Environmental regulation of root growth angle in cereal crops.

Background: In recent years, the root system has emerged as a critical determinant of crop resilience and productivity under soil-related environmental stresses. Among root architectural traits, the root angle is particularly important because it shapes the depth and spatial distribution of roots, thereby influencing access to water and nutrients. A steeper angle can promote deeper soil exploration, whereas a shallower angle can improve topsoil foraging; each can confer advantages under specific stress conditions, including drought, flooding, salinity, and nutrient deficiency. Scope: This review explores the ecological relevance of the root angle, illustrated by adaptations observed in cereal crops across Australia, a continent characterized by diverse climatic and edaphic conditions. We then review the current understanding of how crop plants adjust root angle in response to water-related stresses (drought, flooding, osmotic and salinity) and nutrient (nitrate and phosphate) deficiency on root system and molecular level. We examine the underlying mechanisms and physiological responses, highlighting the roles of key hormonal pathways, particularly auxin, abscisic acid and ethylene and explore how cell wall mechanics potentially acts as a converging point for developmental and environmental signals to fi ne-tune root angle. Conclusions: Root angle is emerging as a critical trait for improving crop resilience and productivity under increasingly variable environmental conditions, and its ecological relevance, genetic variability, and responsiveness to environmental cues underscore its potential as a breeding target. Despite these insights, major knowledge gaps remain, particularly in understanding how multiple tropic responses coordinate under combined stresses and how these responses vary across root classes.