Botanisches Institut und Botanischer Garten, Universität Kiel, D-24098 Kiel, Germany.
Plant Physiol. 2012 Jul;159(3):920-9. doi: 10.1104/pp.112.195024. Epub 2012 May 7.
Homospermidine synthase (HSS) is the first specific enzyme in pyrrolizidine alkaloid (PA) biosynthesis, a pathway involved in the plant's chemical defense. HSS has been shown to be recruited repeatedly by duplication of a gene involved in primary metabolism. Within the lineage of the Boraginales, only one gene duplication event gave rise to HSS. Here, we demonstrate that the tissue-specific expression of HSS in three boraginaceous species, Heliotropium indicum, Symphytum officinale, and Cynoglossum officinale, is unique with respect to plant organ, tissue, and cell type. Within H. indicum, HSS is expressed exclusively in nonspecialized cells of the lower epidermis of young leaves and shoots. In S. officinale, HSS expression has been detected in the cells of the root endodermis and in leaves directly underneath developing inflorescences. In young roots of C. officinale, HSS is detected only in cells of the endodermis, but in a later developmental stage, additionally in the pericycle. The individual expression patterns are compared with those within the Senecioneae lineage (Asteraceae), where HSS expression is reproducibly found in specific cells of the endodermis and the adjacent cortex parenchyma of the roots. The individual expression patterns within the Boraginales species are discussed as being a requirement for the successful recruitment of HSS after gene duplication. The diversity of HSS expression within this lineage adds a further facet to the already diverse patterns of expression that have been observed for HSS in other PA-producing plant lineages, making this PA-specific enzyme one of the most diverse expressed proteins described in the literature.
同型精胺合酶（HSS）是吡咯里西啶生物碱（PA）生物合成中的第一个特定酶，该途径参与植物的化学防御。已经表明，HSS 是通过参与初级代谢的基因重复复制而被多次招募的。在 Boraginales 的谱系中，只有一次基因复制事件导致了 HSS 的产生。在这里，我们证明了在三种滨藜科物种，Heliotropium indicum、Symphytum officinale 和 Cynoglossum officinale 中，HSS 的组织特异性表达在植物器官、组织和细胞类型方面是独特的。在 H. indicum 中，HSS 仅在幼叶和嫩枝下表皮的非特化细胞中表达。在 S. officinale 中，HSS 的表达已在根内皮层的细胞和正在发育的花序下的叶片中检测到。在 C. officinale 的幼根中，仅在内皮层的细胞中检测到 HSS，但在稍后的发育阶段，在外皮层中也检测到。将各个表达模式与 Senecioneae 谱系（菊科）中的表达模式进行比较，在该谱系中，HSS 表达可重复地在根的内皮层和相邻皮层薄壁细胞的特定细胞中检测到。讨论了滨藜科物种中的各个表达模式，因为这是基因复制后成功招募 HSS 的要求。在该谱系中，HSS 的表达多样性增加了在其他产生 PA 的植物谱系中已经观察到的 HSS 表达的多样性，使这种 PA 特异性酶成为文献中描述的最多样化表达的蛋白质之一。
