Abstract
Climate change scenarios predict a higher variability in rainfall and an increased risk of water deficits during summers for the coming decades. For this reason, arbuscular mycorrhizal fungi (AMF) and their mitigating effects on drought stress in plants are increasingly considered in crop management. However, the impact of a decrease in water availability on the development of AMF and their ability to take up and transport inorganic phosphorus (Pi) to their hosts remain poorly explored. Here, Medicago truncatula plantlets were grown in association with Rhizophagus irregularis MUCL 41833 in bi-compartmented Petri plates. The system consisted in associating the plant and AMF in a root compartment (RC), allowing only the hyphae to extend in a root-free hyphal compartment (HC). Water availability in the HC was then lowered by increasing the concentration of polyethylene glycol-8000 (PEG-8000) from 0 to 10, 25, and 50 g L-1 (corresponding to a slight decrease in water potential of -0.024, -0.025, -0.030, and -0.056 Mpa, respectively). Hyphal growth, spore production and germination were severely impaired at the lowest water availability. The dynamics of Pi uptake by the AMF was also impacted, although total Pi uptake evaluated after 24 h stayed unchanged. The percentage of metabolically active extraradical hyphae remained above 70%. Finally, at the lowest water availability, a higher P concentration was observed in the shoots of M. truncatula. At reduced water availability, the extraradical mycelium (ERM) development was impacted, potentially limiting its capacity to explore a higher volume of soil. Pi uptake was slowed down but not prevented. The sensitivity of R. irregularis MUCL 41833 to a, even small, decrease in water availability contrasted with several studies reporting tolerance of AMF to drought. This suggests a species or strain-dependent effect and support the necessity to compare the impact of water availability on morpho-anatomy, nutrient uptake and transport capacities of other, potentially more drought-tolerant (e.g., isolated from dry environments) AMF.
Highlights
Drought associated to extreme heat has been responsible for a significant drop in yield of crops and cultivable surfaces over the last 40 years (Lesk et al, 2016)
Despite the quite large number of studies reporting the effects of arbuscular mycorrhizal fungi (AMF) on plant physiology and growth under drought stress conditions, little is known on the impact of decreased water availability on the development of the fungus and its capacity to take up and transport Pi to plants, an essential attribute of these fungi
We evaluated and compared the effects of PEG-induced decreased water availability on the development and on the Pi uptake and transport of R. irregularis MUCL 41833 associated to Medicago truncatula
Summary
Drought associated to extreme heat has been responsible for a significant drop in yield of crops and cultivable surfaces over the last 40 years (Lesk et al, 2016). Irrigation is a practical solution to increase crop yield, an intensification of this method is not viable on the long term as freshwater resources are limited (Elliott et al, 2014). Soil microorganisms living in close relations with plant roots might play a role in plant water stress mitigation via various mechanisms (see for review Kim et al, 2012). Among these below-ground inhabitants are the beneficial and naturally occurring arbuscular mycorrhizal fungi (AMF) (Cardoso and Kuyper, 2006; Rodriguez and Sanders, 2014)
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