The efficacy of nitrification inhibitors (NIs) varies widely in agricultural soils, influenced by environmental conditions that regulate both NI persistence and microbial activity. In this study, we investigated how soil pH and temperature interact to regulate the efficacy and overall soil ecological impact of three widely used synthetic NIs—dicyandiamide, nitrapyrin, and 3,4-dimethylpyrazole phosphate—and a novel inhibitor, quinone imine (QI). A microcosm experiment was conducted using two agricultural soils with contrasting pH (acidic and alkaline) incubated at 12.5 °C and 25 °C, following urea amendment. In addition to monitoring inorganic N pools and potential nitrification rates, inhibitors persistence and effects on ammonia-oxidizing microorganisms (AOM), nitrite-oxidizing bacteria (NOB), and non-target bacterial and fungal communities were assessed using qPCR and amplicon sequencing. All NIs dissipated faster at higher temperatures and showed greater persistence in the alkaline soil. Despite their reduced persistence at 25 °C, inhibition of nitrification was more pronounced at the higher temperature, reflecting increased nitrification activity under warmer conditions. All NIs significantly reduced potential nitrification rates and nitrate concentrations, particularly in the alkaline soil. Commercial NIs mainly affected ammonia-oxidizing bacteria (AOB), whereas QI exerted a stronger effect on ammonia-oxidizing archaea (AOA) and induced more pronounced shifts in bacterial and fungal communities, indicating stronger off-target effects. Overall, our results suggest that, under the conditions examined, soil pH and temperature jointly regulate NI persistence, functional partitioning of AOM, and microbial community composition, highlighting the importance of environmental context in determining NI performance in agricultural soils.

Soil organic amendments can influence nitrogen fixation in legumes, affecting plant growth. In addition to symbiotic rhizobial bacteria, free-living nitrogen-fixing bacteria, such as Azotobacter vinelandii, play a crucial role in nitrogen dynamics by fixing atmospheric nitrogen into plant-usable forms. This study investigated the effects of Azotobacter vinelandii inoculation, compost and biochar (wood and manure-derived) application when applied individually or in combination, on the growth and nitrogen dynamics of Medicago sativa cultivated under low nitrogen availability conditions, in a pot experiment. Compost and its combinations with both biochars markedly improved shoot and root dry weight, with compost showing the most consistent growth benefits. Inoculated plants treated with manure or wood biochar exhibited increased nitrogen fixation efficiency and higher NH₄⁺-N concentrations, while those treated with wood biochar also showed increased plant height. However, inoculation with A. vinelandii alone negatively affected plant biomass. In both combined compost–biochar treatments, A. vinelandii inoculation led to a reduction in NH₄⁺-N and an increase in NO₃⁻-N levels, accompanied by a decrease in growth substrate pH, indicating stimulation of nitrification. These results highlight the importance of microbial–amendment interactions with organic inputs in modulating nitrogen availability and optimizing legume growth under low N availability conditions.

Salinity adversely impacts soil ecosystems, by inducing osmotic stress, ionic imbalances, water deficit, and oxidative damage in plants. It also alters the composition of plant-associated microbial communities in the rhizosphere and roots, while disrupting microbial processes critical to nutrient cycles. Aloe vera (Aloe barbadensis Miller), a xerophytic succulent plant, produces acemannan, a bioactive polysaccharide in its leaf gel with pharmaceutical applications. Acemannan contributes to drought tolerance by facilitating water storage within the leaf gel tissue. This study examined the effects of soil salinity on rhizosphere properties, plant nutrient acquisition, acemannan accumulation, and plant-associated microbial communities in A. vera plants grown in the field in Laconia, Greece. Both acemannan and sodium (Na) accumulated in the leaf gel in response to soil salinity, showing a strong positive correlation. Significant differences in the composition and structure of the rhizosphere and root microbial communities were also observed under salinity, with the prokaryotic microbial community in the plant roots showing a pronounced shift towards functionally relevant membership and abundance of monoderms. Moreover, we observed significant co-variation of changes in the acemannan and Na concentrations in the leaf gel with changes in the prokaryotic rhizosphere soil community and the fungal community in the roots. Our findings demonstrate enhanced accemanan production and indicate links between osmolyte accumulation and microbial community adaptation in A. vera under soil salinity.

Fusarium solani strain K (FsK) and arbuscular mycorrhizal fungi (AMF) are soilborne symbionts that colonize plant roots and modulate stress responses. While most studies focus on individual microbial partners, understanding multipartite microbial interactions under realistic conditions is essential for designing effective inoculants. Here, we investigated the individual and combined effects of FsK, Funneliformis mosseae (F. mosseae) and Rhizophagus irregularis (R. irregularis) on tomato (Solanum lycopersicum) performance under drought and salinity stress in a greenhouse experimental set up. Under stress conditions, each endophyte showed enhanced root colonization. Co-inoculation with multiple microbes diminished this effect, however the functional outcomes were not directly dependent on the extent of microbial establishment. Under drought, FsK consistently promoted shoot growth, water retention and abscisic acid accumulation, while AMF improved nutrient status. Co-inoculation with FsK and F. mosseae led to synergistic improvements in physiological traits, but only under drought conditions. In contrast, salinity responses were less consistent and revealed functional divergence among microbial partners. These findings demonstrate that context-specific microbial combinations can enhance stress resilience in tomato.

Legumes produce highly nutritious seeds, making them a valuable resource of nutrients for both humans and livestock. In addition to contributing to global food security, they can also provide an ecologically sustainable and cost-effective means of soil fertilization by initiating nutrient cycling in nutrient-limited soils through symbiotic associations with beneficial microbes such as nitrogen-fixing rhizobia and arbuscular mycorrhizal fungi (AMF). However, climate change-induced xerothermic conditions and soil salinity may threaten these beneficial associations between symbionts. Changing environmental conditions and increasing earth’s population require the development of new sustainable products that will support not only plant nutrition but also plant growth and productivity under extreme conditions. This challenge can be addressed by isolating natural beneficial microbes from extreme environments and applying optimized combinations of selected strains that carry interesting and compatible traits to legume crops. To achieve this objective, rhizobial and AMF strains were isolated from wild legumes grown in extreme environments throughout Greece and Cyprus and are currently being evaluated for their application as microbial consortia in pasture legumes for increased rangeland production under climate change. Compatibility between symbiotic partners, functionality of symbiotic relationships and performance of symbiotic plants are investigated at the ecophysiological, molecular and microbial community levels. Furthermore, in order to overcome the lack of knowledge about the molecular mechanisms regulating the interactions between symbiotic partners and to test the efficacy of different rhizobia-AMF co-inoculation strategies, studies are also being conducted in the model legume Lotus japonicus . Roots of 45 different species of wild legumes and their rhizosphere were collected from 22 locations in Greece and Cyprus, and functional nodules were obtained from 12 species. A total of 324 pure rhizobia strains were isolated and the most valuable strains were selected based on their tolerance to abiotic stress (tested in vitro ) and their ability to nodulate the legumes Trifolium resupinatum and Medicago sativa under salinity and drought conditions. In parallel, trap cultures were established to enrich AMF spores from the sampled rhizospheres, and more than 20 AMF spore morphotypes were isolated and re-inoculated to trap cultures for a second reproduction cycle. To optimize the composition of beneficial microbial consortia, legume plants were co-inoculated with different combinations of selected rhizobia and AMF strains and subjected to salinity or drought stress with the aim of achieving compatibility between symbiotic partners and improved plant fitness. Application of microbial consortia is expected to improve the survival, productivity, and nutritive value of pasture plants and promote grazing livestock nutrition and carbon sequestration in pasture soils. This study will contribute to the development of innovative bio-based products, creating new opportunities for sustainable agriculture and investment.

Microbial inocula are considered a promising and effective alternative solution to the use of chemical fertilizers to support plant growth and productivity since they play a key role in the availability and uptake of nutrients. Here, the effect of a beneficial of a fungal root endophyte, Fusarium solani strain K (FsK), on nutrient acquisition efficiency of the legume Lotus japonicus was studied, and putative mode-of-action of the endophyte at a molecular level was determined. Plant colonization with the endophyte resulted in increased shoot and root fresh weight under Fe deficiency compared to control nutrient conditions. Plant inoculation with FsK was associated with a significant increase in macro- and micronutrient concentration in leaves at an early stage of endophyte inoculation and a replenishment of Fe content under prolonged iron starvation. The mechanistic basis of the plant growth promotion capabilities of the endophyte is exerted at the transcriptional level since we recorded changes in the expression levels of genes related to iron uptake in FsK-colonized plants under stress conditions compared to uninoculated plants. In addition, the observed changes in the ethylene biosynthesis-related genes suggest a possible implication of ethylene in the mode of action used by FsK to enhance plant response to nutrient stress conditions. Finally, we demonstrated that the endophyte possesses a reductive high-affinity Fe uptake system and identified a ferric reductase that was induced in planta under Fe deficiency conditions, indicating that this fungal Fe homeostasis mechanism may result in a benefit in nutrient acquisition for the plant as well.

The olive tree is a hallmark crop in the Mediterranean region. Its cultivation is characterized by an enormous variability in existing genotypes and geographical areas. As regards the associated microbial communities of the olive tree, despite progress, we still lack comprehensive knowledge in the description of these key determinants of plant health and productivity. Here, we determined the prokaryotic, fungal and arbuscular mycorrhizal fungal (AMF) microbiome in below- (rhizospheric soil, roots) and above-ground (phyllosphere and carposphere) plant compartments of two olive varieties 'Koroneiki' and 'Chondrolia Chalkidikis' grown in Southern and Northern Greece respectively, in five developmental stages along a full fruit-bearing season. Distinct microbial communities were supported in above- and below-ground plant parts; while the former tended to be similar between the two varieties/locations, the latter were location specific. In both varieties/locations, a seasonally stable root microbiome was observed over time; in contrast the plant microbiome in the other compartments were prone to changes over time, which may be related to seasonal environmental change and/or to plant developmental stage. We noted that olive roots exhibited an AMF-specific filtering effect (not observed for bacteria and general fungi) onto the rhizosphere AMF communities of the two olive varieties/locations/, leading to the assemblage of homogenous intraradical AMF communities. Finally, shared microbiome members between the two olive varieties/locations include bacterial and fungal taxa with putative functional attributes that may contribute to olive tree tolerance to abiotic and biotic stress.

Aims This study aims to identify main factors that influence the tripartite association of legumes with arbuscular mycorrhiza fungi (AMF) and nitrogen-fixing rhizobia. Methods and Results Concurrent inoculations with Mesorhizobium loti and four AMF strains were performed on the model legume Lotus japonicus. Nodulation was significantly enhanced by all AMF strains, under normal conditions, and by specific AMF strains under heat-stress conditions. The impact of rhizobia on mycorrhizal colonization was AMF strain dependent. Co-inoculation trials, where either AMF or rhizobia were restricted outside the root, showed that the symbiotic phenotypes are not influenced by microbial interactions at the pre-symbiotic stage. External application of nutrients showed that P enhances nodulation, while N application does not enhance mycorrhizal colonization. Conclusions Nodulation and mycorhization affect one another during advanced stages of the symbiosis. AMF strains may enhance nodulation under both normal and high environmental temperatures. Rhizobium-AMF compatibility is critical, as rhizobium may positively affect specific AMF strains, an effect that does not derive from increased N uptake.

This work investigates the effects of an organic fertilizer enriched in Ca and Mg and two bacterial inoculants, applied alone and in combination, on soil fertility, plant growth, nutrition, and production of secondary metabolites, namely, acemannan and total phenolic compounds (TPCs), by Aloe vera (Aloe barbadensis Miller), under field cultivation. The first inoculum consisted of five native bacterial strains (Pseudomonas sp., Enterobacter sp., and three strains of Pantoea sp.), characterized in vitro as putative plant growth promoters, isolated from local organic farming fields of Aloe vera. The second inoculant was a commercial product (BACTILIS-S and HUMOFERT) and consisted of three Bacillus species: B. pumilus, B. amyloliquefaciens, and B. subtilis. The organic fertilizer (HUMO-CAL M-8O) was a mixture of humic and fulvic acids, with an additional CaCO3 (40% w/w) and MgO (4% w/w). The most significant increase in the content of acemannan and TPCs was detected under single application of the organic fertilizer, which was linked to enhanced concentration of Mg and Ca in the leaf gel. The concentration of acemannan tended to be increased with the combined application of the organic fertilizer and microbial inoculants. TPCs were significantly increased in both single and combined treatments, seemingly related to Fe concentration in the leaf rinds.

AbstractAimsThis study aims to identify main factors that influence the tripartite association of legumes with arbuscular mycorrhiza fungi (AMF) and nitrogen-fixing rhizobia.Methods and ResultsConcurrent inoculations with Mesorhizobium loti and four AMF strains were performed on the model legume Lotus japonicus. Nodulation was significantly enhanced by all AMF strains, under normal conditions, and by specific AMF strains under heat-stress conditions. The impact of rhizobia on mycorrhizal colonization was AMF strain dependent. Co-inoculation trials, where either AMF or rhizobia were restricted outside the root, showed that the symbiotic phenotypes are not influenced by microbial interactions at the pre-symbiotic stage. External application of nutrients showed that P enhances nodulation, while N application does not enhance mycorrhizal colonization.ConclusionsNodulation and mycorhization affect one another during advanced stages of the symbiosis. AMF strains may enhance nodulation under both normal and high environmental temperatures. Rhizobium-AMF compatibility is critical, as rhizobium may positively affect specific AMF strains, an effect that does not derive from increased N uptake.

Dicyandiamide (DCD) and nitrapyrin (NP) are nitrification inhibitors (NIs) used in agriculture for over 40 years. Recently, ethoxyquin (EQ) was proposed as a novel potential NI, acting through its derivative quinone imine (QI). Still, the specific activity of these NIs on the different groups of ammonia-oxidizing microorganisms (AOM), and mostly their effects on other soil microbiota remain unknown. We determined the impact of QI, and comparatively of DCD and NP, applied at two doses (regular versus high), on the function, diversity, and dynamics of target (AOM), functionally associated (nitrite-oxidizing bacteria-NOB), and off-target prokaryotic and fungal communities in two soils mainly differing in pH (5.4 versus 7.9). QI was equally effective to DCD but more effective than NP in inhibiting nitrification in the acidic soil, while in the alkaline soil QI was less efficient than DCD and NP. This was attributed to the higher activity of QI toward AOA prevailing in the acidic soil. All NIs induced significant effects on the composition of the AOB community in both soils, unlike AOA, which were less responsive. Beyond on-target effects, we noted an inhibitory effect of all NIs on the abundance of NOB in the alkaline soil, with Nitrobacter being more sensitive than Nitrospira. QI, unlike the other NIs, induced significant changes in the composition of the bacterial and fungal communities in both soils. Our findings have serious implications for the efficiency and future use of NIs on agriculture and provide unprecedented evidence for the potential off-target effects of NIs on soil microbiota. IMPORTANCE NIs could improve N use efficiency and decelerate N cycling. Still, we know little about their activity on the distinct AOM groups and about their effects on off-target soil microorganisms. Here, we studied the behavior of a new potent NI, QI, compared to established NIs. We show that (i) the variable efficacy of NIs across soils with different pH reflects differences in the inherent specific activity of the NIs to AOA and AOB; (ii) beyond AOM, NIs exhibit negative effects on other nitrifiers, like NOB; (iii) QI was the sole NI that significantly affected prokaryotic and fungal diversity. Our findings (i) highlight the need for novel NI strategies that consider the variable sensitivity of AOM groups to the different NIs (ii) identify QI as a potent AOA inhibitor, and (iii) stress the need for monitoring NIs' impact on off-target soil microorganisms to ensure sustainable N fertilizers use and soil ecosystem functioning.

Mutualistic relationships of legume plants with, either bacteria (like rhizobia) or fungi (like arbuscular mycorrhizal fungi), have been investigated intensively, usually as bi-partite interactions. However, diverse symbiotic interactions take place simultaneously or sequentially under field conditions. Their collective, but not additive, contribution to plant growth and performance remains hard to predict, and appears to be furthermore affected by crop species and genotype, non-symbiotic microbial interactions and environmental variables. The challenge is: (i) to unravel the complex overlapping mechanisms that operate between the microbial symbionts as well as between them, their hosts and the rhizosphere (ii) to understand the dynamics of the respective mechanisms in evolutionary and ecological terms. The target for agriculture, food security and the environment, is to use this insight as a solid basis for developing new integrated technologies, practices and strategies for the efficient use of beneficial microbes in legumes and other plants. We review recent advances in our understanding of the symbiotic interactions in legumes roots brought about with the aid of molecular and bioinformatics tools. We go through single symbiont-host interactions, proceed to tripartite symbiont-host interactions, appraise interactions of symbiotic and associative microbiomes with plants in the root-rhizoplane-soil continuum of habitats and end up by examining attempts to validate community ecology principles in the legume-microbe-soil biosystem.

Cowpea can effectively form tripartite symbiotic associations with nitrogen-fixing bacteria (NFB) and arbuscular mycorrhizal fungi (AMF) although the selection of compatible AMF species and rhizobial strains which are promoting cowpea growth remains a challenge. The aims of the current study were 1) to evaluate the response of cowpea plants to a symbiotic NFB and a multi-AMF inoculum and 2) to explore any interaction between the symbiotic NFB and the different AMF isolates. In a pot gnotobiotic trial, cowpea plants grown under limited N supply, were inoculated with or without a symbiotic nitrogen-fixing bacterium, Sinorhizobium meliloti, and combinations of three different AMF species namely Dominikia disticha, Claroideoglomus etunicatum and Rhizophagus irregularis. Experimental evaluation was determined through the measurement of above ground biomass, nutrient content and AMF root colonization. The presence of AMF species on cowpea roots was also determined with cloning and sequencing. Inoculation with both AMF and S. meliloti led to increased cowpea biomass production compared to inoculation with AMF only, but the presence of a positive effect depended on the specific AMF partners used. Inoculation with AMF alone had a highly positive impact on the growth and P uptake of cowpea, but the NFB inoculation was needed to address N deficiency in planta. The presence of both symbionts generally led to increased AMF colonization of the cowpea roots, however, plant colonization depended on the AMF species, and became even negative, when all three AMF inocula were used together. The AMF composition in plant roots was also altered in the presence of the S. meliloti. Plant nitrogen content of cowpea plants significantly increased under the presence of both symbionts compared to AMF alone, while phosphorus content was hardly affected by dual inoculations. The results show positive synergistic effects of the different AMF species and S.meliloti. Inoculation with all AMF isolates and S. meliloti led to high above ground biomass production and accumulation of N. The presence of S. meliloti increased P content in plants not inoculated with AMF. Finally, the efficiency of synergism depends on the specific AMF partners used and it is not related to their colonization levels.

Sand dunes of the Mediterranean region constitute drought-stressed, low-fertility ecosystems. Arbuscular mycorrhizal fungi (AMF) are regarded as key components of their biota, that contribute to plant host adaptation and fitness. However, AMF community assembly rules in the roots of the psammophilous plants of coastal sand dunes have not been investigated. We studied the root colonizing AMF communities of four characteristic native plants of eastern Mediterranean coastal foredunes, in nine locations in Greece. Host-specificity (plant identity) was the major driver of AMF community assembly in the plant roots, while geographical distance between locations was not related to differences in the AMF communities. Additionally, colonizer AMF communities were characterized by over-dominance of a single operational taxanomic unit (OTUs), which was remarkably host-specific among locations. Wider dissimilarity in AMF communities was observed in small and disturbed (SD) sites compared to large and undisturbed (LU) sites, a trait that may be attributed to relaxed environmental filtering and facilitated AMF dispersal/immigration in SD sites from surrounding habitats. Overall, our results indicate that the assembly of root-colonizing AMF communities in the eastern Mediterranean sand dunes is characterized by strong biotic filtering (host identity), suggesting that co-adaptation processes may be more pronounced than previously proposed, under extreme environmental conditions.

Arbuscular mycorrhizal fungi (AMF) are soilborne microorganisms that establish a mutualistic symbiotic association with most of land plants. To investigate the effects of AMF symbiosis under different water status conditions, we grew AMF-inoculated and non-inoculated tomato plants in the greenhouse under two irrigation regimes, 70% and 30% of growth-substrate water holding capacity. Two different AMF inoculation strains, Funneliformis mosseae and Rhizophagus irregularis, were applied as single inocula. AMF colonization significantly enhanced plant vegetative growth by 40% and 50–60%, under normal and reduced irrigation respectively. In the presence of the AMF, phosphorous concentrations in the leaves were increased under both watering regimes while K, Ca, Mg, Zn, and Mn were also increased under limited watering to levels similar to those of non-stressed plants. Transpiration and stomatal conductance increased by an average 80% and 65% respectively in the presence of the AMF under full watering, but were kept stable and coupled to reduced leaf area-to-leaf biomass ratios and to increased metabolic water use efficiency under limited irrigation. This indicates a different mode of action induced by AMF colonizers, prioritizing water conservation in tomato plants under drought stress.

Cultivation of olive trees covers large coastal areas of land in Mediterranean regions, many of them characterized by low soil fertility and exposed to salinity and seasonal drought. In this frame, we developed mixed community inocula of arbuscular mycorrhizal fungi (AMF) derived from the extreme, seasonally arid environments of six Mediterranean sand dunes and evaluated their effects, in the form of community inocula, on rooted semi-woody olive tree cuttings ( Olea europaea cv. Koroneiki). The plantlets were grown in the greenhouse for 10 months under 50 mM and 100 mM concentrations of NaCl, successively applied to induce osmotic stress. Inoculation had a positive effect on plant growth and nutrient uptake. However, the three best-performing inocula in early colonization and in plant growth enhancement also resulted in high plant sensitivity to high salinity, which was not observed for the other three inocula. This was expressed by decreased nutrient uptake and drastically lower plant growth, plant photosynthesis, and stomatal conductance (generally an over 50% reduction compared to no salinity application). Amplicon sequencing analysis of the olive plants under salinity stress showed that the AMF communities in the roots were clearly differentiated by inoculation treatment. We could not, however, consistently associate the plant responses observed under high salinity with specific shared AMF community membership or assembly attributes. The observed physiological overreaction to osmotic stress may be an adaptation trait, potentially brought about by host selection coupled to abiotic environmental filtering, in the harsh conditions from which the AMF inocula were derived. The overreaction may, however, be undesirable if conveyed to allochthonous plants at an agronomic level.

A soil, artificially contaminated with Pb and Ni, was amended with a readily decomposable carbon source (glucose - SG) and with olive tree pruning materials (either raw SP, or composted - SC). Pb and Ni partitioning in the soil and CO2 evolution at 0.125, 1, 3, 9, 27, 54, 108, 216 and 301 days post contamination were monitored. SG, SP and SC treatments resulted in gradual reduction of Pb and Ni availability and redistribution of both metals among the soil fractions. Partitioning patterns differed, depending on the metal and the carbon source: The reduction of Ni from the exchangeable fraction, where it initially accumulated, was accompanied by a great progressive increase of Ni concentration in the oxidisable fraction, which was outstanding in the SP treatment. Most of the Pb was recovered in the reducible fraction, however, the SP and SC treatments also resulted in a translocation of Pb in the oxidisable fraction. The addition of all carbon sources in the soil boosted CO2 evolution and appeared to alleviate metal toxicity to microbial activity; however, wasteful respiration indicative of stress may have occurred following metal additions, in the SP treatment in particular. Apart from direct sorption mechanisms, the incorporation of Pb and especially of Ni in proliferating soil microbial cells and in biogenic Mn-oxides appear to be contributing to the impressive Ni transition, and to the lesser but substantial Pb transition from available to oxidisable forms in the soil.

Plants establish symbiotic relationships with soil bacteria or fungi, which colonize the plant root and provide the plant with inorganic nutrients, in exchange for photosynthesis products. Legume plants associate with both arbuscular mycorrhizal fungi (AMF) and the nitrogen-fixing soil bacteria called rhizobia. During the legume-rhizobium symbiosis, biological nitrogen fixation takes place in specific plants organs formed on the root, called nodules. Using the model legume Lotus japonicus, we studied the establishment of the legume-rhizobia-AMF tripartite symbiosis. We examined how the AM fungus Rhizophagus irregularis and the rhizobium Mesorhizobium loti affected one another during the colonization of the same legume roots, by performing co-inoculations. Moreover, we monitored the effect of the co-inoculation on the general plant performance. According to our results, the presence of M. loti had no effect on the root colonization by R. irregularis. However, root colonization by R. irregularis had a positive effect on the formation of root nodules. This study aims to enhance our understanding on how the plant selects, combines and controls its symbionts, towards to a more efficient use of legume plants in agroecosystems.

The endophyte Fusarium solani strain FsK may colonize the roots of tomato plants and protect them against fungal pathogens. Here we show that the fungus can alleviate the strong negative effects of water stress on the development and growth of tomato seedlings that were co-cultivated with FsK under in vitro conditions. In a succeeding pot experiment, the presence of FsK ameliorated physiological stress in plants grown in peat under water deficit irrigation conditions: plants practically restored levels of net photosynthesis rate (P-N), maximal potential yield of PSII (F-v/F-m) and relative water content (% RWC) and displayed a 4-fold increase in stomatal conductivity (g(s)) and a 7-fold decrease in proline levels compared to mock-inoculated plants. These changes indicate a shift towards an anisohydric type response to water stress, aided by mechanisms maintaining efficient water absorption by roots and high-water conductance in planta. Effects on the accumulation of reactive oxygen species and relevant antioxidant enzymes were less dramatic. Our data indicate that the endophyte may reduce stomatal closure response to water stress, and potentially improve water acquisition by roots and/or water conductance in planta, thus preventing reduction in photosynthesis rates and minimizing water loss; this, results in increased tolerance of tomato plants to water deprivation, conferred by the endophyte.