What do Root Hairs Sense in Soil? Examples from Grasses Grown in Different Environments

Root hairs are unicellular extensions of root epidermis cells that enlarge the root surface area into the soil environment. They have been under investigation for more than a century. Barley plants with and without roots hairs demonstrated that root hairs enhance phosphorous (P) (Zhang et al. 2018) and water uptake from soil (Carminati et al. 2017). Studies of root hair development are generally conducted in agar media. These show that longer and denser hairs are produced in P deficient media, while shorter and sparser hairs develop in sufficient and starvation levels of P. Our recent experiments in different soil conditions show, however, that root hairs may sense multiple components of a soil environment, in addition to P and water. For example, rice plants grown in nutrient solution, as expected, formed longer and more root hairs in P deficient than in P sufficient nutrient solution. However this effect of P was not observed for the same genotypes when grown in soil (Nestler et al. 2016). In another experimental system with wheat (Schreiber et al. 2018), plants grown in a sand or an organic-based soil developed root hairs differently regardless of P application. Further, the P source (inorganic fertilizer or organic living or dead algae) influenced root hair formation in addition to soil type. To study the influence of soil solution components on root hair development, we are using a newly developed growth setup engineered for Arabidopsis thaliana and Brachypodium distachyon. We found, as expected, an effect of inorganic P application on root hair elongation; however soil extract had different effects. Taken together, these experiments suggest that root hair development in grass species can be influenced by many properties that distinguish soil from a sterile nutrient solution, including penetration resistance, presence of microbes, and others. Results will be presented and discussed.