There are many ways to live a parasitic lifestyle. But according to a new review in the Annals of Botany by Balios et al, all parasitic plants depend on a single specialized organ for infection, the haustorium.
“The haustorium is a unique organ that is not found in non-parasitic plant species,” write Balios et al, who explain that the term haustorium was coined in the early 19th century “to describe suction structures in spermatophytes that live off other plants” and today is commonly used to describe the “entire composite infection structure” of the plant parasite, with the exclusion of host tissue.
An estimated 1% of land plants use an haustorium to acquire water, inorganic nutrients and organic molecules from their hosts. It is Nature’s perfect graft — a complete continuity between the host and parasitic vascular systems. Balios et al have focused their review on the Cuscuta genus, which consists of over 200 parasitic species of red, orange or yellow plants and belongs to the morning glory family, Convolvulaceae.
Cuscuta, commonly known as dodder or amarbel, but also having more apt folk names such as strangleweed, wizard’s net and witch’s hair, live throughout temperate and tropical regions, with greatest diversity in the subtropics and tropics. Cuscuta are interesting because they develop cytoplasmic continuity and vascular connections with a large range of diverse species, including herbs, bushes and trees, and these host species do not recognize the parasitic cells as foreign, calling into question issues of cell identity and defence. Host species can also include crops. And when infections occur in agricultural settings, consequences on yield and quality can be dire, say Balios et al.
This is “why considerable research efforts have been, and still are, devoted to pinpoint how the parasite’s distribution can be confined, and infection impact mitigated. Haustorium initiation and maturation represent critical stages in the lifecycle of a parasitic plant, and their understanding is pivotal to the ability to control the parasite,” write Balios et al.
Cuscuta are prolific parasites that can simultaneously connect via tens or even hundreds of haustoria to a single or several host plants of different species. In their review, Balios et al focus on what is known about haustoriogenesis in the Cuscuta genus, which has been the subject of decades of research on haustorium architecture as well as newer molecular studies and genome publications. The review covers evolutionary origin, initiation and maturation, cell wall dynamics and solute transport.
One of the more interesting questions addressed is how Cuscuta finds a host to infect. Cuscutainfects host shoots and so cannot rely on root exudates in the soil to find its prey. Balios et al present evidence that Cuscuta uses touch stimulus (thigmotropism) to detect the presence of plants. In support of this notion, Cuscuta will attempt infection of metal rods and sticks when given the opportunity and will even infect itself in an act of self-cannibalism.
Balios et al also present strong evidence for the involvement of phototropism. Far-red light, which is naturally enriched under dense foliage, induces movement in Cuscuta shoots as well as coiling around host stems. Additionally, germinating Cuscuta seedlings grow toward far-red light. Balios et al write that Cuscuta is attracted to far-red light because it “provides the parasite with an efficient means to detect areas with high host abundance, or in other words, denser vegetation”.
After Cuscuta detects its prey, infection is precipitated when “long and flexible tentacle-like cells termed ‘hyphae’ start to grow outward from its tip and sides into the host tissue,” write Balios et al. These hyphae establish connections with the host’s vascular cells and consequently there is an “emerging view of the haustorium as being a natural graft and, thus, a valuable resource for crop improvement by grafting,” explain Balios et al.
The authors go on to compare the mechanics and molecular mechanisms of haustorium infection with artificial grafting to shed light on how we might expand our capabilities to artificially graft across diverse plant species.
“The fact that parasitic plants like Cuscuta are further able to connect with a wide variety of host plants while [artificial] grafting is successful with only a very limited range of plant species renders the processes that unfold during haustorial infection into a potential treasure trove for genes that could widen grafting compatibilities,” write Balios et al.
But while Nature’s graft might be perfect, our understanding is not.
“Much remains to be done to elucidate how the enzymatic arsenal in Cuscuta interacts with developmental and immunity components to drive successful haustorial growth and host invasion,” write Balios et al.
READ THE ARTICLE
Balios, V.A., Fischer, K., Bawin, T. and Krause, K. (2025) “One organ to infect them all: the Cuscuta haustorium,” Annals of Botany, 135(5), pp. 823–840. Available at: https://doi.org/10.1093/aob/mcae208.
Cover image: Canva.
The post Curse of the perfect graft: how the haustorium allows parasitic plants to infect appeared first on Botany One.
