Since fall is my favorite season, I decided to see if there’s a microbe that’s unique to pumpkins. A quick search told me no, not really, but I also found that Pseudomonas syringae causes warts to form on pumpkins (to temper expectations, the bacterium is not behind the intentionally warty pumpkins we see at farmstands or in stores). This name sounded familiar as I wrote about the bacterium before for ASM about how this bacterium acts as an ice nucleus, which is a particle which ice forms around. While that’s super cool (literally), P. syringae is more than that. It’s a common plant pathogen affecting many crops worldwide.
How was P. syringae discovered?
P. syringae was first isolated in 1899 by Martinus Beijerinck, a microbiologist and botanist who discovered viruses. It gets its name because it was found on a diseased lilac, which has the scientific name Syringa vulgaris.
Pseudomonas syringae‘s superpower: Making snow
So P. syringae doesn’t “make snow” per se, but it helps get the process started. The bacterium acts as an “ice nucleus” which means that it helps water condense and form ice around it. In 2016, scientists found out how this actually works. A protein on its surface called InaZ changes the arrangement of water molecules so that they are in a lattice pattern, like how they are in ice. P. syringae has been found in ice, snow, and hail and is adapted to survive at high altitudes, tolerating solar radiation.
Did you know?
Pure water can be supercooled below its freezing point and remain a liquid as low as -37°C. Adding P. syringae to it makes it nearly instantly freeze as it provides a nucleation site for ice to form around it. See this for yourself:
How Pseudomonas syringae infects plants
P. syringae‘s snowy superpower is cool, but the bacterium is also a common plant pathogen. It can infect many agricultural crops including tomatoes, soybean, kiwi, and other fruits and vegetables, causing billions of dollars in economic loss. It can spread through the air which makes it especially difficult to contain.
P. syringae have two different phases of growth:
- Epiphytic phase: This occurs when the bacterium stays on the surface like leaves, stems or fruits. In this phase, the bacterium doesn’t cause disease. Some strains primarily grow in this phase only.
- Endophytic phase: This occurs when the bacterium gets inside the plant through surface wounds or stomata, natural pore on the leaf’s surface. Once bacteria enters the plant, it colonizes the space between the plant cells. This is where the bacteria multiply and cause lesions on the plant.
Infection with P. syringae causes spots on plants, which can let other harmful bacteria into the plant. Source: Alan Collmer, Cornell University.
Fun fact: P. syringae infects pumpkins and makes them warty
P. syringae infections in pumpkins does give them warts, but these warts aren’t the ones we seek out for our front stoops. Those pumpkins are rather bred for their wartiness. For P. syringae infected pumpkins, the infection can lead to the colonization of other pathogens. Researchers at Washington State University wanted to understand how this pathogen affects pumpkins. By infecting different kinds of squashes, the team noticed that thick-skinned pumpkins were resistant to infection while thinner-skinned gourds were most sensitive.
What microbe should we feature next? Let us know in the comments below!
If you love this, check out my post on the pumpkin microbiome.
Featured image source: jansku136.
