SPECIAL - SPECIfying the mechanisms of activation of cALcium signalling in root legume endosymbiosis

Nitrogen (N) and Phosphorus (P) are major macronutrients impacting plant productivity. They are essential for all aspects of plant growth and are required in large quantities for crop maturation and seed production. However, P and N are poorly available in soils, leading to costly chemical fertiliser applications to sustain crop yields. This extensive, and expensive agrochemical practice comes at a severe cost to our environment and human health via soil, water and air pollution, and contributes substantially to global warming. Lowering chemical fertiliser inputs is essential to protect our health and environment. But how can we reduce the use of chemical fertilizer without decreasing crop yields?

Plants have evolved the ability to interact with root-associated endosymbionts to access N from atmospheric N2 and to overcome P limitation of growth. The Arbuscular Mycorrhizal (AM) symbiosis, which delivers phosphate, nitrogen, and other nutrient, is present in 80 % of land plants, including legumes, bread wheat and maize. Legume roots can also associate with nitrogen-fixing bacteria (rhizobia), which reduce N2 to the plant-usable form NH3, within root nodules. Thus, the use of root endosymbionts as a natural source of fertilizer can be part of the solution to reduce fertilizer use without decreasing yield. Although biofertilizer products based on root endosymbionts have been developed, their efficiency remains poor because the capacity of plants to establish endosymbioses is affected by soil quality, prevailing climate and genotype.

In recent years it has become clear that new genetic solutions to enhance endosymbioses are essential for endosymbiont based biofertilizers. Notably activation of nuclear calcium signals is essential for the development of endosymbioses. Although the mechanism of activation is unknown, this signalling is impaired by environmental factors (abiotic and biotic stresses). Thus, understanding how nuclear calcium signals are activated can open the way for engineering crops that are more resistant to the environmental stresses that inhibit the activation of root endosymbioses.

In this project we aim to identify the mechanism of nuclear calcium signalling activation.

Based on extensive preliminary work, we have identified a novel nuclear membrane component which is directly required for the activation of nuclear calcium signalling. This nuclear component has all the characteristics of a nuclear "receptor"; 1) capable of perceiving macromolecular factors induced by endosymbionts, and 2) capable of modulating the activity of ion channels via phosphorylation. We will characterise this nuclear "receptor" further and identify the macromolecular component(s) that activate it, to generate nuclear calcium signals, that initiate endosymbioses. Understanding this new molecular component is essential to understand how environmental factors inhibit endosymbioses. Thus, in the longer term, this project will provide the basis for methods that enhance endosymbioses in crops and reduce the use of chemical fertilizers.

Reducing the fertilizer input in farming systems without impacting crop yield is one of the major challenges needed to protect our environment and health. Soil microbes such as endosymbionts provide important sources of natural fertilizers for crops. Biofertilizer products based on root endosymbionts have been produced, but their efficiency and notably the capacity of the plant to perform endosymbioses is limited by soil quality, climate and genotype.

Our strategy is to understand the molecular mechanisms in plants essential to activate root endosymbioses and thus identify solutions to generate crops that are more resistant to the environmental stresses that otherwise limit root endosymbioses.

The objective of this proposal is to identify the mechanism of activation of nuclear calcium signalling which is essential for root endosymbioses. Our extensive preliminary work has led to the discovery that the activation of nuclear-localized ion channels requires a nuclear membrane localized "receptor", AUK. In this proposal, our aim is to identify the mechanisms of activation and regulation of AUK that contribute to nuclear calcium signalling. To achieve this, we will combine genetic, molecular, biochemical, cell biology and structural approaches to 1) identify how AUK activates the ion channels, 2) identify the interactors of AUK, 3) the mechanism which switches AUK from an inactive to an active state, 4) determine the molecular interface between AUK and its interactors. Further, we will identify key residues that, when mutated, will enhance affinity and/or protect endosymbiosis from inhibition. This work will pave the way for enhanced endosymbiosis and reduced reliance on Chemical N and P fertilizers in crops.