Bacteria Trick Plant Cells into Digesting Themselves - Here’s How

By Jenna Gallegos

Plant and animal cells have two ways of getting rid of proteins: autophagy and the ubiquitin proteasome system (UPS). The UPS does most of the work, but cells rely on autophagy for getting rid of extra-large protein complexes.

Proteins degraded by the UPS are marked with a special tag, like a postal address, and delivered to protein-degrading centers called proteasomes. Autophagy literally means “self-eating.” Proteins degraded by autophagy get packaged into little membrane-bound sacks called autophagosomes. The autophagosomes fuse with other membrane-bound sacks full of protein-degrading enzymes and acids.

Autophagy in animals: both friend and foe

Autophagy is important for trapping and destroying invading bacteria and viruses. But in animal cells, the system sometimes goes awry. The food-borne pathogens Brucella and Listeria use the autophagosome-building machinery to create little pockets where the bacteria can grow. This is an example of the evolutionary “arms race” between pathogens and whatever they infect. Bacteria and viruses have been evolving alongside plants and animals for eons: we evolve to resist pathogens, and pathogens evolve counter-attacks, sometimes even using our own defense systems against us.

Autophagy in plants: self-eating turned cannibal

We don’t know as much about autophagy in plants. But this month, The Plant Cell published work by scientists at The Swedish University of Agricultural Sciences (Üstün et al., 2018) showing something very strange.

Autophagy was kick-started when plant cells were infected by a bacterium called Pseudomonas. But mutant plants that couldn’t perform autophagy were actually more resistant to infection. That meant the bacterium might be somehow using autophagy to make the infection worse.

To figure out what was going on, the researchers looked at which plant genes become hyperactive following Pseudomonas infection. They identified genes that are important for autophagy and genes that are active when the UPS is shut down. So in infected plants, autophagy was cranked up and the UPS system was turned down.

To find out if changes to the UPS and autophagy were related, the researchers used fluorescent proteins. The proteasome was fused with a green-fluorescent protein, so that it would glow green. Autophagy proteins were fused with a red-fluorescent protein, so that they would glow red. The red and green glowing proteins localized to the same part of the cell. It looked like UPS proteins were being trapped and degraded by autophagosomes: not a good sign for the plants.

Understanding autophagy to create tougher plants

The researchers concluded that when plant cells are infected by Pseudomonas, two thing happen:

1. The plant cells recognize the infection and use autophagy to trap the pathogen, and

2. The bacteria hijack the plant’s autophagy system and use it to attack the plant’s proteosome.

This discovery is important, because if we can figure out exactly how the bacteria is killing the plant, we may be able to help the plant defend itself.

The scientists discovered two key players in the battle over autophagy. A bacterial protein called HopM1 helps the pathogen hijack the autophagy machinery and attack the proteasome. A plant protein called NBR1 helps the plant destroy the pathogen via autophagy. If scientists can figure out how to disable HopM1 or to help NBR1 do its job, they could put the plants back in control, and prevent the bacteria from using autophagy against them.

Pseudomonas creates nasty “water-soaked lesions” in plants like tomatoes (as shown in the photo above), so protecting plants from infection means protecting our food supply.

Jenna Gallegos

Chemical & Biological Engineering

Colorado State University

Jenna.E.Gallegos@gmail.com

ORCID: 0000–0003–4875–8163

Read the research paper on which this story is based:

Suayib Üstün, Anders Hafrén, Qinsong Liu, Richard S Marshall, Elena A. Minina, Peter Bozhkov, Richard D. Vierstra, Daniel Hofius (2018). Bacteria exploit autophagy for proteasome degradation and enhanced virulence in plants. Plant Cell Published March, 2018. DOI: https://doi.org/10.1105/tpc.17.00815.

Further reading:

Mostowy, S. (2013). Autophagy and bacterial clearance: a not so clear picture. Cellular Microbiology. 15: 395–402.

Image credit: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org