Exogenous auxins for proline regulation in heat-stressed plants
Microbial indole-3-acetic acid (IAA) has long been recognized as a driver of plant growth and developmental plasticity. Recent studies show that microbial IAA production is sustained or even enhanced at elevated temperatures, suggesting that microbial auxins may contribute to plant thermotolerance by stabilizing auxin signalling and supporting metabolic adaptation. Here, we synthesize emerging molecular and physiological evidence linking microbial IAA to proline turnover during thermomorphogenesis. We propose that microbial IAA establishes a regulatory window in which proline metabolism transitions between early osmoprotective synthesis and later catabolism that fuels elongation and redox balance. This integration involves crosstalk between the HSP90-TIR1 auxin perception module, heat-responsive MPK-IAA8 signalling, mitochondrial redox regulators (SSR1, HSCA2), and hormonal interactions with abscisic acid and ethylene. We outlined four mechanistic hypotheses and associated experiments to test how microbial IAA modulates proline homeostasis. This framework highlights microbial auxins as metabolic integrators during heat stress and provides a conceptual basis for leveraging auxin-producing microbes to enhance plant resilience under global warming. HighlightMicrobial auxins modulate the proline cycle during heat stress by integrating hormonal, redox, and mitochondrial signals, offering a mechanistic framework for how microbial IAA enhances thermomorphogenesis and plant heat resilience.