Ethylene as a Systems-level Integrator of Plant responses to Drought and Heat stress.

Ethylene (ET) functions as a central integrator of plant responses to drought and heat stress by linking hormone signaling, metabolism, and chromatin regulation. Its biosynthesis and perception are tightly controlled through ACC synthase/oxidase activity and ER-localized receptors that regulate the ETR-CTR1-EIN2-EIN3 signaling cascade. Under drought, ethylene modulates growth restraint, root architecture, and stomatal behavior, often acting in coordination or opposition with ABA to balance water conservation and carbon assimilation. Soil drying and compaction further influence ethylene diffusion, shaping local hormone signaling and root growth responses. Under heat stress, ethylene promotes thermotolerance by enhancing ROS buffering, activating HSF-HSP pathways, and stabilizing cellular homeostasis. These contrasting roles reflect distinct physiological priorities: drought emphasizes water economy, whereas heat prioritizes proteostasis and survival. Ethylene's effects are therefore highly context dependent, governed by stress intensity, tissue specificity, and developmental stage. Effective crop improvement strategies should focus on tuning ethylene sensitivity and downstream transcriptional modules rather than globally suppressing ethylene signaling, enabling optimized resilience to combined climate stresses.