Localized micronutrient patches induce lateral root foraging and chemotropism in Nicotiana attenuata

Nutrients are distributed unevenly in the soil.Phenotypic plasticity in root growth and proliferation mayenable plants to cope with this variation and effectivelyforage for essential nutrients. However, how micro-nutrients shape root architecture of plants in their naturalenvironments is poorly understood. We used a combina-tion of field and laboratory-based assays to determine thecapacity of Nicotiana attenuata to direct root growthtowards localized nutrient patches in its native environ-ment. Plants growing in nature displayed a particular rootphenotype consisting of a single primary root and a fewlong, shallow lateral roots. Analysis of bulk soil surroundingthe lateral roots revealed a strong positive correlationbetween lateral root placement and rnicronutrientgradients, including copper, iron and zinc. In laboratoryassays, the application of localized micronutrient saltsclose to lateral root tips led to roots bending in thedirection of copper and iron. This form of chemotropismwas absent in ethylene and jasmonic acid deficient lines,suggesting that it is controlled in part by these twohormones. This work demonstrates that directed rootgrowth underlies foraging behavior, and suggests thatchemotropism and micronutrient-guided root placementare important factors that shape root architecture innature.