Connecting polyphosphate translocation and hyphal water transport points to a key of mycorrhizal functioning

The role of arbuscular mycorrhizal symbiosis in improving plant water status under drought

Arbuscular mycorrhizal fungi (AMF) have been presumed to ameliorate crop tolerance to drought. Here, we review the role of AMF in maintaining water supply to plants from drying soils and the underlying biophysical mechanisms. We used a soil-plant hydraulic model to illustrate the impact of several AMF mechanisms on plant responses to edaphic drought. The AMF enhance the soil's capability to transport water and extend the effective root length, thereby attenuating the drop in matric potential at the root surface during soil drying. The synthesized evidence and the corresponding simulations demonstrate that symbiosis with AMF postpones the stress onset limit, which is defined as the disproportionality between transpiration rates and leaf water potentials, during soil drying. The symbiosis can thus help crops survive extended intervals of limited water availability. We also provide our perspective on future research needs and call for reconciling the dynamic changes in soil and root hydraulics in order to better understand the role of AMF in plant water relations in the face of climate changes.

AMF species do matter: Rhizophagus irregularis and Funneliformis mosseae affect healthy and PVY-infected Solanum tuberosum L. in a different way

Arbuscular mycorrhizal fungi (AMF) were documented to positively influence plant growth and yield, which is extremely important for the production of many crops including potato. However, the nature of the interaction between arbuscular mycorrhiza and plant virus that share the same host is not well characterized. In this study, we examined the effect of different AMF, Rhizophagus irregularis and Funneliformis mosseae, on healthy and potato virus Y (PVY)-infected Solanum tuberosum L. The analyses conducted included the measurement of potato growth parameters, oxidative stress indicators, and photosynthetic capacity. Additionally, we evaluated both the development of AMF in plant roots and the virus level in mycorrhizal plants. We found that two AMF species colonized plant roots to varying degrees (ca. 38% for R. irregularis vs. 20% for F. mosseae). Rhizophagus irregularis had a more positive effect on potato growth parameters, causing a significant increase in the total fresh and dry weight of tubers, along with virus-challenged plants. Furthermore, this species lowered hydrogen peroxide levels in PVY-infected leaves and positively modulated the levels of nonenzymatic antioxidants, i.e., ascorbate and glutathione in leaves and roots. Finally, both fungal species contributed to reduced lipid peroxidation and alleviation of virus-induced oxidative damage in plant organs. We also confirmed an indirect interaction between AMF and PVY inhabiting the same host. The two AMF species seemed to have different abilities to colonize the roots of virus-infected hosts, as R. irregularis showed a stronger drop in mycorrhizal development in the presence of PVY. At the same time, arbuscular mycorrhiza exerted an effect on virus multiplication, causing increased PVY accumulation in plant leaves and a decreased concentration of virus in roots. In conclusion, the effect of AMF-plant interactions may differ depending on the genotypes of both symbiotic partners. Additionally, indirect AMF-PVY interactions occur in host plants, diminishing the establishment of arbuscular mycorrhiza while changing the distribution of viral particles in plants.

Impact of arsenic on phosphate solubilization, acquisition and poly-phosphate accumulation in endophytic fungus Serendipita indica

Symbiotic interactions play a crucial role in the phosphate (Pi) nutrient status of the host plant and offer resilience during biotic and abiotic stresses. Despite a competitive behavior of arsenic (AsV) with Pi, Serendipita indica association promotes plant growth by reducing arsenic bioavailability in the rhizosphere. Reduced arsenic availability is due to the adsorption, accumulation, and precipitation of arsenic in the fungus. The present investigation focused on the fitness and performance of Pi acquisition and utilization in S. indica for growth and metabolism under arsenic stress. The fungus accumulates a massive amount of arsenic up to 2459.3 ppm at a tolerable limit of arsenic supply (1 mM) with a bioaccumulation factor (BAF) 32. Arsenic induces Pi transporter expression to stimulate the arsenic acquisition in the fungus. At the same time, Pi accumulation was also enhanced by 112.2 times higher than the control with an increase in poly-P (polyphosphate) content (6.69 times) of the cell. This result suggests arsenic does not hamper poly-P storage in the cell but shows a marked delocalization of stored poly-P from the vacuoles. Furthermore, an enhanced exopolyphosphatase activity and poly-P storage during arsenic stress suggest induction of cellular machinery for the utilization of Pi is required to deal with arsenic toxicity and competition. However, at high arsenic supply (2.5 and 5 mM), 14.55 and 22.07 times reduced Pi utilization, respectively, was observed during the Pi uptake by the fungus. The reduction of Pi uptake reduces the cell growth and biomass due to competition between arsenic and phosphate. The study suggests no negative impact of arsenic on the Pi acquisition, storage, and metabolism in symbiotic fungus, S. indica, under environmental arsenic contamination.

Effect of the diverse combinations of useful microbes and chemical fertilizers on important traits of potato

The belowground soil environment is an active space for microbes, particularly Arbuscular Mycorrhizal Fungi (AMF) and P hosphate Solubilizing Bacteria (PSB) that can colonize with roots of higher plants. In the present experiment, we evaluated the combination of microbial inoculants with the different doses of urea and superphosphate in a complete randomized block design (CRBD). Three different doses of urea and superphosphate were tested, i.e., recommended dose, 75% of the recommended dose and 125% of the recommended dose, independently and in combination with three microbial groups viz. Glomus mosseae (AMF), Bacillus subtilis (PSB) and Nitrifying microorganisms (Nitrosomonas + Nitrobacter, NN). Overall, there were 16 treatment combinations used, and studied the number of tubers per plant, the weight of tubers, moisture content, and the number of nodes per tubers which were best in treatment comprising of AMF + PSB + NN + 75% of urea + superphosphate. From our results, it is suggested for the growers to use a lesser quantity of fertilizers from the recommended dose along with some bioinoculants to maintain the soil fertility and also to achieve the yield targets by decreasing the cost of chemical fertilizers.

Facilitation of plant water uptake by an arbuscular mycorrhizal fungus: a Gordian knot of roots and hyphae

Arbuscular mycorrhizal (AM) fungi play a positive role in plant water relations, and the AM symbiosis is often cited as beneficial for overcoming drought stress of host plants. Nevertheless, water uptake via mycorrhizal hyphal networks has been little addressed experimentally, especially so through isotope tracing. In a greenhouse study conducted in two-compartment rhizoboxes, Medicago truncatula was planted in the primary compartment (PC), either inoculated with Rhizophagus irregularis or left uninoculated. Plant roots were either allowed to enter the secondary compartment (SC) or were restricted to the PC by root-excluding mesh. Substrate moisture was manipulated in the PC such that the plants were grown either in high moisture (15% of gravimetric water content, GWC) or low moisture (8% GWC). Meanwhile, the SC was maintained at 15% GWC throughout and served as a water source accessible (or not) by roots and/or hyphae. Water in the SC was labeled with deuterium (D) to quantify water uptake by the plants from the SC. Significantly, increased D incorporation into plants indicated higher water uptake by mycorrhizal plants when roots had access to the D source, but this was mainly explained by generally larger mycorrhizal root systems in proximity to the D source. On the other hand, AM fungal hyphae with access to the D source increased D incorporation into plants more than twofold compared to non-mycorrhizal plants. Despite this strong effect, water transport via AM fungal hyphae was low compared to the transpiration demand of the plants.

Arbuscular Mycorrhiza Changes the Impact of Potato Virus Y on Growth and Stress Tolerance of Solanum tuberosum L. in vitro

Under the field conditions crop plants interact with diverse microorganisms. These include beneficial (symbiotic) and phytopathogenic microorganisms, which jointly affect growth and productivity of the plants. In last decades, production of potato (Solanum tuberosum L.) suffers from increased incidence of potato virus Y (PVY), which is one of most important potato pests. Arbuscular mycorrhizal fungi (AMF) are common symbionts of potato, however the impact of mycorrhizal symbiosis on the progression of PVY-induced disease is scarcely known. Therefore, in the present study we investigated the effect of joint PVY infection and mycorrhizal colonization by Rhizophagus irregularis on growth traits of the host potato plant (cv. Pirol). The tested PVY isolate belonged to N-Wilga strain group, which is considered to be predominant in Europe and many other parts of the world. The viral particles were concentrated in the leaves, but decreased the root growth. Furthermore, the infection with PVY evoked prolonged oxidative stress reflected by increased level of endogenous H₂O₂. AMF alleviated oxidative stress in PVY-infected host plants by a substantial decrease in the level of shoot- and root-derived H₂O₂, but still caused asymptomatic growth depression. It was assumed that mycorrhizal symbiosis of potato might mask infection by PVY in field observations.

Arbuscular Mycorrhiza Alleviates Restrictions to Substrate Water Flow and Delays Transpiration Limitation to Stronger Drought in Tomato

Arbuscular mycorrhizal fungi (AMF) proliferate in soil pores, on the surface of soil particles and affect soil structure. Although modifications in substrate moisture retention depend on structure and could influence plant water extraction, mycorrhizal impacts on water retention and hydraulic conductivity were rarely quantified. Hence, we asked whether inoculation with AMF affects substrate water retention, water transport properties and at which drought intensity those factors become limiting for plant transpiration. Solanum lycopersicum plants were set up in the glasshouse, inoculated or not with Funneliformis mosseae, and grown for 35 days under ample water supply. After mycorrhizal establishment, we harvested three sets of plants, one before (36 days after inoculation) and the second (day 42) and third (day 47) within a sequential drying episode. Sampling cores were introduced into pots before planting. After harvest, moisture retention and substrate conductivity properties were assessed and water retention and hydraulic conductivity models were fitted. A root water uptake model was adopted in order to identify the critical substrate moisture that induces soil derived transpiration limitation. Neither substrate porosity nor saturated water contents were affected by inoculation, but both declined after substrates dried. Drying also caused a decline in pot water capacity and hydraulic conductivity. Plant available water contents under wet (pF 1.8-4.2) and dry (pF 2.5-4.2) conditions increased in mycorrhizal substrates and were conserved after drying. Substrate hydraulic conductivity was higher in mycorrhizal pots before and during drought exposure. After withholding water from pots, higher substrate drying rates and lower substrate water potentials were found in mycorrhizal substrates. Mycorrhiza neither affected leaf area nor root weight or length. Consistently with higher substrate drying rates, AMF restored the plant hydraulic status, and increased plant transpiration when soil moisture declined. The water potential at the root surface and the resistance to water flow in the rhizosphere were restored in mycorrhizal pots although the bulk substrate dried more. Finally, substrates colonized by AMF can be more desiccated before substrate water flux quantitatively limits transpiration. This is most pronounced under high transpiration demands and complies with a difference of over 1,000 hPa in substrate water potential.

Phosphorus- and nitrogen-acquisition strategies in two Bossiaea species (Fabaceae) along retrogressive soil chronosequences in south-western Australia

During long-term ecosystem development and its associated decline in soil phosphorus (P) availability, the abundance of mycorrhizal plant species declines at the expense of non-mycorrhizal species with root specialisations for P-acquisition, such as massive exudation of carboxylates. Leaf manganese (Mn) concentration has been suggested as a proxy for such a strategy, Mn concentration being higher in non-mycorrhizal plants that release carboxylates than in mycorrhizal plants. Shifts in nitrogen (N)-acquisition strategies also occur; nodulation in legumes is expected at low N availability, when sufficient P is available. We investigated whether two congeneric legume species (Bossiaea linophylla and Bossiaea eriocarpa) occurring along two long-term chronosequences on the south-western Australian coast and grown in a glasshouse at varying N and P supply exhibited plasticity in nutrient-acquisition strategies. We hypothesised that the shifts in nutrient limitation and nutrient-acquisition strategies at the community level would also be found at the species level. Leaf N: P ratios and the responses to nutrient availability suggested that growth of both species exhibited P-limitation in all treatments, due to the very high leaf [N] of legumes afforded by symbiotic N-fixation. Mycorrhizal colonisation was not greater at higher P supply, and root exudation of carboxylates was not stimulated at low P supply; both were unrelated to leaf [Mn]. However, nodule production declined with increasing N supply. We conclude that intraspecific variation in nutrient-acquisition and use is low in these species, and that the variation at the community level, observed in previous studies, is likely driven by high-species turnover.