Plants can get sick in a variety of ways. Perhaps one of the strangest is when a plant produces a large number of branches or shoots close together to form what is referred to as a “witches’ broom”. The plants stop reproducing and become locked into a “forever young” state – repeating their growth of shoots or branches from the same part of the plant again and again.
Such plants have fallen victim to parasitic phytoplasma bacteria which have reduced them to “zombies” – a husk of their former selves fit only to be the habitats and hosts of the bacteria. These bacteria can affect a wide range of food crops and other economically important plants like sugar cane and may cause a range of different diseases with similar symptoms, such as phyllody (where flower structures become leaves), virescence (where parts of the plant that are not usually green become green) and chlorosis (where plants do not produce enough chlorophyll and become pale or yellow in colour).
The way that phytoplasma bacteria enslave their host plant was not clear until very recently, when researchers found that the protein SAP05 produced by the bacteria manipulates the host plant’s own cellular machinery. SAP05 tags the host plant’s growth and development proteins in such a way that the plant believes it is marked for termination, and its own proteasome dismantles it. The plant therefore becomes stuck in a mode where it will not reproduce and favours continuously growing shoots.
Because the bacteria that produces SAP05 infects plants via insect hosts, the researchers also investigated whether SAP05 interacts in a similar way with these insects. They discovered that the protein could not bind to the cellular recycling machinery (the proteasome) in these insects because of two amino acid differences between the insect and plant proteasomes. This means that if the plant proteasome is edited to use these two amino acid changes instead it will no longer be able to be targeted by SAP05 and the host plant would be immune to the curse of eternal youth offered by the phytoplasma bacteria. Furthermore, one would probably be able to use gene editing techniques to introduce these two amino acid changes to important crops, making them resistant to infection by phytoplasma bacteria.
Text: Dr Cora Stobie (Department of Animal and Plant Systematics).
Image 1: Botanical illustration of Arabidopsis thaliana. Credit: Wikipedia.
Image 2: A comparison of transgenic Arabidopsis thaliana plants producing either Green Fluorescent Protein (GFP – left) or SAP05 (right) grown under Long-Day (LD) conditions for 10 weeks. This figure was cropped from Figure S1. Reproduced under Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/) from the publication https://doi.org/10.1016/j.cell.2021.08.029
For more information, please refer to the press release by one of the researcher’s host institutions: https://www.jic.ac.uk/press-release/the-microbial-molecule-that-turns-plants-into-zombies/
Or the academic paper for all the details:
Huang, W. et al. (2021). Parasitic modulation of host development by ubiquitin-independent protein degradation. Cell, 184(20), 5201-5214. https://doi.org/10.1016/j.cell.2021.08.029