Drought accentuates the role of mycorrhiza in phosphorus uptake

Highlights

• Medicago truncatula plants were inoculated or not with Rhizophagus irregularis.

• Mycorrhizal phosphorus uptake was quantified under 15-step moisture gradient.

• Radioisotopes enabled to distinguish cumulative and immediate-term phosphorus uptake.

• Mycorrhiza increased cumulative phosphorus uptake throughout the whole gradient.

• Mycorrhiza increased immediate-term phosphorus uptake at medium and low moistures.

Abstract

Arbuscular mycorrhizal fungi (AMF) greatly facilitate uptake of phosphorus (P) by most plant species. Whether or how this changes if water becomes limiting remained little explored. Medicago truncatula plants were grown in previously sterilized cultivation substrate and inoculated or not with AMF isolate Rhizophagus irregularis ‘PH5’ (M or NM, respectively) in two-compartment rhizoboxes. The compartments were separated or not with root-excluding meshes. A 15-step soil moisture gradient (maintained over 5 weeks) spanned generous water supply through strong water deficiency. Two radioisotopes (³³P and ³²P) were injected into the plant and distant compartments, respectively, to distinguish cumulative and immediate-term plant P uptake from either of the compartments at each moisture level. M plants were larger than NM plants at high and medium moisture levels and accumulated markedly more P throughout the whole gradient. The immediate-term P uptake did not differ between M and NM plants at high moisture, but at medium and low moistures, M plants acquired significantly more ³³P than NM plants, even after accounting for differences in root biomass. Increased immediate-term P uptake by M plants grown under medium and low substrate moistures was probably due to complementary effect of direct hyphal uptake and indirect alterations of substrate hydraulic properties by AMF. This research illustrates that AMF remain functional part of plants even under severe drought stress and thus they should be considered when plant functioning under water deficiency is to be fully understood.

Introduction

Drought brings significant stress to ecosystem stability, agricultural production and many other aspects of human society (Turner and Kramer, 1980; Bray, 1997; Dai et al., 1998; Rowell and Jones, 2006; Vergni and Todisco, 2011). In order to sustain plant production under increasingly adverse conditions caused by environmental changes, including shifting rainfall patterns and ongoing soil degradation, it is essential to fully understand the whole complex of processes and interactions involved in plant growth and nutrition under conditions of soil-moisture deficiency.

Phosphorus (P) is a macronutrient, hence its deficiency also limits growth of plants (Schachtman et al., 1998; Bucher, 2007; Vitousek et al., 2009). Plants acquire P from soil solution in a form of phosphate anions (Raghothama and Karthikeyan, 2005). These anions generally have very low mobility, because they interact with soil cations, mainly Ca²⁺, Al³⁺ and Fe³⁺, and they also sorb to clay minerals by electrostatic bonds and thus show low solubility and high sorption in soils (Tinker and Nye, 2000; Hinsinger, 2001). The amount of dissolved P in soil solutions (i.e., immediately plant available P) is often lower than 10 μM (Bieleski, 1973) and represents only a small fraction of the total amount of P in the soil (Frossard et al., 1995). Transport of phosphate ions through the soil by diffusion is slower than is its transfer across root cell membranes, and therefore depletion zones develop around plant roots (Fitter et al., 2011). For that reason, the uptake of P by plant roots largely depends on diffusion of P in the soil solution and, ergo, on the soil water content around roots (Bhadoria et al., 1991; Gahoonia et al., 1994). Diffusional cross-sections and P diffusion coefficients decline with soil desiccation, which reduces size of the zones around roots from which P can be exploited (Bhadoria et al., 1991; Gahoonia et al., 1994). This limits plant P availability in dry soils even more.

Plants, however, need not rely solely on their roots to take up P. It has been firmly established that arbuscular mycorrhizal fungi (AMF) facilitate P uptake from soils to the majority of terrestrial plant species on Earth (Smith and Read, 2008; Brundrett and Tedersoo, 2018). In exchange for reduced carbon assimilated by the plants (Bücking and Shachar-Hill, 2005), AMF provide their hosts with mineral nutrients, and particularly with those having low mobility, especially P (Jansa et al., 2003, 2011; Kiers et al., 2011). The plants hosting AMF acquire part, or even most, of their P via the so-called mycorrhizal pathway (Smith et al., 2004; Smith and Smith, 2011) which is often more effective in scavenging P from the soil than is the direct uptake by plant roots (Smith et al., 2011). This is achieved thanks to the network of extraradical AMF mycelia that proliferates from colonized roots. Mycorrhizal hyphae reach several centimeters into the surrounding soil, far beyond the P depletion zones around plant roots (Zhu et al., 2001), thereby increasing P absorption area by as much as two orders of magnitude as compared to plant roots alone (Raven and Edwards, 2001). Further, because the hyphae often are only about 2 μm in diameter (Friese and Allen, 1991), they can penetrate and exploit even some soil pores physically inaccessible to the roots directly, i.e., pores <30 μm in diameter (Allen, 2007). The hyphal network of AMF can effectively bypass diffusion limitation by acquiring P beyond the depletion zone of roots and transporting it back to plants through their mycelia (Fitter et al., 2011). Hyphae may also maintain high P diffusivity in the cytosol, resembling more the diffusivity of P in water or gelatin than that in bulk soil (Bieleski, 1973). Most likely, this is because P is moved as polyphosphate in the cytosol (Ezawa and Saito, 2018), and the advective flow inside hyphae seems to integrate into the plant transpiration stream (Bitterlich and Franken, 2016; Kikuchi et al., 2016).

Does all this imply that P acquisition by mycorrhizal plants employing extensive hyphal networks is less dependent on soil water availability around the roots, or on soil water availability in general? Is P supply via the mycorrhizal pathway equally effective at various soil moisture levels, or is it reduced by low water availability? If so, then how? It might be surprising to learn that the question of how the actual soil moisture level affects acquisition of P by mycorrhizal plants has been largely neglected heretofore, although it has previously been recognized that P uptake by plants could be more compromised under conditions of drought than is N uptake (He and Dijkstra, 2014; Mariotte et al., 2020). We should thus urgently explore and understand plant responses and performances when exposed to both low moisture and low P availability (Suriyagoda et al., 2014).

To fill this gap in knowledge, it is necessary to distinguish reliably the long-term, cumulative uptake of P during the whole growth period of plants from the immediate-term P uptake during a drought spell or event. While the former is almost inherently reported in most mycorrhizal studies, data on the latter is almost nonexistent, even though previous research has indicated that mycorrhizal functioning under conditions of drought may actually be more important than it is under conditions of ample water supply (Neumann and George, 2004; Voříšková et al., 2019).

We set out to test the hypothesis that AMF increase immediate-term plant uptake of P under conditions of medium substrate moisture compared to both ample water supply and severe drought. To accomplish this, we conducted a greenhouse experiment using the model plant Medicago truncatula either inoculated with cultured Rhizophagus irregularis or left non-mycorrhizal. We choose this model plant because of its high demand for P due to the biological nitrogen fixation (Püschel et al., 2017) and also because we previously studied mycorrhizal responsiveness of this very plant under different environmental stresses (Voříšková et al., 2019). The plants were grown in two-compartment rhizoboxes, wherein root growth into the second compartment was either allowed or restricted by root-excluding mesh. The novelty of the study consisted in the establishment and maintenance of a 15-step soil moisture gradient and subsequent employment of pulse labeling with radioisotopes ³³P and ³²P (Frossard et al., 2011) injected into one or the other compartment 5 days before harvest. This provided us with a unique “snapshot” of the actual plant acquisition of P via root and mycorrhizal pathways at a considerably wide range of soil moisture levels.