Protuberances are organized distinct regions of long-term callus: histological and transcriptomic analyses in kiwifruit

“…The regular cell composition on the surface in comparison with regions beyond the PT, the presence of starch granules, and the shoot buds arising on the PT can be mentioned as the most relevant. These features are similar to the histology of protocorm-like bodies or the multiple meristem-like tissues observed in Orchidaceae (Zhao et al 2008 ; Chen et al 2019 ) and nodular cultures in Bromeliaceae (Corredor-Prado et al 2015 ). However, the histological organization of PT in kiwifruit also resembles PT observed in other woody vines— Passiflora edulis (Fernando et al 2007 ).…”

“…Structures, described as protrusions or outgrowths of the callus, have been observed during tissue culturing of different explants and species. The morphology, especially the shape of the PTs in kiwifruit examined here, seems to be similar to the protocorm-like bodies in Dendrobium (Zhao et al 2008 ) and Phalaenopsis (Chen et al 2019 ).…”

“…The relationship between the appearance of PTs or outgrowths and the emergence of primordia-like structures or shoot buds has been observed during tissue culturing of different species (Nakano and Maeda 1974 ; Jarret et al 1981 ; Fernando et al 2007 ; Daffalla et al 2011 ; Qiao et al 2012 ; Tekdal and Cetiner 2013 ; Manivannan et al 2015 ; Lee and Pijut 2017 ). However, morpho-histological data about organogenic calluses are focused mainly on structures referred to as protocorm-like bodies (Zhao et al 2008 ; Palama et al 2010 ; Chen et al 2019 ) or nodular structure formation (Oka et al 1982 ; Rocha et al 2012 ; Corredor-Prado et al 2015 ). Histological and ultrastructural studies describing the origin and different stages of in vitro morphogenesis have contributed significantly to the understanding and optimization of regeneration systems (Rocha et al 2012 ).…”

“…Transcriptome profiling is a powerful tool that has been used to estimate the importance of gene products in tissues under different conditions. Moreover, molecular studies could help in distinguishing the structures which are morphologically similar (Chen et al 2019 ). Recently, there was increased interest in gene expression during callus induction and proliferation, and then during the regeneration processes from the callus (Xu et al 2012 ; Ikeuchi et al 2013 , 2019 ; Li et al 2014 ; Kumari et al 2017 ; Nakano et al 2018 ; Fu et al 2019 ; Gao et al 2019 ; Lee and Huang 2019 ).…”

Transcriptome atlas ofPhalaenopsis equestris

“…Genome assembly of Phalaenopsis equestris (the horse phalaenopsis) (Cai et al, 2015) provided novel opportunities for evolutionary and functional studies of Orchidaceae. Genome assembly was used for the functional studies of transcription factor families (Lin et al, 2016; Valoroso et al, 2019), somatic embryogenesis (Chen et al, 2019), retrotransposon insertions (Hsu et al, 2019), as well as for evolutionary studies of ancient polyploidy (Barrett et al, 2019). However, transcriptome resources of P. equestris remain limited even though the de novo transcriptome assembly was performed based on RNA sequencing of 11 organs (Niu et al, 2016).…”