Is Parasitaxus Parasitic?

Parasitaxus usta (Viellard) de Laubenfels* is a rare gymnosperm of the family Podocarpaceae. It is found only in New Caledonia where it occurs in remote, densely forested areas. It was first discovered and described by Vieillard (1861).

Parasitaxus is a woody shrub or small tree, up to 1.5 m in height, with fleshy deep wine red to purple scale leaves. The plant lacks roots and is always found attached to the roots of Falcatifolium taxoides (Brongn. & Gris) de Laubenfels, which (strangely) is also a member of Podocarpaceae. There has been much debate about the parasitic status of this plant for several reasons. First, a typical haustorium is not formed and the connection to the "host" resembles a root graft. The xylem-to-xylem connections were documented by Köpke, Musselman and DeLaubenfels (1981) and described as an "obligate root graft."

Woltz et al. (1994) made additional observations and their findings are summarized as follows:

"...Penetration of the parasite up to the cambium of the host is accomplished under the form of fascicular system. The haustorial apparatus is localized at the level of the cambium. Host and parasite are infected by a mycelian endophyte showing appressoria and VA [vessicular arbuscular mycorrhizae]. Fungal fruiting bodies (fructifications) show a characteristic conidiocarp of the Fungi Imperfecti (Deuteromycotina). It seems that this unique parasite is linked to a mycelian symbiosis."

So at that time, Parasitaxus was considered to be a mycotrophic epiphyte. More recently, Taylor Field and Tim Brodribb (2005) clarified the parasitic nature of Parasitaxus. In that paper they state:

"Here we show that Parasitaxus is definitively parasitic, but it displays a physiological habit unlike any known angiosperm parasite. Despite possessing chloroplasts, it was found that the burgundy red shoots of Parasitaxus lack significant photosynthetic electron transport. However unlike nonphotosynthetic angiosperm parasites (holoparasites), tissues of Parasitaxus are considerably enriched in13carbon relative to its host. In line with anatomical observations of fungal hyphae embedded in the parasite/host union, stable carbon isotopic measurements indicate that carbon transport from the host to Parasitaxus most likely involves a fungal partner. Therefore, Parasitaxus parallels fungus feeding angiosperms (mycoheterotrophs) that steal carbon from soil mycorrhizal fungi. Yet with its tree-like habit, association with fungi residing within the host union, high stomatal conductance, and low water potential, it is demonstrated that Parasitaxus functions unlike any known angiosperm mycoheterotroph or holoparasite. Parasitaxus appears to present a unique physiological chimera of mistletoe- like water relations and fungal-mediated carbon trafficking from the host."

Photographs of Parasitaxus usta

References

De Laubenfels, D. J. 1959. Parasitic conifer found in New Caledonia. Science, N.Y. 130: 97.

De Laubenfels, D. J. 1972. Flore de la Nouvelle Caledonia et Dependances. No. 4. gymnospermes. Muséum national d`histoire naturelle, Paris.

Feild, T.S. T. J. Brodribb.  2005. A unique mode of parasitism in the conifer coral tree Parasitaxus ustus (Podocarpaceae) Plant, Cell and Environment 28: 1316–1325.

Gray, N. E. 1953. A taxonomic revision of Podocarpus. VIII. The African species of section Eupodocarpus, subsection A and E. J. Arnold Arb. 34:163-175.

Gray, N. E. 1955. A taxonomic revision of Podocarpus. IX. The South pacific species of section Eupodocarpus, subsection F. J. Arnold Arb. 36:199-206.

Gray, N. E. 1960. A taxonomic revision of Podocarpus. XII. Section Microcarpus. J. Arnold Arb. 41:36-39.

Gray, N. E. and J. T. Bucholz. 1948. A taxonomic revision of Podocarpus. III. The American species of Podocarpus. J. Arnold Arb. 29:117-122.

Griffith, M. M. 1957. Foliar ontogeny in Podocarpus macrophyllus, with special reference to transfusion tissue. Amer. J. Bot. 44:705-715.

Hair, J. B. and E. J. Beuzenberg. 1958. Chromosomal evolution in the Podocarpaceae. Nature (London) 181:1584-1586.

Jackman, V. H. 1960. The shoot apex of some New Zealand gymnosperms. Phytomorphology 10:145-157.

Köpke, E., L. J. Musselman, and D. J. De Laubenfels. 1981. Studies on the anatomy of Parasitaxus ustus and its root connections. Phytomorophology 31:85-92.

Sinclair, W. T., R. R. Mill, M. F. Gardner, P. Woltz, T. Jaffre, J. Preston, M. L. Hollingsworth, A. Ponge, and M. Moller. 2002. Evolutionary relationships of the New Caledonian heterotrophic conifer, Parasitaxus usta (Podocarpaceae), inferred from chloroplast trnL-F intron/spacer and nuclear rDNA ITS2 sequences. Plant Systematics and Evolution 233: 79-104.

Viellon, J. R. 1976. Archtecture vegetative de quelques arbres de l'Archipel Neo-Caledonien. Ph.D. Thesis, Academie de Montpellier, Hérault, France.

Woltz, P. R. A. Stockey, M. Gondran and J. F. Cherrier. 1994. Interspecific parasitism in the gymnosperms: unpublished data on two endemic New Caledonian Podocarpaceae using scanning electron microscopy. Acta Bot. Gallica 141 (6/7):731-746.


*  The correct spelling of the scientific name of this plant is Parasitaxus usta, not P. ustus. When David de Laubenfels (1972) transferred this species from Podocarpus to Parasitaxus he erroneously wrote the specific epithet as it had been under Podocarpus.  Since then, many authors (see above) have copied the name as given by de Laubenfels (but with some exceptions, e.g. see Sinclair et al. 2002). The name is derived from Podocarpus ustus and when it was transferred to Parasitaxus, the genus (like Taxus) must be treated as feminine, not masculine. Normally the gender of the specific epithet (when an adjective) must agree with the gender of the genus. But with trees this rule is not followed, thus trees are considered feminine (e.g. Quercus alba, not Q. albus).  Under the International Code of Botanical Nomenclature Art. 21.2, the correct gender must be used and, if published wrongly, it has to be corrected under Art. 32.5.  Thanks to Aljos Farjon for pointing this out to me!  DLN


 SIUC / College of Science / The Parasitic Plant Connection: http://www.science.siu.edu/parasitic-plant/parasitaxus.html
Last updated: 27-Sept-11 / dln