Pseudomonas syringae is a widespread bacterium that causes disease in a variety of plants, impacting agricultural crops and natural ecosystems. It is a significant concern for plant health globally, leading to substantial economic losses in farming.
Understanding Pseudomonas syringae
Pseudomonas syringae is a rod-shaped, Gram-negative bacterium with whip-like flagella, allowing it to move through water and soil. It is commonly found in various natural environments, including on plant surfaces, in soil, and in water bodies.
The bacterium exhibits considerable diversity, with over 50 distinct types, known as pathovars. Each pathovar specializes in infecting a particular group of host plants, though some can affect a broad range of species. For instance, P. syringae pv. syringae was originally isolated from lilac but can infect many dicotyledonous and monocotyledonous hosts.
Diseases It Causes
Pseudomonas syringae causes a variety of visible symptoms on infected plants. Common signs include necrotic leaf spots—dark, dead areas on leaves that can expand into brown lesions with yellow halos. These spots may lead to a “shot-hole” appearance as the dead tissue falls out.
The bacterium can also cause blights, characterized by the rapid browning and death of flowers, buds, and young shoots, often leading to blossom blast. On woody plants, Pseudomonas syringae can lead to the formation of cankers, which are sunken, discolored areas on stems and branches. These cankers can girdle stems, disrupting the flow of water and nutrients, and potentially causing dieback of branches or even the entire plant. In some cases, a gummy substance may exude from cankers on stone fruits, a symptom referred to as “gummosis”.
Specific examples of diseases include:
- Bacterial blight on soybeans
- Halo blight on beans
- Bacterial canker on cherry and plum trees
- Bacterial speck on tomatoes (P. syringae pv. tomato)
- Bleeding canker on horse chestnut trees (P. syringae pv. aesculi)
- Spots and blisters on fruit
How Pseudomonas syringae Attacks Plants
Pseudomonas syringae employs several strategies to infect plants and establish disease. The bacteria commonly enter plants through natural openings like stomata on leaves or through wounds, as they are generally unable to breach the intact plant cell wall. Once inside, they multiply in the intercellular spaces of plant tissues, known as the apoplast.
To overcome plant defenses, Pseudomonas syringae injects various effector proteins into plant cells using a specialized delivery system. These proteins suppress the plant’s immune responses, allowing the bacteria to proliferate. For example, effectors like HopAF1 can target plant processes such as methionine recycling, involved in ethylene biosynthesis. Other effectors, such as AvrPto and AvrPtoB, directly interfere with plant immune receptors.
The bacterium also produces toxins that injure plant cells and contribute to disease development. One such toxin is coronatine, which mimics a plant hormone called jasmonic acid isoleucine. Coronatine helps the bacteria enter the plant by preventing stomata from closing and promotes bacterial growth within plant tissues. Another group of toxins, syringomycins, can form pores in plant cell membranes, leading to leakage of ions and eventual cell death, releasing nutrients for the bacteria.
A unique virulence mechanism of some Pseudomonas syringae strains is their ability to produce ice-nucleating proteins (INPs). These proteins, located on the bacterial surface, act as catalysts for ice crystal formation at temperatures warmer than normal freezing points, sometimes as high as -1.8 °C (28.8 °F). The formation of ice crystals damages plant tissues, creating wounds and releasing nutrients that the bacteria can then utilize for growth and further infection. This freezing injury can significantly increase the severity of disease, particularly in sensitive plants like stone fruits.
Factors Influencing Disease Development
The development and severity of diseases caused by Pseudomonas syringae are influenced by environmental conditions. Wet and cool conditions are favorable for disease outbreaks, with optimal temperatures often ranging from 12°C to 25°C (54°F to 77°F). High humidity and rainfall facilitate bacterial spread between plants, often through rain splash, and promote bacterial multiplication on leaf surfaces.
Physical damage to plants provides easy entry points for the bacteria. Wounds from mechanical injury, such as pruning cuts, or environmental stressors like frost damage, can predispose plants to infection. Frost injury is especially impactful, as the ice crystals formed by the bacteria’s ice-nucleating proteins directly wound plant cells, increasing susceptibility. Plant dormancy can also increase susceptibility, with dormant peach trees, for example, showing greater vulnerability than actively growing ones.
The plant’s surrounding microbial community, or microbiome, also plays a role in disease outcomes. Interactions with other microbes can influence disease severity. Soil factors like pH imbalances or nutrient deficiencies can further weaken plants, making them more susceptible to Pseudomonas syringae infections.