背景回顾：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. 

提出问题：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. 

主要发现：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. 

结果1-BBM诱导的YUC3/8表达：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. 

结果2-YUC驱动的生长素合成作用于BBM介导的体胚发生：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. 

结果3-YUC驱动的生长素合成作用于体胚发生后期胚胎身份维持和生长：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. 

结论：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.

 摘 要 

体胚发生是植物细胞全能性的一种类型，指的是非生殖细胞不经受精作用而发育成胚胎的过程。体外生长素处理能够诱导体胚发生，或者通过异位表达胚胎中表达的转录因子，比如含有AP2/ERF结构域的蛋白BBM也可以诱导体胚发生。不同的途径都可以促进体胚发生，研究者普遍认为这些不同的通路最后都会通过促进生长素响应和生物合成来作用于体胚发生，但是内源的生长素通路到底在体胚发生过程中发挥何种作用还不是很清楚。本文中，作者发现在BBM介导的拟南芥体胚发生中，BBM能够转录调控生长素合成基因YUC3和YUC8。在拟南芥幼苗中，BBM能够诱导YUC3和YUC8的局域和异位表达，这与DR5生长素响应和IAA生物合成以及胚胎报告基因WOX的异位表达相一致。YUC驱动的生长素合成对于BBM介导的体胚发生是必须的，在yuc3 yuc8双突变体中BBM介导的体胚发生诱导效率降低了大概一半，并且通过化学抑制所有YUC的活性能够完全废除BBM介导的体胚发生。但是，作者通过详细的YUC抑制时间序列研究发现，依赖于YUC的IAA生物合成并不是幼苗中全能细胞重新起始的必须条件。相比而言，YUC可能是作用于后期的体胚发育过程，用以维持体胚发生后期胚胎的发育和生长。本文的研究解决了有关内源生长素合成在BBM等转录因子介导的体胚发生中的作用这一长期存在的问题，同时也为理解内源生长素合成在其它植物体内和体外胚胎发生系统中的作用提供了新的试验框架。