The endophytic fungus Piriformospora indica enhances Arabidopsis thaliana growth and modulates Na+/K+ homeostasis under salt stress conditions

The endophyte Serendipita indica reduces the sodium content of Arabidopsis plants exposed to salt stress: fungal ENA ATPases are expressed and regulated at high pH and during plant co-cultivation in salinity

Serendipita indica (formerly Piriformospora indica) is an endophytic fungus that colonizes plant roots producing a beneficial effect on plant growth and development under optimal and suboptimal conditions provoked by any biotic or abiotic stress, such as salt stress. Salinity induces osmotic and ionic imbalances in plants, mainly by altering the Na⁺ and K⁺ contents. However, the mechanism by which Serendipita improves plant growth has yet to be elucidated. Previous works suggest that this fungus improves the plant osmotic state but not much is known about whether it participates in readjustment of the ionic imbalance in plants. Here, we report that co-cultivation with Serendipita reduces the Na⁺ content of Arabidopsis plants under saline conditions. Additionally, we describe the functional characterization of the two Serendipita ENA ATPases, which are homologous to the main proteins involved in the salt tolerance of other fungi. Their heterologous expression in salt-sensitive yeast mutants shows that SiENA1 is involved in Na⁺ and K⁺ efflux, while SiENA5 seems to only be involved in Na⁺ detoxification. Both are induced and might have a relevant function at alkaline pH, condition in which there are few chlamydospores in the mycelium, as well as during co-cultivation with plants exposed to saline conditions.

The Role of Soil Fungi in K+ Plant Nutrition

K⁺ is an essential cation and the most abundant in plant cells. After N, its corresponding element, K, is the nutrient required in the largest amounts by plants. Despite the numerous roles of K in crop production, improvements in the uptake and efficiency of use of K have not been major focuses in conventional or transgenic breeding studies in the past. In research on the mineral nutrition of plants in general, and K in particular, this nutrient has been shown to be essential to soil-dwelling-microorganisms (fungi, bacteria, protozoa, nematodes, etc.) that form mutualistic associations and that can influence the availability of mineral nutrients for plants. Therefore, this article aims to provide an overview of the role of soil microorganisms in supplying K⁺ to plants, considering both the potassium-solubilizing microorganisms and the potassium-facilitating microorganisms that are in close contact with the roots of plants. These microorganisms can influence the active transporter-mediated transfer of K⁺. Regarding the latter group of microorganisms, special focus is placed on the role of endophytic fungus. This review also includes a discussion on productivity through sustainable agriculture.

Phylogenetically diverse endophytic bacteria from desert plants induce transcriptional changes of tissue-specific ion transporters and salinity stress in Arabidopsis thaliana

Salinity severely hampers crop productivity worldwide and plant growth promoting bacteria could serve as a sustainable solution to improve plant growth under salt stress. However, the molecular mechanisms underlying salt stress tolerance promotion by beneficial bacteria remain unclear. In this work, six bacterial isolates from four different desert plant species were screened for their biochemical plant growth promoting traits and salinity stress tolerance promotion of the unknown host plant Arabidopsis thaliana. Five of the isolates induced variable root phenotypes but could all increase plant shoot and root weight under salinity stress. Inoculation of Arabidopsis with five isolates under salinity stress resulted in tissue-specific transcriptional changes of ion transporters and reduced Na⁺/K⁺ shoot ratios. The work provides first insights into the possible mechanisms and the commonality by which phylogenetically diverse bacteria from different desert plants induce salinity stress tolerance in Arabidopsis. The bacterial isolates provide new tools for studying abiotic stress tolerance mechanisms in plants and a promising agricultural solution for increasing crop yields in semi-arid regions.

Antarctic Extremophiles: Biotechnological Alternative to Crop Productivity in Saline Soils

Salinization of soils is one of the main sources of soil degradation worldwide, particularly in arid and semiarid ecosystems. High salinity results in osmotic stress and it can negatively impact plant grow and survival. Some plant species, however, can tolerate salinity by accumulating osmolytes like proline and maintaining low Na+ concentrations inside the cells. Another mechanism of saline stress tolerance is the association with symbiotic microorganism, an alternative that can be used as a biotechnological tool in susceptible crops. From the immense diversity of plant symbionts, those found in extreme environments such as Antarctica seems to be the ones with most potential since they (and their host) evolved in harsh and stressful conditions. We evaluated the effect of the inoculation with a consortium of plant growth-promoting rhizobacteria (PGPB) and endosymbiotic fungi isolated from an Antarctic plant on saline stress tolerance in different crops. To test this we established 4 treatments: (i) uninoculated plants with no saline stress, (ii) uninoculated plants subjected to saline stress (200 mM NaCl), (iii) plants inoculated with the microorganism consortium with no saline stress, and (iv) inoculated plants subjected to saline stress. First, we assessed the effect of symbiont consortium on survival of four different crops (cayenne, lettuce, onion, and tomato) in order to obtain a more generalized response of this biological interaction. Second, in order to deeply the mechanisms involved in salt tolerance, in lettuce plants we measured the ecophysiological performance (Fv/Fm) and lipid peroxidation to estimate the impact of saline stress on plants. We also measured proline accumulation and NHX1 antiporter gene expression (involved in Na+ detoxification) to search for possible mechanism of stress tolerance. Additionally, root, shoot, and total biomass was also obtained as an indicator of productivity. Overall, plants inoculated with microorganisms from Antarctica increased the fitness related traits in several crops. In fact, three of four crops selected to assess the general response increased its survival under salt conditions compared with those uninoculated plants. On the other hand, saline stress negatively impacted all measured trait, but inoculated plants were significantly less affected. In control osmotic conditions, there were no differences in proline accumulation and lipid peroxidation between inoculation treatments. Interestingly, even in control salinity, Fv/Fm was higher in inoculated plants after 30 and 60 days. Under osmotic stress, Fv/Fm, proline accumulation and NHX1 expression was significantly higher and lipid peroxidation lower in inoculated plants compared to uninoculated individuals. Moreover, inoculated plants exposed to saline stress had a similar final biomass (whole plant) compared to individuals under no stress. We conclude that Antarctic extremophiles can effectively reduce the physiological impact of saline stress in a salt-susceptible crops and also highlight extreme environments such as Antarctica as a key source of microorganism with high biotechnological potential.

Arabidopsis thaliana responds to colonisation of Piriformospora indica by secretion of symbiosis-specific proteins

Plants interact with a wide variety of fungi in a mutualistic, parasitic or neutral way. The associations formed depend on the exchange of nutrients and signalling molecules between the partners. This includes a diverse set of protein classes involved in defence, nutrient uptake or establishing a symbiotic relationship. Here, we have analysed the secretomes of the mutualistic, root-endophytic fungus Piriformospora indica and Arabidopsis thaliana when cultivated alone or in a co-culture. More than one hundred proteins were identified as differentially secreted, including proteins associated with growth, development, abiotic and biotic stress response and mucilage. While some of the proteins have been associated before to be involved in plant-microbial interaction, other proteins are newly described in this context. One plant protein found in the co-culture is PLAT1 (Polycystin, Lipoxygenase, Alpha-toxin and Triacylglycerol lipase). PLAT1 has not been associated with plant-fungal-interaction and is known to play a role in abiotic stress responses. In colonised roots PLAT1 shows an altered gene expression in a stage specific manner and plat1 knock-out plants are colonised stronger. It co-localises with Brassicaceae-specific endoplasmic reticulum bodies (ER-bodies) which are involved in the formation of the defence compound scopolin. We observed degraded ER-bodies in infected Arabidopsis roots and a change in the scopolin level in response to the presence of the fungus.

Cadmium Tolerance of Perennial Ryegrass Induced by Aspergillus aculeatus

Cadmium (Cd) pollution is becoming increasingly prevalent, posing a global environmental hazard due to its negative effects on plants growth and human health. Phytoremediation is a green technology that involves uptake of Cd from the soil by a combination of plants and associated microbes. The objective of this study was to investigate the role of Aspergillus aculeatus in perennial ryegrass Cd tolerance. This fungus produced indole-3-acetic acid, siderophores, and 1-aminocyclopropane-1-carboxylate deaminase. Physiological traits including growth rate, turf quality and chlorophyll content were measured to evaluate the physiological responses of perennial ryegrass to Cd stress. These physiological traits were improved after inoculated with A. aculeatus. Inoculation of A. aculeatus actively reduced DTPA-Cd concentration in the soil and Cd translocation to plant shoots. Chlorophyll a fluorescence transient and the C/N ratio in shoots were elevated by A. aculeatus, which implied that the fungus could protect the photosystem II against Cd stress and increase the photosynthetic efficiency. These results suggested that A. aculeatus is beneficial in improving Cd tolerance of perennial ryegrass and reducing Cd-induced injuries, thus, it has promising potential for application of phytostabilization in Cd contaminated soil.

Piriformospora indica inoculation alleviates the adverse effect of NaCl stress on growth, gas exchange and chlorophyll fluorescence in tomato (Solanum lycopersicum L.)

Salinity is now an increasingly serious environmental issue that affects the growth and yield of many plants. In the present work, the influence of inoculation with the symbiotic fungus, Piriformospora indica, on gas exchange, water potential, osmolyte content, Na/K ratio and chlorophyll fluorescence of tomato plants under three salinity levels (0, 50, 100 and 150 mm NaCl) and three time periods (5, 10 and 15 days after exposure to salt) was investigated. Results indicate that P. indica inoculation improved growth parameters of tomato under salinity stress. This symbiotic fungus significantly increased photosynthetic pigment content under salinity, and more proline and glycine betaine accumulated in inoculated roots than in non-inoculated roots. P. indica further significantly improved K⁺ content and reduced Na⁺ level under salinity treatment. After inoculation with the endophytic fungus, leaf physiological parameters, such as water potential, net photosynthesis, stomatal conductance and transpiration, were all higher under the salt concentrations and durations compared with controls without P. indica. With increasing salt level and salt treatment duration, values of F₀ and qP increased but F_m , F_v /F_m , F'_v /F'_m and NPQ declined in the controls, while inoculation with P. indica improved these values. The results indicate that the negative effects of NaCl on tomato plants were alleviated after P. indica inoculation, probably by improving physiological parameters such as water status and photosynthesis.

The Arabidopsis AtUNC-93 Acts as a Positive Regulator of Abiotic Stress Tolerance and Plant Growth via Modulation of ABA Signaling and K+ Homeostasis

Potassium (K⁺) is one of the essential macronutrients required for plant growth and development, and the maintenance of cellular K⁺ homeostasis is important for plants to adapt to abiotic stresses and growth. However, the mechanism involved has not been understood clearly. In this study, we demonstrated that AtUNC-93 plays a crucial role in this process under the control of abscisic acid (ABA). AtUNC-93 was localized to the plasma membrane and mainly expressed in the vascular tissues in Arabidopsis thaliana. The atunc-93 mutants showed typical K⁺-deficient symptoms under low-K⁺ conditions. The K⁺ contents of atunc-93 mutants were significantly reduced in shoots but not in roots under either low-K⁺ or normal conditions compared with wild type plants, whereas the AtUNC-93-overexpressing lines still maintained relatively higher K⁺ contents in shoots under low-K⁺ conditions, suggesting that AtUNC-93 positively regulates K⁺ translocation from roots to shoots. The atunc-93 plants exhibited dwarf phenotypes due to reduced cell expansion, while AtUNC-93-overexpressing plants had larger bodies because of increased cell expansion. After abiotic stress and ABA treatments, the atunc-93 mutants was more sensitive to salt, drought, osmotic, heat stress and ABA than wild type plants, while the AtUNC-93-overexpressing lines showed enhanced tolerance to these stresses and insensitive phenotype to ABA. Furthermore, alterations in the AtUNC-93 expression changed expression of many ABA-responsive and stress-related genes. Our findings reveal that AtUNC-93 functions as a positive regulator of abiotic stress tolerance and plant growth by maintaining K⁺ homeostasis through ABA signaling pathway in Arabidopsis.

Induction of abiotic stress tolerance in plants by endophytic microbes

Endophytes are micro-organisms including bacteria and fungi that survive within healthy plant tissues and promote plant growth under stress. This review focuses on the potential of endophytic microbes that induce abiotic stress tolerance in plants. How endophytes promote plant growth under stressful conditions, like drought and heat, high salinity and poor nutrient availability will be discussed. The molecular mechanisms for increasing stress tolerance in plants by endophytes include induction of plant stress genes as well as biomolecules like reactive oxygen species scavengers. This review may help in the development of biotechnological applications of endophytic microbes in plant growth promotion and crop improvement under abiotic stress conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

Increasing human populations demand more crop yield for food security while crop production is adversely affected by abiotic stresses like drought, salinity and high temperature. Development of stress tolerance in plants is a strategy to cope with the negative effects of adverse environmental conditions. Endophytes are well recognized for plant growth promotion and production of natural compounds. The property of endophytes to induce stress tolerance in plants can be applied to increase crop yields. With this review, we intend to promote application of endophytes in biotechnology and genetic engineering for the development of stress-tolerant plants.