Rosario Haro

Salinity is a major constraint to crop productivity. Beneficial plant–fungus interactions represent a promising strategy to enhance stress resilience. Here, we investigated fungal endophytes isolated from the roots of Oryza sativa cultivated in saline-prone marshlands of the Guadalquivir River, Spain. From a collection of 38 isolates, five salt-tolerant strains exhibiting plant growth-promoting activity were identified, including a previously uncharacterized Reticulascus sp. strain S5. Co-cultivation assays with the non-native host plant Arabidopsis thaliana demonstrated that S5 increased the root and shoot biomass under salt stress. To elucidate the underlying molecular mechanisms, a comprehensive RNA-Seq analysis of the roots and shoots under control and saline conditions was performed. Fungal colonization induced pronounced transcriptomic changes, particularly in the shoots, including rewiring of the auxin- and abscisic acid-related pathways and the induction of genes associated with cell wall remodeling. Concurrently, defense-related processes, including glucosinolate biosynthesis and ethylene signaling, were broadly repressed, suggesting attenuated stress perception in colonized plants. In the roots, S5 inoculation suppressed the expression of genes involved in root hair development and cell wall organization, indicating a fungus-driven reconfiguration of root development. Moreover, comparative analysis with Fusarium sp. K-23, a fungus that has previously been demonstrated to promote plant growth under salinity stress, revealed distinct root-associated mechanisms but convergence on a shared regulatory module in shoots involving ABA-responsive transcription factors and osmotic stress regulators. Collectively, our findings demonstrate that Reticulascus sp. S5 enhances plant salt stress tolerance through the coordinated transcriptional reprogramming of growth, hormone signaling, and stress responses, highlighting a possible potential of habitat-adapted endophytes for sustainable crop improvement.

Salinity is a major constraint to crop productivity. Beneficial plant–fungus interactions represent a promising strategy to enhance stress resilience. Here, we investigated fungal endophytes isolated from Oryza sativa cultivated in saline-prone marshlands of the Guadalquivir River, Spain. From a collection of 38 isolates, five salt-tolerant strains exhibiting plant growth-promoting activity were identified, including a previously uncharacterized Reticulascus sp. strain S5. Co-cultivation assays with Arabidopsis thaliana demonstrated that S5 increased root and shoot biomass under salt stress. To elucidate the underlying mechanisms, a comprehensive RNA-seq analysis of roots and shoots under control and saline conditions was performed. Fungal colonization induced pronounced transcriptomic changes, particularly in shoots, including the rewiring of auxin- and abscisic acid-related pathways and the induction of genes associated with cell wall remodelling. Concurrently, defence-related processes, including glucosinolate biosynthesis and ethylene signalling, were broadly repressed, suggesting attenuated stress perception in colonized plants. In roots, S5 infection suppressed genes involved in root hair development and cell wall organization, indicating a fungus-driven reconfiguration of root development. Comparative analysis with the growth-promoting Fusarium sp. K-23 revealed distinct root-associated mechanisms but convergence on a shared regulatory module in shoots involving ABA-responsive transcription factors and osmotic stress regulators. Collectively, our findings demonstrate that Reticulascus sp. S5 enhances plant salt stress tolerance through coordinated transcriptional reprogramming of growth, hormone signalling, and stress responses, highlighting the potential of habitat-adapted endophytes for sustainable crop improvement.

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.

Summary: 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.

Main conclusion Arabidopsis plants in NaCl suffering half growth inhibition do not suffer osmotic stress and seldom shoot Na ⁺ toxicity; overaccumulation of Na ⁺ plus K ⁺ might trigger the inhibition. It is widely assumed that salinity inhibits plant growth by osmotic stress and shoot Na⁺ toxicity. This study aims to examine the growth inhibition of Arabidopsis thaliana by NaCl concentrations that allow the completion of the life cycle. Unaffected Col-0 wild-type plants were used to define nontoxic Na⁺ contents; Na⁺ toxicities in shoots and roots were analyzed in hkt1 and sos1 mutants, respectively. The growth inhibition of Col-0 plants at 40 mM Na⁺ was mild and equivalent to that produced by 8 and 4 mM Na⁺ in hkt1 and sos1 plants, respectively. Therefore, these mutants allowed to study the toxicity of Na⁺ in the absence of an osmotic challenge. Col-0 and Ts-1 accessions showed very different Na⁺ contents but similar growth inhibitions; Ts-1 plants showed very high leaf Na⁺ contents but no symptoms of Na⁺ toxicity. Ak-1, C24, and Fei-0 plants were highly affected by NaCl showing evident symptoms of shoot Na⁺ toxicity. Increasing K⁺ in isotonic NaCl/KCl combinations dramatically decreased the Na⁺ content in all Arabidopsis accessions and eliminated the signs of Na⁺ toxicity in most of them but did not relieve growth inhibition. This suggested that the dominant inhibition in these conditions was either osmotic or of an ionic nature unspecific for Na⁺ or K⁺. Col-0 and Ts-1 plants growing in sorbitol showed a clear osmotic stress characterized by a notable decrease of their water content, but this response did not occur in NaCl. Overaccumulation of Na⁺ plus K⁺ might trigger growth reduction in NaCl-treated plants.

The function of HAK transporters in high-affinity K⁺ uptake in plants is well established; this study aims to demonstrate that some transporters of the same family play important roles in endomembranes. The PpHAK2–PpHAK4 genes of Physcomitrella patens encode three transporters of high sequence similarity. Quantitative PCR showed that PpHAK2 and PpHAK3 transcripts are expressed at approximately the same level as the PpACT5 gene, while the expression of PpHAK4 seems to be restricted to specific conditions that have not been determined. KHA1 is an endomembrane K⁺/H⁺ antiporter of Saccharomyces cerevisiae, and the expression of the PpHAK2 cDNA, but not that of PpHAK3, suppressed the defect of a kha1 mutant. Transient expression of the PpHAK2–green fluorescent protein (GFP) and PpHAK3–GFP fusion proteins in P. patens protoplasts localized to the endoplasmic reticulum and Golgi complex, respectively. To determine the function of PpHAK2 and PpHAK3 in planta, we constructed ΔPphak2 and ΔPphak2 ΔPphak3 plants. ΔPphak2 plants were normal under all of the conditions tested except under K⁺ starvation or at acidic pH in the presence of acetic acid, whereupon they die. The defect observed under K⁺ starvation was suppressed by the presence of Na⁺. We propose that PpHAK2 may encode either a K⁺–H⁺ symporter or a K⁺/H⁺ antiporter that mediates the transfer of H⁺ from the endoplasmic reticulum lumen to the cytosol. PpHAK2 may be a model of the second function of HAK transporters in plant cells. The disruption of the PpHAK3 gene in ΔPphak2 plants showed no effect.

This study aims to increase our understanding of the functions of CHX transporters in plant cells using the model plant Physcomitrella patens, in which four CHX genes have been identified, PpCHX1–PpCHX4. Two of these genes, PpCHX1 and PpCHX2, are expressed at approximately the same level as the PpACT5 gene, but the other two genes show an extremely low expression. PpCHX1 and PpCHX2 restored growth of Escherichia coli mutants on low K⁺-containing media, suggesting that they mediated K⁺ uptake that may be energized by symport with H⁺. In contrast, these genes suppressed the defect associated with the kha1 mutation in Saccharomyces cerevisiae, which suggests that they might mediate K⁺/H⁺ antiport. PpCHX1–green fluorescent protein (GFP) fusion protein transiently expressed in P. patens protoplasts co-localized with a Golgi marker. In similar experiments, the PpCHX2–GFP protein appeared to localize to tonoplast and plasma membrane. We constructed the ΔPpchx1 and ΔPpchx2 single mutant lines, and the ΔPpchx2 ΔPphak1 double mutant. Single mutant plants grew normally under all the conditions tested and exhibited normal K⁺ and Rb⁺ influxes; the ΔPpchx2 mutation did not increase the defect of ΔPphak1 plants. In long-term experiments, ΔPpchx2 plants showed slightly higher Rb⁺ retention than wild-type plants, which suggests that PpCHX2 mediates the transfer of Rb⁺ either from the vacuole to the cytosol or from the cytosol to the external medium in parallel with other transporters. The distinction between these two possibilities is technically difficult. We suggest that K⁺ transporters of several families are involved in the pH homeostasis of organelles by mediating either K⁺/H⁺ antiport or K⁺–H⁺ symport.

Control of intracellular Na⁺ and K⁺ concentrations in plant tissues under saline conditions is crucial for salt tolerance. In this study, two Na⁺ ‐selective class I‐HKT transporters, HKT1;1 and HKT1;2, from tomato were found to be closely linked in proximity to the position with the maximum LOD score for major QTLs in chromosome 7 (at a distance of 35 kb), thus contributing to stem and leaf Na⁺ and K⁺ in two populations of tomato RILs derived from two salt‐tolerant lines from wild tomato species S. pimpinellifolium and S. cheesmaniae. This result, together with their patterns of expression and phenotypic effects on a pair of near isogenic lines (NILs) differing in terms of HKT1‐like alleles, support the hypothesis that HKT1;1 and/or HKT1;2 genes underlies a major tomato QTL for shoot Na⁺ / K⁺ .