南澳洲植物筆記 002.｜Harlequin Mistletoe ——與聖誕節密不可分的榭寄生與它的夥伴們
By: Kantine Liu
Tags: Flora, Australia, 植物筆記, Loranthaceae, 澳洲

Harlequin mistletoe and its parasitic plant companions

On a hot summer afternoon, I came across a few Native Cherry (Excarpos cupressiformis) individuals growing in a circle along the trail while hiking in the Belair National Park. Although having ‘cherry’ in its common name, Native Cherry in fact is evolutionarily distant from the cherry we buy from a supermarket or farmer’s market, as the later is from the Rosaceae family while the former belongs to the Santalaceae family.

Upon a closer look, I soon noticed there were several stems with elongated and broader light green leaves and eye-catching tube-like red flowers standing out from all the tiny, scaly leaves of Native Cherry that evolved to cope with dry forest and woodland condition. These seemly out-of-place stems are from the plant we are going to share in this article — — Lysiana exocarpi, commonly called Harlequin mistletoe, is one of the most common parasitic plants we can find in the Adelaide hills.

What is a parasitic plant? How is it different from other plants?

Before diving into the mistletoe itself, let’s begin by knowing what defines a parasitic plant and why they are different to normal plants. For a plant to be parasitic, it will have a specialised structure called ‘Haustoria’ by botanists and plant scientists and it is by having haustoria that parasitic plants can penetrate through its host’s tissues and connect with xylem and or phylum, the channels that transport water and nutrients in plants. Depending on where it parasites on the host, parasitic plants can be divided into stem parasites or root parasites. Furthermore and maybe one of the most striking differences is that some parasitic plants evolved not only to steal resources from others but also to become completely dependent on the host to the point that they no longer retain their own leaves to produce their own sugar and only appear when it is time for them to reproduce. ‘Holoparasite’ is the term used to describe such parasitic traits, whereas ‘Hemiparasite’ refers to parasitic plants that are only partially dependent on the host and still have their leaves for producing their own sugars and other nutrients.

The Harlequin mistletoe that grew on the Native Cherry is one of the stem hemiparasites. However, most interestingly its host, the Native Cherry, is also a parasitic plant itself stealing water from its surrounding host plants through their roots. Plant scientists call this phenomenon ‘Hyperparasitism’ because for a parasitic plant to successfully parasite another parasitic plant it requires further physiological adaptations to achieve this outcome.

The origin of parasitic plants

In order to understand the evolution of parasitic plants, scientists used mitochondria DNA to reconstruct their evolutionary relationships and found that ‘parasitism’ had evolved independently in flowering plants at least eleven times (Barckman et al. 2007). In other words, this means that while we use parasitic plants as a collective term these species are in fact originated from different ancestors and evolved to be parasitic separately. But nonetheless, the process of using haustoria to enter their hosts can be summed up into the following steps: location, attachment, invasion and connection. Each stage involves a complex series of chemical signalling and utilisation so that the parasitic plant can pass through the host defence system accessing water and nutrients for its own benefit.

Lysiana exocarpi

This species belongs to Loranthaceae and is endemic to and wide-spread in Australia. Its hosts range from different families but not include Eucalyptus species. In South Australia, it can be found from coastal areas to further inland except the Kangaroo Island. It is one of the most common mistletoes you will come across in Adelaide, another common one is Amyema miquelii.

Perhaps the most well-known fact about mistletoes’ relationships with animals is that they have a close interaction with the mistletoe bird which the bird feeds on its fruit and help spread the seeds by wiping off the seeds covering in sticky seed coats onto the branches they perch on. This is not an exclusive relationship since other birds also feed on mistletoe fruits and disperse the seeds in a similar way, but it still highlights the ecological importance the parasitic plants serve as a food source. Another key ecological role played by the parasitic plants is that they help redistribute nutrients in the ecosystem where they are living in by concentrating resources in reach leave litters as they accumulate valuable molecules in their tissues.

So while they may appear to be the ultimate villain in nature, the parasitic plants are indeed a non-dispensable part of the ecosystem and provide various important ecological functions to other living organisms. Next time in the wild why not looking around your surroundings you never know if there is a parasitic plant right next to you!

參考資料 Reference

Barkman, T. J., McNeal, J. R., Lim, S.-H., Coat, G., Croom, H. B., Young, N. D., and dePamphilis, C. W. (2007). Mitochondrial DNA suggests at least 11 origins of parasitism in angiosperms and reveals genomic chimerism in parasitic plants. BMC evolutionary biology 7, 248–248. doi: 10.1186/1471–2148–7–248.

Clarke, I. (2003) ‘Name that flower : the identification of flowering plants.’ 2nd ed. edn. (Melbourne University Publishing: Carlton, Vic.)

Department for Environment and Water (2019) ‘Understand the connection between some of SA’s flora and fauna.’ Available at https://www.anbg.gov.au/leptospermum/ [Accessed 24 December 2020]

Heide-Jørgensen, H. S. (2008) ‘Parasitic flowering plants.’ (Brill: Leiden ;.)

State Herbarium of South Australia (2007) ‘Lysiana exocarpi’ Available at https://bie.ala.org.au/species/https://id.biodiversity.org.au/node/apni/2921161 [Accessed 24 December 2020]