Ignored or disdained as a “weed tree” for many years by forest industrialists, Pacific or western yew received an unsustainable amount of interest in the last few decades of the 20th century due to the presence of paclitaxel in its bark.
Paclitaxel is used in the treatment of a range of cancers, including “ovarian cancer, breast cancer, lung cancer, Kaposi sarcoma, cervical cancer, and pancreatic cancer”. Prior to the discoveries of total synthesis of the compound in the early 1990s, paclitaxel had to be extracted from Pacific yew bark. This resulted in the death of the tree. Between exploitation for the chemical compound and forestry practices, the global population of Taxus brevifolia is estimated to have declined between 10 and 30% within its last three generations (a generation being roughly 30 years); this resulted in a IUCN Red List status of Near Threatened.
Taxus brevifolia has a range that spans both the coastal and inland temperate rainforests of western North America. At lower and wetter elevations, it is a small understory tree in forests otherwise dominated by larger conifers. Occasionally, in drier conditions, it will form a sprawling shrub. See the Gymnosperm Database for an excellent write-up and additional photographs: Taxus brevifolia.
Today’s photographs highlight two features of Pacific yew that are uncommon in conifers. The first is the fleshy red cone or aril. Encompassing a single seed, the aril is derived from a single modified seed scale which does not lignify (become woody). It is notably mucilaginous when crushed between one’s fingers. The second photograph is my attempt at showing how even the trunks of established trees can layer; when trunks or branches of Taxus brevifolia touch the ground (or in this case the tree trunk of a larger fallen tree), adventitious roots can be formed in order to increase nutrient and water uptake.
Yes, until the 1990’s we could sell or get rid of a lot of Taxus (baccata) shearings on our hedges and topiaries as these were pruned annually… Well now they go on the compost heap. Cheers from arboretum Wespelaar (Belgium). Philippe
Why symbiotic fungi within yews (endophytes) also produce paclitaxel may be quite interesting – https://www.sciencedirect.com/science/article/pii/S0960982215011793
more comprehensive – https://www.sciencedirect.com/science/article/pii/S0960982215009987
Is there any suggestion of transmission of paclitaxel gene(s) between the tree & the fungus?
I haven’t spent much time on this, but the question had came up in several papers I looked at:
https://www.sciencedirect.com/science/article/pii/S0960982215009987
“As to why yew selected an endophyte that uses Taxol as its fungicidal mechanism, both partners might have been primed—sharing metabolic precursors for Taxol [12] and possibly having compatible signaling pathways.”
https://link.springer.com/article/10.1007/s13225-013-0228-7
(conclusion of this paper differs from previous and later papers)
“The presence of these genes would require the extensive horizontal gene transfer (HGT) between the yew trees and multiple endophytic fungi, representing a pathway with more than 20 steps (Croteau et al. 2006). We find it difficult to believe that this entire pathway could have transferred in an arbitrary manner, and therefore we searched for evidence of DNA transfer involving potential taxane-synthesis gene clusters originating from Taxus plants. Whereas biosynthetic gene clusters are a common features in bacterial genomes and have also been described in fungi (Tudzynski and Hölter 1998; Zhang et al. 2004), there have been few reports of clustered metabolic pathways in plants, and those that do exist tend to be spread over larger genomic regions than their microbial counterparts (Field and Osbourn 2008; Field et al. 2011; Chu et al. 2011). The existence of taxane gene clusters in fungi and plants raises intriguing questions about the origin and evolution of these highly-specialized biosynthetic pathways, and the potential for HGT from fungi to Taxus trees. However, HGT between distantly-related organisms is a rare evolutionary event which is also constrained by the amount of genetic information transferred and genetic barriers involving incompatible regulation and codon usage. This contrasts sharply with the widespread observation of Taxol biosynthesis in many different endophytic fungi (Kurland et al. 2003).”
https://www.ncbi.nlm.nih.gov/pubmed/24460898
“We found that paclitaxel biosynthetic gene candidates in P. aurantiogriseum NRRL 62431 have evolved independently and that horizontal gene transfer between this endophytic fungus and its plant host is unlikely.”
Many thanks for the information Richard. I gather that the answer is probably no. Sorry for the delayed reply.