Phosphorus facilitation in the cluster root neighborhood

Phosphorus (P) is a limited resource and an important plant macronutrient characterized by low bioavailability in soils. Therefore, strong ecological and socioeconomic interest exists to increase P use efficiency of agricultural practices and crop species. Cluster roots are specialized structures with an extraordinary P mobilization and acquisition efficiency. This project aims at understanding how P is shared among cluster-rooted and non-cluster rooted plant species in natural and agricultural contexts. When a non-cluster rooted species grows next to a plant that builds those bottle-brush-like root structures, P facilitation may occur, during which the neighboring species benefits of the P mobilized by cluster roots. As the lifetime of cluster roots is restricted to several days, it seems necessary that neighboring species direct their root growth in order to profit of cluster root activity. How this precision in root placement is achieved remains unknown. In contrast to current crop species and model plants, which are well adapted to P rich environments, cluster-rooted species evolved in P poor habitats. Some of the worlds most P impoverished habitats are found in the Southwest Australian Ecoregion, where cluster-rooted and non-cluster rooted species evolved next to each other. The aim of this project is (i) to determine whether and how non-cluster rooted native Australian species sense the emergence of cluster roots in their neighborhood to direct root growth and (ii) to evaluate the relevance of the identified processes in an agricultural intercropping system consisting of non-cluster rooted wheat and cluster-rooted white lupin. For this purpose multispecies rhizobox experiments and bioassays involving cluster root exudates will be used to determine whether neighboring species direct root growth in response to cluster root emergence. Second, an in-depth analysis of the wheat secreted rhizospheric proteins will be performed to understand systemic changes in response to neighbors or varying P availability. This proteomic information will also be used to design a targeted mass spectrometric assay for the relative quantification of wheat P mobilizing enzymes, which will help in identifying the organismal contribution to overall phosphatase activity in soil. Finally, two dimensional chemical imaging techniques will be used to determine the spatiotemporal dynamics of P facilitation between wheat and lupin plants and evaluate the previously identified processes in intercropping. This project will increase our understanding of direct and indirect root-root interactions in plant communities. It will reveal new insights into the mechanisms involved in P facilitation, which will in turn favor efforts to increase P use efficiency in crop species and management practices.

This project aimed to investigate how phosphorus (P) is dynamically shared among non -cluster root bearing and cluster root forming plant species. Cluster roots are specialized structures with an extraordinary P mobilization and acquisition efficiency. When a non-cluster rooted species grows next to a plant that builds those bottle-brush-like root structures, P facilitation may occur, during which the neighboring species benefits of the P mobilized by cluster roots. Whether and how directed root growth occurs in response to the emergence of neighboring cluster roots remains unclear. Within this project, we conducted lab experiments with native Australian plant species and analyzed root positioning in response to neighbors as well as leaf nutrient content and relevant anatomical parameters of roots. The presence of a cluster root bearing neighbor induced changes in root positioning, total nutrient content and plant development in a non-cluster rooted species, which developed thinner roots and had hig her leaf nutrient content than when grown next to a non -cluster rooted species. Previously identified major organic anions could be excluded as potential direct or indirect signaling compounds. In another part of the project, the relevance of these mechanisms were evaluated in the agricultural context. We compared the processes occurring in mixed cultures of wheat and white lupin relative to processes occurring in the respective mono -cultures using proteomic and imaging techniques. In contrast to the rhizosphere of white lupin cluster roots, extracellular wheat phosphatases are tightly associated with root cell walls. The contribution of wheat enzymes to the overall rhizosphere phosphatase activity beyond the immediate root surface therefore appears to be minimal. This work shows that cluster rooted species can influence root positioning, root diameter and plant development of their neighbors, thereby contributing to facilitative interactions under nutrient poor conditions.