Northern Light Reviews - Rethinking Plant Stress Biology in a Multifactorial World.

The accelerated pace of climate change, soil degradation, environmental pollution, and pathogen outbreaks subjects trees and crops growing in different parts of the world to multiple abiotic/biotic stress conditions, simultaneously or sequentially. Unfortunately, most studies conducted to date, including studies of two-stress combinations, do not mimic the complex multifactorial stress combination (MFSC) environment plants experience in nature. Here, we summarize evidence from recent MFSC studies of three or more stresses simultaneously impacting plants, and argue that environmental complexity, i.e., a high number of interacting stress factors, functions as a system-level variable shaping plant performance. Across different model and crop plants, the progressive increase in stress complexity results in non-additive effects that cause marked declines in growth, photosynthetic rates and yield, even when the level of each individual stress involved in such MFSC remains moderate. Multi-omics analyses reveal that MFSC does not simply induce stress-response pathways but redistributes regulatory control within hormonal networks, most prominently within abscisic acid signaling, accompanied by reduced physiological performance and repression of photosynthesis-related genes. These responses scale nonlinearly with increasing complexity and are consistent with threshold-like shifts in regulatory function, rather than simple additive effects. We further present a complexity-driven regulatory model in which rising combinatorial load progressively narrows regulatory flexibility and promotes survival-oriented strategies over plant growth. Finally, we discuss how recognizing stress complexity as an organizing variable could reshape breeding priorities and predictive crop modeling under increasingly complex environmental scenarios.