Contribution of arbuscular mycorrhiza and exoenzymes to nitrogen acquisition of sorghum under drought

Rosepiah Munene ^1,2* Osman Mustafa ^1,3,4 Sara Loftus ¹ Callum C Banfield ² Reimund Paul Roetter ^5,6 Ezekiel Bore ⁷ Bernard Mweu ⁸ Kevin Mganga ⁹ Otieno O Dennis ^10,2 Mutez A Ahmed ¹¹ Michaela A Dippold ^1,12

• ¹ University of Göttingen, Göttingen, Germany
• ² Geo-Biosphere Interactions, Department of Geosciences, University of Tuebingen, Germany, Tübingen, Germany
• ³ Institute of Bio- and Geosciences, Julich Research Center, Helmholtz Association of German Research Centres (HZ), Juelich, North Rhine-Westphalia, Germany
• ⁴ University of Khartoum, Khartoum, Sudan
• ⁵ Tropical Plant Production and Agricultural Systems Modelling (TROPAGS), University of Goettingen, 37077 Goettingen, Germany, Göttingen, Germany
• ⁶ Centre of Biodiversity and Sustainable Land Use, Faculty of Agricultural Sciences, University of Göttingen, Göttingen, Lower Saxony, Germany
• ⁷ University of Helsinki, Helsinki, Uusimaa, Finland
• ⁸ South Eastern Kenya University, Kitui, Kenya
• ⁹ Utrecht University, Utrecht, Netherlands, Netherlands
• ¹⁰ School of Biological and Physical sciences Jaramogi Oginga Odinga University of Science and Technology, Bondo, Kenya
• ¹¹ Root-Soil Interaction, TUM School of Life Sciences, Technical University of Munich, Freising, Germany, Freising, Germany
• ¹² Geo-Biosphere Interactions, Department of Geosciences, University of Tuebingen., Tübingen, Germany

Introduction: For low-fertile and degraded soils of sub-Saharan Africa, nitrogen (N) is often the most growth limiting factor restricting crop yields. The often-suggested exploitation of advantageous rhizosphere traits such as enzyme secretion and/or the symbiosis with arbuscular mycorrhizal fungi (AMF) remains to be validated as a potential strategyies to overcome N limitation, especially when N deficiency co-occurs with further abiotic stresses such as water scarcity.Methods Three sorghum genotypes were cultivated in soil mesocosms with a root-exclusion compartment, where solely AMF could scavenge for nutrients under drought and optimal conditions. Plant carbon (C) investment into rhizosphere and N uptake were tracked by 15 N application coupled with 13 CO2 labeling Results: Under drought, uptake of mineral 15 N by arbuscular mycorrhizal fungi (AMF) from the root-exclusion compartment increased 4-12 times compared to well-watered conditions. In addition, water stress enhanced below-ground allocation of recently assimilated C into microbial biomass. Drought reduced enzymatic potential (Vmax) of chitinase while increasing leucine aminopeptidase (LAP) activity. This suggests that N acquisition via protein mineralization in soil was relatively enhanced to that of chitin following moisture limitation. LAP substrate affinity (Km) was reduced by drought compared to that of chitinase with genotype-specific shifts in rhizosphere enzyme systems observed.Our findings suggest that belowground C allocation activated AMF symbiosis and its associated microbiome. This not only led to a shift in enzyme-driven exploitation of distinct organic N sources but also induced a strong increase in AMF-based mineral N acquisition from the mycosphere. This trait plasticity in response to drought may be harnessed for stabilizing food production from low-fertile soil under the increasingly negative impacts of droughts under climate change.