Silent Battles: How Plants Detect and Defeat Molecular Sabotage

Plants may look peaceful, but at the microscopic level, they are constantly fighting invisible enemies. Bacteria that infect plants are not simple invaders; they bring specialised molecular weapons, the type III secreted effectors, designed to sabotage plant cells. In response, plants have evolved sophisticated surveillance systems that can detect these attacks and trigger efficient defence responses. One of the most dramatic immune reactions is the hypersensitive response (HR), a process where infected cells rapidly die to prevent the pathogen from spreading further.

YopJ family effectors are powerful weapons facilitating bacterial infection in animals and plants. They are delivered by several important crop pathogens, including the causative agents of the bacterial speck, bacterial spot, and bacterial wilt diseases of tomato. These bacterial enzymes can attach a small chemical group (acetyl) to plant proteins or, in some cases, cleave them in order to block the host defence responses.

But plants are not defenceless. In fact, they harbour a vast array of immune receptors that monitor such unusual activity as the modifications catalysed by the YopJ family effectors. One of these immune receptors, ZAR1, detects effector activity and turns on robust immune responses, including the HR.

In the model plant Nicotiana benthamiana, NbZAR1 functions together with the accessory protein JIM2 to detect several YopJ family effectors. Surprisingly, two YopJ family effectors are also detected by another immune receptor, NbPtr1. Can this plant recognise any YopJ family effectors? What is the telltale that sets off NbZAR1 and NbPtr1?

To answer these questions, PhD candidate Injae Kim in the group of Prof. Cécile Segonzac at Seoul National University used the versatile Agrobacterium-mediated transformation system to express YopJ family effectors and monitored the HR in Nicotiana benthamiana plants equipped or not with NbZAR1 and NbPtr1. Besides a clear demonstration of the genetic bases of YopJ family effector recognition, this work confirmed that both immune receptors monitor the modifications resulting from the YopJ effector activity. Unexpected findings came from experiments where researchers used a plant deacetylase enzyme called SOBER1. SOBER1 could specifically block NbPtr1 or NbZAR1 activation, depending on the group of the recognised YopJ family effectors. This suggests that YopJ family effectors are recognised through modifications of diverse targets that are monitored independently by the two immune receptors.

This work will accelerate the search for the plant proteins that are “guarded” by NbZAR1 and NbPtr1, allowing us to develop strategies aimed at transferring the resistance to bacterial diseases to important horticultural crops such as tomato.