The Wheat NLR Protein PM3b Localises to Endoplasmic Reticulum–Plasma Membrane Contact Sites and Interacts With AVRPM3b2/c2 Through Its LRR Domain

Plants and pathogens are locked in a constant arms race, with each side evolving new strategies to outsmart the other. In this study, the authors explore how wheat defends itself against a fungal pathogen called powdery mildew. Powdery mildew uses special “effector” proteins, such as AVRPM3^b2/c2, to enter plant cells and weaken their defenses. However, wheat fights back using an immune protein called PM3b.

PM3b belongs to a group of proteins called NLRs (nucleotide-binding leucine-rich repeat proteins), which act like security sensors inside plant cells. These sensors detect invaders like AVRPM3^b2/c2 and trigger the plant’s immune response to stop the attack. What makes PM3b unique is its location within the cell. Instead of moving freely, it is found at specific contact points between the endoplasmic reticulum (ER), the cell’s protein factory, and the plasma membrane, the cell’s outer layer. These contact points appear to act as hubs for signaling and defense.

Our research showed that PM3b interacts with AVRPM3^b2/c2 using part of its structure called the leucine-rich repeat (LRR) domain. However, this interaction is temporary. Once PM3b is activated, it releases the effector. This suggests that activation causes PM3b to change shape or behavior, allowing it to trigger immune responses. Interestingly, when PM3b is mutated so that it cannot activate, it continues to bind the effector, showing that activation is critical for breaking this connection.

This study highlights the incredible precision of plant immune systems and how specific proteins like PM3b help wheat defend itself. By better understanding how PM3b and similar proteins function, scientists can develop new wheat varieties that are resistant to powdery mildew, helping to protect crops and ensure food security in the future.

TITLE IMAGE: PM3b localises into puncta at SYT1-type endoplasmic reticulum (ER)-plasma membrane (PM) contact sites (S-EPCSs) and is excluded from the VAP27-1-type EPCSs (V-EPCSs) in Nicotiana benthamiana. (a) Images of PM3b- eGFP co-expressed with markers for the ER (SP-CFP-HDEL) and the V-EPCS marker VAP27-1-tagRFP are shown individually. Image of a
merge of PM3b- eGFP with both markers SP-CFP-HDEL and VAP27-1-tagRFP is also shown. Pictures were taken 48-72 h post- infiltration. (b) Images of PM3b-eGFP co- expressed with markers for the ER (SP-CFP-HDEL) and the S-EPCS marker SYT1-tagRFP are shown individually. Images of a merge of PM3b-eGFP both markers SP-CFP-HDEL and SYT1-tagRFP is also shown. Pictures were taken 48–72 h post- infiltration (hpi). Images were taken 48–72 hpi using confocal laser scanning microscopy. Each confocal image represents a single plane of a cell. Scale bars are as indicated.