A bacterial effector targets a chromatin-associated regulator to suppress plant immunity

Plants are not passive victims in the face of disease; they possess a sophisticated immune system that acts as a constant surveillance network. When a plant detects the presence of a bacterial invader, it triggers a rapid defense response known as PAMP-triggered immunity. This process begins when the plant recognizes specific molecular patterns common to bacteria, much like a security system identifying a familiar intruder's uniform. This recognition sets off a chain reaction, including the production of reactive oxygen species—chemical bursts that help neutralize the threat—and a reorganization of the plant's internal genetic instructions to produce more defenses. However, successful bacteria have evolved their own countermeasures. They inject proteins called effectors into the plant cells to sabotage these defenses, often by targeting the plant's signaling pathways or by hiding in the cytoplasm to disrupt communication. While scientists have long known that some bacterial invaders can enter the plant's nucleus to interfere with gene activity, the specific mechanisms by which they manipulate the complex machinery of chromatin—the material that packages DNA inside the nucleus—have remained largely a mystery.

A team of researchers at Shanghai Jiao Tong University and other institutions has now uncovered a precise method used by a specific bacterium to disable plant immunity. They focused on Pseudomonas syringae, a common plant pathogen, and its effector protein called HopT1-1. Through a series of experiments involving tobacco, tomato, and Arabidopsis plants, the scientists discovered that HopT1-1 does not simply destroy or hide the plant's defense proteins. Instead, it targets a specific nuclear regulator called LHP1. This protein acts as a chromatin-associated regulator, meaning it helps organize the DNA and control which genes are turned on or off. The researchers found that LHP1 plays a surprisingly positive role in plant immunity; when LHP1 is missing, the plant's ability to detect bacteria and launch a defense response is severely weakened. The bacterium exploits this by having HopT1-1 bind directly to the C-terminal chromo-shadow domain of LHP1. This binding interferes with LHP1's ability to associate with itself, effectively jamming the molecular machinery required for a robust immune response without destroying the protein or moving it out of the nucleus.

The journey to this discovery began with a search for the specific targets of HopT1-1. The researchers knew this protein could suppress the plant's early immune signals, such as the production of reactive oxygen species, but they did not know how. By screening a library of plant genes, they identified LHP1 as a primary target. To confirm this, they used multiple methods, including yeast-based interaction screens and experiments inside living plant cells, which showed that HopT1-1 and LHP1 physically stick together. Crucially, they mapped the interaction to a specific region of LHP1 called the chromo-shadow domain. This domain is known to help LHP1 proteins bind to one another, forming complexes that are essential for their function. The researchers demonstrated that when HopT1-1 is present, it blocks LHP1 from binding to itself. This disruption happens without changing the amount of LHP1 in the cell or moving it to a different location, suggesting the bacterium is subtly altering the protein's behavior rather than removing it.

The biological importance of this interaction was confirmed through genetic experiments in both Arabidopsis and tomato plants. When the researchers created tomato plants that lacked a functional version of LHP1, these plants became significantly more susceptible to bacterial infection. They produced fewer reactive oxygen species when exposed to bacterial signals and allowed bacteria to grow much faster than in normal plants. Similarly, in Arabidopsis, the loss of LHP1 led to a weakened immune response. The study showed that the presence of HopT1-1 in the bacteria was directly responsible for this increased susceptibility. When the bacteria were engineered to lack the HopT1-1 gene, they could no longer cause the same level of damage to plants that were missing LHP1, proving that the bacterium relies on this specific interaction to overcome the plant's defenses.

Further analysis of the plant's genetic activity revealed that LHP1 is essential not just for turning on defense genes when an attack occurs, but also for maintaining a baseline level of readiness. In plants lacking LHP1, many defense-related genes were expressed at lower levels even before any infection took place. This suggests that LHP1 helps prepare the plant's genetic machinery to respond quickly to threats. The researchers also ruled out several alternative explanations for the observed effects. For instance, they showed that the immune defects were not caused by a simple imbalance in salicylic acid, a hormone often associated with plant immunity, nor were they due to the protein being degraded or moved out of the nucleus. The evidence pointed consistently to a model where HopT1-1 acts as a molecular wedge, preventing LHP1 from forming the necessary self-associations required for full immune competence.

This work highlights a sophisticated strategy used by pathogens to manipulate their hosts. Rather than destroying the plant's immune components, the bacterium hijacks a specific interaction-dependent property of a key regulator. By interfering with how LHP1 proteins organize themselves, HopT1-1 compromises the plant's ability to mount a defense. The findings were consistent across two different plant species, Arabidopsis and tomato, indicating that this mechanism is a conserved and effective virulence strategy. The study provides a clear example of how a bacterial effector can exploit the structural dynamics of a nuclear regulator to suppress plant immunity, offering new insights into the ongoing evolutionary arms race between plants and the pathogens that seek to infect them.