Rhizosphere traits as key for P efficiency in upland rice

Rice is staple food for more than half of the earths population. Particularly in upland systems its growth is frequently challenged by low P availability due to high P-fixing soils and the lack of P fertilizers. Consequently, understanding which processes are responsible for high P acquisition efficiency (PAE) in plants is crucial to sustain crop yields in low-input systems. In this project, we aim to elucidate the role and interplay of biogeochemical processes in the rhizosphere (i.e. soil volume influenced by the presence of living plant roots) of four rice genotypes with contrasting PAE and root hair properties. We hypothesize that high PAE is driven either by (i) a high release of plant root derived carbon metabolites, so-called root exudates, into the soil, (ii) a rhizosphere microbiome composition favoring high abundances of P solubilizing and plant growth promoting microorganisms or by (iv) a high degree of mycorrhization. In a collaboration between ZALF (Germany: Maire Holz), BOKU (Austria: Eva Oburger) and JIRCAS (Japan: Matthias Wissuwa), we will investigate root exudation, the degree of genotype-specific mycorrhization as well as the rhizosphere microbial community composition of the selected rice lines under low and high P conditions in a field trial typical for upland growth conditions in Tsukuba, Japan. In a second step, we will thoroughly investigate the P solubilization capacity of genotype specific root exudates collected from the field trial in batch experiments to conclude whether differences in root exudate composition between the genotypes can explain differences in P acquisition efficiency. We will also apply various chemical imaging approaches (phosphor imaging, zymography, planar optode measurements) as well as radioactive 33P labelling in laboratory trials under controlled conditions to study and visualize the spatio-temporal interplay of plant P acquisition and related soil processes. Taken together our results will elucidate which rhizosphere processes confer high P acquisition efficiency in upland rice. This will enable the transfer of rhizosphere traits to breeding programs of upland rice lines and improve selection of crops with highly efficient P acquisition to sustain yields in low-input agricultural systems.

Rice (Oryza sativa) is a staple food for more than half of the world's population. In upland systems, however, its growth is often constrained by low phosphorus (P) availability due to highly P-fixing soils and limited application of P fertilizers. In this project, we investigated how root and rhizosphere traits - including root exudation, mycorrhizal colonization, and the associated rhizosphere microbiome - influence genotypic P acquisition efficiency under both high and low P fertilization conditions. Our results confirmed the crucial role of an extensive root system in promoting efficient P acquisition, particularly when P availability is limited. Interestingly, we observed a trade-off between root exudation and the degree of mycorrhization with root biomass production. This suggests that although root exudation and mycorrhizal associations may support a range of valuable soil ecosystem services, investing in a more extensive root system appears to be a more effective strategy for rice to acquire P in highly P-fixing soils. These findings offer valuable insights for the selection and development of crop varieties with enhanced P acquisition efficiency, contributing to sustained yields in low-input agricultural systems.