Perception and integration of nutritional signals in plant root systems: Solving the mystery of K-Fe-P interactions.

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Mineral elements are essential to human nutrition. For example, potassium (K) is the major electrolyte in the human body and is required for kidney, muscle, nerve and heart functions. Iron (Fe) is a component of redox enzymes enabling cellular energy metabolism and of hemoglobin carrying oxygen to the brain and to the peripheral tissues. Minerals are introduced into the food chain through plants. Their root systems actively forage the soil for beneficial mineral nutrients and extract them with the help of specialized transport proteins. As for humans, mineral nutrition is essential for plant health. The importance of the root system for yield and nutritional value of food crops has been recognized, and root research has taken a center stage in food security.Plants can perceive signals about mineral nutrient availability in the soil and translate them into developmental and physiological processes that adapt root shape and transport activity, thereby maximizing foraging and uptake capacity. If we want to enhance nutrient usage efficiency of crops we need to understand the signaling pathways that mediate between soil conditions and root adaptations. The underlying mechanisms are complicated. Root systems act simultaneously as receptors perceiving nutrient availability and as effectors carrying out nutrient uptake. To achieve the best result they need to differentially regulate growth of individual root parts and transport in different cells. Without a centralized brain this involves both local and systemic signals and responses. Roots also need to integrate information on different nutrients and prioritize their responses, which requires crosstalk between individual nutrient signaling pathways.We have recently made several discoveries that should enable a better understanding of how plants process multiple nutrient signals and regulate root system architecture. We identified two different ecotypes of the model species Arabidopsis thaliana that respond differently to low K supply. The Columbia (Col-0) accession maintains growth of the primary root but halts lateral root extension, thus displaying a long, narrow root system. By contrast, Catania (Ct-1) halts main root growth but extends lateral roots thus displaying a short, bulky root system. Both accessions look very similar when K supply is sufficient. Surprisingly, we could transform the Ct-1 root phenotype into the Col-0 root phenotype by subjecting the plants to low Fe together with low K - both accessions now developed long, narrow root architectures. Fe is known to play a role in root responses of Col-0 to low P, nevertheless, both accessions showed a similar response to low P (inhibition of main root only). Clearly, the Col-0/Ct-1 pair provides us with an excellent experimental model to discover the molecular processes that underpin developmental decisions of plants under nutrient stress, and to unravel nutrient-nutrient interactions.In this project, we will combine electrophysiological methods and confocal microscopy with molecular genetics and automated root phenotyping to address the following questions: How is low-K perceived by root cells and what is the link to Fe redox metabolism? Which cellular processes underlie main root inhibition? Which signals link developmental responses of the main root with those of the lateral roots? How do different root architectures impact on nutrient uptake and on final nutrient contents in the leaves? Which genes determine root architectural responses to nutrient signals? The results from this study can be expected to lead to a detailed understanding of the fundamental biological processes and genetic components that link soil-derived nutrient signals with root development and nutrient uptake. In particular we will provide new information on the functional relationship between three essential nutrients, K, P and Fe, which will be invaluable for future efforts to improve crop performance and nutritional quality.