Distinct cell-specific expression of homospermidine synthase involved in pyrrolizidine alkaloid biosynthesis in three species of the boraginales.

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.