Anne has experience in science research and writing. She has a graduate degree in nutrition (gut microbiome & nutritional microbiology) and undergraduate degrees in microbiology (immunology & medical microbiology) and English (myth & folklore). She has also worked as an ocean & Earth science educator.
What is Pseudomonas Syringae?
Pseudomonas syringae is a plant and water-dwelling species of bacteria with more than 60 known pathogenic variants, also known as pathovars. Only a few are non-pathogenic variants that benefit plants by competitive inhibition. One of the distinguishing features of Pseudomonas species is their ability to produce fluorescent pigments.
P. syringae is very well-studied species and serves as a model microorganism for understanding the processes of bacterial virulence and infection in plants.
Like all Pseudomonas species, P. syringae is an obligate anaerobe with flagellated, motile rods that stain Gram-negative. P. syringae is an opportunistic pathogen that infects plant psyllospheres, which include leaves and other parts of the plant that grow above ground. P. syringae is found throughout the world and infects a wide variety of plants, including agricultural crops like green beans, tomatoes, and fruit trees.
A unique ability of P. syringae is ice nucleation, which is the ability to cause water to freeze at temperatures slightly warmer than freezing. The bacteria achieve this feat by producing proteins that catalyze the formation of ice crystals from water droplets on their outer cellular membrane.
Spread and Colonization
Spread of the bacteria occurs primarily via insects, wind, human use of tools on infected and noninfected plants, and by rain via bioprecipitation.
Bioprecipitation is when rain and other precipitation is actually caused by living organisms like bacteria. When P. syringae becomes airborne, it can be pulled into the atmosphere. Using ice nucleation of water droplets in clouds, P. syringae causes bioprecipitation, and so their spread to plants is then facilitated by rain.
Initially, the colonization of leaf surfaces by P. syringae is harmless. Once established on the surface of leaves, P. syringae starts reproducing via binary fission. Most pathovars are limited in specificity to only one host plant, but P. syringae pv. syringae can infect more than 80 different species of plants.
Infection and Disease
Plant apoplasts are the intracellular spaces below the leaf surface that include the xylem and plant cell wall. When humidity rises despite low temperatures during the spring season, P. syringae use their type III secretion system (T3SS), a syringe-like protein appendage, to inject proteins into plant apoplasts. Infiltration of apoplasts disrupt water and nutrient transport as well as the function of various plant proteins, hormones, and chemical messengers.
P. syringae also produces phytotoxins that inhibit plant protein synthesis and creates holes in plant cell membranes. Damage from phytotoxins combined with ice nucleation leads to leaf necrosis, along with frost damage from ice crystal formation at temperatures where a plant would not normally freeze.
The infective process continues throughout the year, with brown spot disease emerging on leaves in the summer during periods of rain or high humidity. Flowers may whither and blacken in a condition called blossom blast. As the bacteria infiltrate branches, the formation of new buds is prevented, and lesions called stem cankers form. Cankers cause another condition called gummosis where a sticky, gooey substance bleeds from the cankers, eventually killing stems, branches, or the entire plant during the upcoming fall and winter seasons. During the winter or other times inhospitable to their growth, P. syringae may hide in buds. As the bacteria spread to uninfected plants, the cycle begins again.
Pseudomonas syringae is a well-studied Gram-negative bacteria and plant pathogen that infects plants worldwide. It is unique for its ability to perform ice nucleation, which causes water to freeze at temperatures warmer than freezing. Ice nucleation also plays a key role in bioprecipitation, which is the seeding of clouds and subsequent rain.
After making contact with plants via the wind, rain, or insects, P. syringae pathovars colonize the leaves of plants. Initially, they do not cause disease. However, during the humid, cold springtime weather, they become infectious and use their type III secretion system (T3SS), a syringe-like protein appendage, to inject phytotoxins into plants that inhibit plant protein synthesis and creates holes in plant cell membranes.
In the summer months, plants start developing brown spot disease followed by stem cankers and gummosis, where a gooey substance bleeds from the cankers. The plant may suffer significant necrotic damage and possibly die during the upcoming fall and winter months. The bacteria spread to uninfected plants for the cycle to start anew.
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