The microbial molecule that turns plants into zombies

Arabidopsis gets witches' broom from Phytoplasma by SAP05 Credit: John Innes Centre
Parasitic bacteria has discovered a new manipulation method to slow down the aging process of plants. This could be a way to help protect food crops from disease.

Parasites can manipulate the organisms they live with to their liking, sometimes in very drastic ways. Parasites can cause plants to undergo drastic changes, which is why they are called "zombies". They cease reproducing and become a host and habitat for parasitic pathogens.

This is a complex process that has been poorly understood at both a molecular level and on a mechanistic level.

The John Innes Centre's Hogenhout research group and its collaborators published in Cell has revealed a manipulating molecule that Phytoplasma bacteria produces to hijack plant growth. This protein triggers abnormal growth by causing key growth regulators to become ineffective.

The Phytoplasma bacteria is a microbe that can reprogramme host plants' development. These bacteria are responsible for the "witches' brooms" seen in trees where too many branches grow together.

These bushy outgrowths occur because the plant is stuck in a vegetative zombie state and is unable to reproduce, thus preventing it from becoming a 'forever old' species.

Phytoplasma bacteria can also be a devastating crop disease like Aster Yellows. This causes substantial yield losses in both leaf and grain crops such as lettuce, carrots and cereals.

Professor Saskia Hout, the corresponding author of this study said that phytoplasmas were a remarkable example of how genes can impact the environment beyond their own organisms.

"Our findings shed new light on the molecular mechanism that underlies this extended phenotype. This could be a solution to a major problem in food production. A promising strategy to engineer plants to resist phytoplasmas is highlighted in this paper.

These new findings reveal how SAP05, a bacterial protein that manipulates plants using some of its own molecular machinery.

The proteasome is a machinery that breaks down proteins no longer required by plant cells. SAP05 takes over this process and causes plant proteins, which are critical in controlling growth and development to be effectively thrown in a molecular recycle centre.

These proteins are essential for the plant's growth. They favor bacteria and trigger multiple vegetative shoots, tissues, and stop the plant from ageing.

The team discovered the exact role of SAP05 through biochemical and genetic experiments on the model Arabidopsis Thaiana.

SAP05 is able to bind directly to both plant developmental proteins as well as the proteasome. Direct binding is a new way to destroy proteins. The proteasome normally tags proteins with a molecule called Ubiquitin before they are destroyed. However, this is not the case in this instance.

SAP05 targets plant developmental proteins similar to those found in animals. The team was curious to find out if SAP05 also affected the insects that carry the bacteria from plant to plant. The structure of the host proteins in the animals was so different that SAP05 did not interact with them, which meant that it didn't affect the insects.

This investigation led to the discovery of two amino acids within the proteasome unit which are required to interact with SAP05. The team discovered that SAP05 can be switched to make plant proteins have the two amino acid found in insect protein. This prevents abnormal growth.

This discovery opens up the possibility to tweak just these amino acids in crops, such as using gene-editing technology, to improve the durability of phytoplasmas and to reduce the impact of SAP05.

Parasitic modulation of host development by ubiquitin-independent protein degradation, appears in Cell DOI:10.1016/j.cell.2021.08.029

More information: Parasitic modulation of host development by ubiquitin-independent protein degradation, Cell DOI: 10.1016/j.cell.2021.08.029 Journal information: Cell Parasitic modulation of host development by ubiquitin-independent protein degradation,