Auxin biosynthesis maintains embryo identity and growth during BABY BOOM-induced somatic embryogenesis

Abstract

Somatic embryogenesis is a type of plant cell totipotency where embryos develop from non-reproductive (vegetative) cells without fertilization. Somatic embryogenesis can be induced in vitro by auxins, and by ectopic expression of embryo-expressed transcription factors like the BABY BOOM (BBM) AINTEGUMENTA-LIKE APETALA2/ETHYLENE RESPONSE FACTOR (AP2/ERF) domain protein. These different pathways are thought to converge to promote auxin response and biosynthesis, but the specific roles of the endogenous auxin pathway in somatic embryogenesis induction have not been well-characterized. Here we show that BBM transcriptionally regulates the YUCCA3 (YUC3) and YUC8 auxin biosynthesis genes during BBM-mediated somatic embryogenesis in Arabidopsis (Arabidopsis thaliana) seedlings. BBM induced local and ectopic YUC3 and YUC8 expression in seedlings, which coincided with increased DR5 auxin response and indole-3-acetic acid (IAA) biosynthesis and with ectopic expression of the WOX2 embryo reporter. YUC-driven auxin biosynthesis was required for BBM-mediated somatic embryogenesis, as the number of embryogenic explants was reduced by ca. 50% in yuc3 yuc8 mutants and abolished after chemical inhibition of YUC enzyme activity. However, a detailed YUC inhibitor time-course study revealed that YUC-dependent IAA biosynthesis is not required for the re-initiation of totipotent cell identity in seedlings. Rather, YUC enzymes are required later in somatic embryo development for the maintenance of embryo identity and growth. This study resolves a long-standing question about the role of endogenous auxin biosynthesis in transcription factor-mediated somatic embryogenesis and also provides an experimental framework for understanding the role of endogenous auxin biosynthesis in other in planta and in vitro embryogenesis systems.

Integrated transcriptomic and metabolic analyses provide insights into the maintenance of embryogenic potential and the biosynthesis of phenolic acids and flavonoids involving transcription factors in Larix kaempferi (Lamb.) Carr.

“…The obvious differences in color, texture, morphology or histology between EC and NEC of different species have been reported in several studies ( Nagmani and Bonga, 1985 ; Maadon et al., 2016 ; Bravo et al., 2017 ; Gautier et al., 2019 ; Xue et al., 2022 ), but the most significant difference between EC and NEC is that EC cells have the potential to acquire totipotency, which can furtherly regenerate into a complete organism ( Condic, 2014 ; Horstman et al., 2017 ; Li et al., 2022b ). A number of plant TFs have been identified that can covert somatic cells into embryogenic, totipotent cells, in which BBM, WUS, and LEC1 have been highlighted in many studies ( Su et al., 2009 ; Horstman et al., 2017 ; Su et al., 2021 ; Wang et al., 2022a ).…”

“…SE is a complex process of embryo development involving cell dedifferentiation and reprogramming to regenerate whole plants, which is regulated by a network of hundreds of genes ( Mordhorst et al., 1997 ; Wang et al., 2020 ). Many studies focusing on plant SE were conducted in some short-lived angiosperms such as Cucumis sativus ( Xue et al., 2022 ), Gossypium hirsutum ( Guo et al., 2019 ; Kumar et al., 2021 ) , and Fragaria vesca ( Liu et al., 2022 ), and most in-depth knowledge of SE can be only obtained from Arabidopsis ( Horstman et al., 2017 ; Karami et al., 2021 ; Li et al., 2022b ; Paul et al., 2022 ). However, little is known from perennials including gymnosperm.…”

“…However, the potential regulatory networks involved in these processes are poorly understood. In angiosperms, genes including SOMATIC EMBRYOGENESIS RECEPTOR KINASE ( SERK ) family members, BABY BOOM ( BBM ), WUSCHEL ( WUS ), and LEAFY COTYLEDON (LEC) are involved in the transition from NEC to EC ( Gruel et al., 2018 ; Mendez-Hernandez et al., 2019 ) and the overexpression of these genes can increase embryo formation frequency ( Lotan et al., 1998 ; Boutilier et al., 2002 ; Zuo et al., 2002 ; Lowe et al., 2016 ; Horstman et al., 2017 ; Li et al., 2022b ), but the degree of the increase differed between monocot and dicot systems ( Srinivasan et al., 2007 ; Zhang et al., 2014b ; Florez et al., 2015 ; Kumar et al., 2021 ). Moreover, it has been reported that genes related to stress response such as abscisic aldehyde synthesis enzyme 2 ( ABA2 ), abscisic acid insensitive 3 ( ABI3 ), jasmonate ZIM-domain 1 ( JAZ1 ), late embryogenesis abundant protein 1 ( LEA1 ), and transcription factors like AUX/IAAs, NACs, WRKYs, MYBs, and ERFs were also involved in callus induction ( Jin et al., 2014 ; Fehér, 2015 ; Guo et al., 2019 ; Salaün et al., 2021 ).…”

BABY BOOM regulates early embryo and endosperm development

“…The FIS-PRC2 complex is responsible for the deposition of H3K27me3 marks in the female gametophyte and in the endosperm, and mutations in genes coding for its subunits leads to the failure of endosperm cellularization (40,55). Our previous chromatin immunoprecipitation sequencing data (6,56) showed that BBM directly targets the promoter of one of the FIS-PRC2 subunits, FERTILIZATION INDEPENDENT ENDOSPERM (FIE), a WD-40 protein that is homologous to Drosophila enhancer of zeste [E(z)] and that is expressed in both sporophytic and gametophytic tissues (57,58). BBM binds the FIE gene close to the protein translation start site and the bound region contains three BBM binding motifs (SI Appendix, Fig.…”

One-Week Scutellar Somatic Embryogenesis in the Monocot Brachypodium distachyon

“…BBM was first identified as a marker gene expressed in microsporederived Brassica napus embryos [32]. When ectopically expressed, the BBM gene induces embryo development from the cotyledon, leaf, and shoot apex in A. thaliana and from the hypocotyls of germinated seedlings in Brassica, without the application of exogenous hormones [32,36,37].…”