BioRxiv：水稻胚發生觸發器BABY BOOM1通過生長素生物合成基因的上調促進體細胞胚發生

體細胞胚發生是克隆繁殖和植物轉化的有力工具，涉及分化的體細胞的細胞重編程以獲得多能性。可以通過用植物生長調節劑處理外植體來誘導體細胞胚發生。但是，包括植物學上重要的穀類作物在內的幾種植物種類，除胚芽組織外，對去分化和轉化均保持頑固性。

體細胞胚發生也可以通過選擇性胚胎因子的異位表達來誘導，包括在穀物中通過BABY BOOM（BBM）轉錄因子表達。 BBM基因如何繞過對外源激素的需求尚不清楚。

在這裡，我們調查了OsBBM1（（Oryza sativa BABY BOOM1））誘導水稻體細胞胚發生過程中的下遊目標。OsBBM1的瞬時誘導導致植物生長素生物合成OsYUCCA基因的上調。OsBBM1的持續誘導導致體細胞胚發生而無需外源性對OsBBM1下遊靶標OsYUCCA6，OsYUCCA7和OsYUCCA9的遺傳突變分析表明，它們是正常水稻發育（包括根和莖發育）所必需的，儘管存在外源性生長素，OsYUCCA三重突變體的體細胞胚發生能力也受到很大損害。我們發現外源生長素誘導水稻體細胞胚發生需要功能性BBM基因，

因此，OsBBM1介導的細胞重編程和體胚發生可能通過直接上調OsYUCCA基因來增加局部生長素，這項研究揭示了如何在分化中建立體胚發生的機制細節水稻中的組織，單子葉植物模型和具有重要農藝意義的穀物作物，未來可能會改善植物的組織培養再生能力。

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.

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