Rice embryogenic trigger BABY BOOM1 promotes somatic embryogenesis by upregulation of auxin biosynthesis genes

Somatic embryogenesis, a powerful tool for clonal propagation and for plant transformation, involves cellular reprogramming of differentiated somatic cells to acquire pluripotency. Somatic embryogenesis can be induced by treating explants with plant growth regulators. However, several plant species including agronomically important cereal crops remain recalcitrant to dedifferentiation and transformation except from embryonic tissues. Somatic embryogenesis can also be induced by ectopic expression of select embryonic factors, including in cereals by BABY BOOM (BBM) transcription factors. How BBM genes bypass the need for exogenous hormones is not well understood. Here, we investigated downstream targets during induction of somatic embryogenesis in rice by OsBBM1 ((Oryza sativa BABY BOOM1). Transient induction of OsBBM1 led to the upregulation of auxin biosynthesis OsYUCCA genes. Continued induction of OsBBM1 resulted in somatic embryogenesis without the need for exogenous auxins. Genetic mutant analysis of OsBBM1 downstream targets, OsYUCCA6, OsYUCCA7 and OsYUCCA9, show that they are required for normal rice development including root and shoot development. Somatic embryogenic potential of OsYUCCA triple mutants was highly compromised despite the presence of exogenous auxin. Additionally, we show that somatic embryogenesis induction by exogenous auxin in rice requires functional BBM genes. Thus, OsBBM1 mediated cellular reprogramming and somatic embryogenesis likely involves increased localized auxin through direct upregulation of OsYUCCA genes. This study reveals mechanistic details of how somatic embryogenesis is established in differentiated tissues in rice, a monocot model and agronomically important cereal crop, with the potential utility to improve regeneration from tissue culture for recalcitrant plants in future. One-sentence summaryRice BABY BOOM1 induces somatic embryogenesis from differentiated tissues by promoting auxin biosynthesis through direct upregulation of YUCCA genes.