Activation of Seminal Root Primordia during Wheat Domestication Reveals Underlying Mechanisms of Plant Resilience

Seminal roots constitute the early root system of major crops of the Poaceae family. Although variation in seminal root number was described in several crops, mechanisms through which seminal root number (SRN) are controlled and in turn contribute to environmental adaptation are poorly understood. Here, we show that SRN increased upon wheat domestication due to the activation of root primordia which are suppressed in wild wheat, a trait controlled by factors expressed in the germinating embryo. We used variation in seminal root number (SRN) between wild and domesticated wheat to investigate its bearing on water uptake and seedling resilience. The persistence of wild roots at their primordial state promoted seedling recovery from episodic water-stress through re-activation of root primordia following rehydration. In spite of their lower root number, wild seedlings transpired more than domesticated seedlings. Additionally, transpiration rate was associated with higher shoot:root ratio in wild wheat, indicating contrasting strategies of resource allocation between wild and domesticated wheat. Our findings suggest that under well-watered conditions, lower root number enables direction of resources to aboveground without limiting water uptake. Furthermore, the maintenance of roots at their primordial state and their re-activation following rehydration maybe regarded as seedling protective mechanism against episodic water-stress. The results underscore SRN as an adaptive trait that was reshaped upon domestication. Identification of factors associated with the plasticity of the SRN phenotype expands our understanding on the evolutionary dynamics of wheat and may serve to optimize root number in future breeding efforts.