When a plant cell is under severe stress, it faces a decision: repair itself or die. This study identifies a protein that helps make that decision—and can push the cell toward either outcome. Inside a plant cell, the compartment that folds newly made proteins can become overwhelmed, causing a potentially lethal condition called ER stress.

The cell then starts a repair response that restores balance, or activates programmed cell death when the stress cannot be resolved. The difficult question is how that response decides between survival and death, especially in plants. BAP2 is described as a cellular rheostat: like a dimmer switch in a room, it adjusts the strength of the cell’s response rather than simply turning it on or off.

During ER stress, BAP2 monitors whether the IRE1-bZIP60 repair pathway is sufficient and acts as a pro-death effector when that pathway is insufficient. The researchers found that natural Arabidopsis varieties differed greatly in how strongly their growth was affected by ER stress.

Those varieties also differed in the activity of genes controlled mainly by the IRE1-bZIP60 arm of the repair response. Natural plant varieties differ dramatically in how well they keep growing during this form of cell stress: one retains nearly all its growth, while another is reduced to roughly one-fifth.

That spread gives researchers a way to find the genes controlling stress tolerance. Compared with normal plants, the BAP2 knockout showed reduced shoot fresh weight and chlorophyll, while the IRE1 mutant showed a strong reduction under the same stress.

Under TM treatment, IRE1 loss caused a strong reduction, whereas BAP2 loss caused a significant reduction in growth and chlorophyll. The same pattern appeared with DTT, another ER-stress inducer: IRE1 loss was drastic, while BAP2 loss was moderate but significant. These results indicate that BAP2 is required to enhance stress tolerance when the repair response is sufficient.

Est-0 carried a BAP2 change that replaced one amino acid, changing aspartate to serine at position sixty-seven in its conserved C2 functional domain. The Est-0 allele showed reduced function and was associated with the greater ER-stress sensitivity observed in that variety under ER stress.

By contrast, the Est-0 BAP2 version left plants as stress-sensitive as the mutant, demonstrating reduced function of that allele under ER stress. Together, the results point to reduced function of that BAP2 version as a major cause of the variety’s stress sensitivity.

The altered BAP2 version produced levels of cell damage and harmful peroxide similar to plants lacking BAP2 altogether. This supports the idea that an intact functional region of BAP2 is needed for its survival-promoting role during ER stress. When the cell’s main stress-response pathway is missing, removing BAP2 protects seedlings: they retain more growth, leak fewer cell contents, and build up less damaging peroxide.

This identifies BAP2 as a driver of cell death specifically when IRE1 cannot function. When the repair response becomes insufficient during unresolved ER stress, BAP2 helps ignite programmed cell death. In that situation, BAP2 leads to harmful peroxide buildup and cell death, an irreversible step that outweighs the cell’s survival efforts.

The final choice under stress is therefore a tug-of-war between processes that keep the cell alive and processes that end it. BAP2 acts like a stress-sensitive switch: with a working repair response, it helps cells survive; when that response fails, it helps trigger cell death.

Understanding this balance may reveal how damaged plant cells are managed.