Root type contribution to phosphate nutrition of rice during asymbiosis and interaction with symbiotic fungi.

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Phosphate (Pi) is an essential plant macronutrient and occurs in scarce amounts in most soils, which frequently limits plant growth. Plant fitness and crop productivity is directly influenced by the plant root system's efficiency in exploring soil nutrient resources. Plant root systems are composed of distinct root types (RTs), and their ratio and spatial arrangement define root system architecture and nutrient foraging efficiency. RTs may individually respond to their abiotic and biotic soil microenvironment, thereby asymmetrically contributing to plant nutrient acquisition. Consistently, there is a growing interest of scientist and breeders in classifying and quantifying root system architectural traits (reviewed in Rogers and Benfey, 2015) towards 'designer crops with optimal root system architecture for nutrient acquisition' (Kong et al., 2014). Surprisingly however, RTs are 'under-investigated' in particular in adult crops where RT-focused research is missing. The Paszkowski group has addressed this gap in knowledge by studying the RTs of adult rice plants at V8 stage (before flowering). The root system of adult rice is composed of three RTs, the crown, large and fine lateral roots (CR, LLR, FLR, respectively) with distinct developmental and anatomical characteristics. Unique transcriptional signatures for each rice RT were revealed, which suggested distinct roles in nutrient acquisition (Gutjahr et al., 2015). Plants acquire Pi either directly or in association with naturally prevalent arbuscular mycorrhizal (AM) fungi. In rice, the different RTs exhibit variable extent of colonization with LLR fully and CR partially colonized, whereas FLRs remain non-colonized (Gutjahr et al., 2009). Exposing rice roots to a beneficial AM fungus led to the profound modulation of each RT transcriptome, indicative of a switch in their functional relationship. The contribution of individual RTs to rice Pi nutrition in asymbiosis or during interaction with AM fungi remains at present unclear and represents the main objective of this proposal. In addition, the Paszkowski group has recently discovered that exclusively vacuoles of fungus-containing plant cells accumulated structures resembling polyphosphate-storage bodies (Roth & Paszkowski, unpublished), previously not described for higher plants. Colonized and non-colonized RTs thus appear to differ in the spatial distribution and speciation of tissue phosphorus, which may impact on the nutritional physiology of the plant. The proposed work aims at quantitatively and spatially determining Pi uptake and partitioning into tissue and cellular pools across the RTs of adult rice root systems. The application of a unique combination of interdisciplinary techniques in analytics, imaging and molecular genetics will deliver an insight into in situ phosphorus fluxes at unprecedented spatio-temporal resolution. As a staple food for more than half of the human population, rice is central for food security. The study will inform about RT functioning, important for rational breeding approaches towards improved plant stress tolerance and crop productivity.