Parasite organ formation
Parasitic plants need another plant to survive. They latch with a special organ, named an haustorium, to their host plants, wriggling themselves inside towards the veins. There they siphon away the nutrients they need.
Seeds of parasitic plants germinate often only when they perceive specific signals that indicate that there is a host plant nearby. When the germinating plants also receive a subsequent signal, a so called haustorium inducing factor, then they start developing a haustorium.
But not much is known about the development of haustorium. After perceiving a haustorium inducing factor the cell sends out a calcium signal. This eventually results in a local auxin maximum stimulating the development of the haustorium. But what is causing the auxin biosynthesis genes to be activated is unknown.
Now do Japanese researchers show in “Neofunctionalized RGF pathways drive haustorial organogenesis in parasitic plants” that peptide growth factors have a role in this.
Signal molecules
In the Japanese study the researchers studied the development of haustoria in the parasitic plant: Phtheirospermum japonicum. They where especially interested in signal molecules that are activated after perception of the haustorium inducing factor. Specifically, they looked at possible peptide signal molecules.
In this way the researchers discovered that the root meristem growth factor RGF1, 2, and 5 were more abundant after exposure to the haustorium inducing factor. Subsequently the researchers analysed if RGF1, 2, or 5 could activate auxin biosynthesis. This turned out to be the case.
Now the researchers wanted to know where in the rood RGF1, 2, or 5 are located. Through coupling to a fluorescent protein, the researchers could see where the RGFs accumulated. Under normal conditions RGF1 is present everywhere in the root, RGF2 was absent, and RGF5 was located only in specific cells. After exposure to a haustorium inducing factor RGF2 and 5 accumulated in large quantities in the developing haustorium. The location of RGF1 did not change.
Perceiving RGFs
The next question the researchers had was which receptors are perceiving these RGFs, as RGFs by themselves can not directly activate genes. Therefore, the researchers studied RGF-receptors. First, they looked at which RGF-receptors were active in haustoria. This turned out to be RGF-receptor1, 2, 3, 4, and 5. Subsequently the researchers analysed which of those RGF-receptors bind RGF2 and RGF5. For RGF2 this was RGF-receptor1, 3, and 4. While RGF5 only bound to RGF-receptor3 and 4.
The ultimate test to confirm that the discovered RGFs and their corresponding RGF-receptors are indeed involved in the regulation of the development of haustoria is showing that in absence of these growth factors and receptors no haustoria develop. Therefore, the researchers developed plants without one of these growth factors or receptors. To the disappointment of the researchers this did not give a clear answer. In absence of one of these RGFs or RGF-receptors the parasitic plant could still develop haustoria. Although at a slightly lower frequency.
This suggests that the different RGFs and RGF-receptors can take over each other’s function. To confirm to the researchers tried to develop parasitic plants in which lacked multiple RGFs or RGF-receptors. Unfortunately, these did not survive. So further research is needed.
Literature
Maxwell R. Fishman et al., (2025) Neofunctionalized RGF pathways drive haustorial organogenesis in parasitic plants.Sci. Adv.11, eadw3965. https://www.science.org/doi/10.1126/sciadv.adw3965
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