Plant roots have to overcome a broad range of mechanical hindrances besides biological and chemical impacts. As an example, roots have to evaluate whether they sidestep an obstacle or continue growing in the original growth direction, tolerating conditions impaired to varying degrees. Root tips possess a special organ, the root cap, which initiates growth deviation.

Sketch of the gadget for measuring mass (in grams) of roots of 3-day-old maize seedlings towards horizontal obstacles
Sketch of the gadget for measuring mass (in grams) of roots of 3-day-old maize seedlings towards horizontal obstacles. For sufficient water supply, the middle column was filled with water-imbibed filter paper, delivering sufficient liquid to the fixed seedling (not shown); the measured values were continuously electronically recorded.

In support of Darwin’s ‘brain hypothesis’ of the root cap, Dreyer and Edelmann investigate this organ’s capacity in assessing the root’s different mechanical surroundings. On encountering resistance, roots with intact caps emit ethylene, which is not observed in decapped roots. The results disclose a root cap dependent fine-tuning of soil resistance and support the involvement of ethylene.