Plants share their habitats with a multitude of different microbes. This close vicinity promoted the evolution of interorganismic interactions between plants and many different microorganisms that provide mutual growth benefits both to the plant and the microbial partner. The symbiosis of Arabidopsis thaliana with the beneficial root colonizing endophyte Serendipita indica represents a well‐studied system. Colonization of Arabidopsis roots with S. indica promotes plant growth and stress tolerance of the host plant. However, until now, the molecular mechanism by which S. indica reprograms plant growth remains largely unknown. This study used comprehensive transcriptomics, metabolomics, reverse genetics, and life cell imaging to reveal the intricacies of auxin‐related processes that affect root growth in the symbiosis between A. thaliana and S. indica. Our experiments revealed the sustained stimulation of auxin signalling in fungus infected Arabidopsis roots and disclosed the essential role of tightly controlled auxin conjugation in the plant–fungus interaction. It particularly highlighted the importance of two GRETCHEN HAGEN 3 (GH3) genes, GH3.5 and GH3.17, for the fungus infection‐triggered stimulation of biomass production, thus broadening our knowledge about the function of GH3s in plants. Furthermore, we provide evidence for the transcriptional alteration of the PIN2 auxin transporter gene in roots of Arabidopsis seedlings infected with S. indica and demonstrate that this transcriptional adjustment affects auxin signalling in roots, which results in increased plant growth.

Plants are constantly exposed to a variety of different environmental stresses, including drought, salinity, and elevated temperatures. These stress cues are assumed to intensify in the future driven by the global climate change scenario which we are currently experiencing. These stressors have largely detrimental effects on plant growth and development and, therefore, put global food security in jeopardy. For this reason, it is necessary to expand our understanding of the underlying mechanisms by which plants respond to abiotic stresses. Especially boosting our insight into the ways by which plants balance their growth and their defense programs appear to be of paramount importance, as this may lead to novel perspectives that can pave the way to increase agricultural productivity in a sustainable manner. In this review, our aim was to present a detailed overview of different facets of the crosstalk between the antagonistic plant hormones abscisic acid (ABA) and auxin, two phytohormones that are the main drivers of plant stress responses, on the one hand, and plant growth, on the other.

Serendipita indica, a widely studied beneficial root-colonizing fungal endophyte, promotes plant growth under saline conditions by reducing Na⁺ accumulation in host plants, including Arabidopsis thaliana. This reduction in Na ⁺ levels likely contributes to salt detoxification, but the underlying mechanisms remain unclear. Previous studies have demonstrated that SOS1, a key Na⁺ transporter and major determinant of salt tolerance in plants, is not involved in this reduction. To explore whether other plant transporters might participate in Na⁺ reduction, we first characterized the full Arabidopsis transportome, with putative substrates and subcellular localizations, and performed a comprehensive transcriptomic analysis of the full set of transporter proteins. In this study, we investigated and excluded the possible contribution of HKT1, another relevant Na⁺ transporter implicated in salt adaptation in Arabidopsis. By examining differentially expressed transporters under salt stress, we identified a subset of candidate genes potentially involved in Na⁺ transport. Among these, we evaluated the role of CNGC10 and CNGC13 using mutant lines under both S. indica- and non-colonized conditions. Interestingly, both transporters appeared to be involved in the endophyte-induced decrease in Na⁺ accumulation although, other, yet-unidentified transporters may also contribute to this phenomenon.

Summary: Plants orchestrate their developmental processes and responses to environmental stimuli through a sophisticated network of small signaling molecules, termed phytohormones. Among these, auxins are recognized for their role in promoting plant growth. However, indole‐3‐acetamide (IAM), an auxin precursor, has been observed to inhibit primary root elongation. The molecular mechanism underlying this inhibitory effect remains largely unexplored. A comprehensive genome‐wide association study (GWAS) conducted on a highly diverse collection of 166 wild Arabidopsis accessions from the Iberian Peninsula has identified several genomic regions associated with reduced IAM sensitivity under controlled in vitro conditions. This study highlighted ABA3 and GA2ox2 as possible candidate genes. Molecular and structural analyses suggest that the inhibition of primary root elongation induced by IAM is intricately associated with the enhanced production of abscisic acid (ABA) involving ABA3. Studies employing mutant and reporter lines have confirmed that IAM activates ABA signaling, thereby revealing a novel interaction between the auxin precursor IAM and ABA and suggesting an independent role for IAM as a signaling molecule in plant hormone crosstalk.

Plants are constantly exposed to a variety of different environmental stresses, including drought, salinity, and elevated temperatures. These stress cues are assumed to aggravate in the future driven by the global climate change scenario which we are currently experiencing. These stressors have largely detrimental effects on plant growth and development and, therefore, put global food security in jeopardy. For this reason, it is necessary to expand our understanding of the underlying mechanisms by which plants respond to abiotic stresses. Especially boosting our insight into the ways by which plants balance their growth and their defense programs appears to be of paramount importance, as this may lead to novel perspectives that can pave the way to increase agricultural productivity in a sustainable manner. In this review, our aim was to present a detailed overview of different factettes of the crosstalk between the antagonistic plant hormones abscisic acid (ABA) and auxin, two phytohormones that are the main drivers of plant stress responses, on the one hand, and plant growth, on the other.

World population growth in the context of climate change is threatening food security. Advances in increasing agricultural yields are not sufficient to satisfy the growing demand for food, which calls for holistic approaches to optimize crop yields. Plants coexist with a wide variety of microorganisms, some beneficial and some pathogenic. Understanding beneficial interactions within the plant microbiome is key to identifying molecular mechanisms that can enhance crop productivity and address the future challenges of food production. The mutualistic interaction between Serendipita indica and Arabidopsis thaliana is a relevant model for the study of plant-microbe interactions. However, the molecular mechanisms by which S. indica increases root growth in A. thaliana seedlings are unknown. In this work, transcriptomic analysis, metabolomic analysis, reverse genetics, and confocal microscopy were used to decipher the details of auxin-related processes in root growth of S. indica-colonized seedlings. The results showed increased auxin signalling in roots inoculated with the fungus and highlighted the essential role of auxin conjugation in the hormonal homeostasis of colonized seedlings. In particular, the GRETCHEN HAGEN 3 (GH3), GH3.5 and GH3.17 genes are critical for the growth-promoting effect triggered by the fungus. In addition, a transcriptional alteration of the gene coding for the auxin transporter PIN2 was observed in fungus-colonized roots, which favoured the creation of local auxin maxima in root tips. This process, combined with GH3.5 and GH3.17-mediated auxin conjugation, contributed to the promotion of root growth. In addition to promoting plant growth, S. indica enhances abiotic stress tolerance in its hosts. Although it has not co-evolved with most of the plants it colonizes, its ability to induce tolerance suggests that the molecular mechanisms underlying these interactions must be conserved. Therefore, the interaction of three endophytic fungi was studied: Penicillium chrysogenum, Penicillium minioluteum, and S. indica, isolated from three very different arid geographical areas: Antarctica, Atacama Desert, and Thar Desert, respectively, on Solanum lycopersicum, which is a non-native host for any of the fungi. All three fungi colonized tomato roots and enhanced tolerance to water deficit. Using transcriptomic analysis, we identified a genetic module shared by S. indica and P. chrysogenum-infected plants. Subsequent hierarchical clustering and functional interaction network analysis allowed us to identify genes related to abscisic acid (ABA) signalling, ABA transport, proline biosynthesis, auxin signalling, ion homeostasis, and jasmonic acid signalling, providing information on the molecular basis of drought tolerance mediated by two endophytic fungi isolated from geographically unrelated arid environments in tomato. Finally, the study of the enhancement of drought stress tolerance by three endophytic fungi in tomato plants, including a P. chrysogenum isolated in Antarctica, led to the investigation of genomic differences between this endophytic strain and other species of the Penicillium genera, characterized by their high antibiotic production. Genome sequencing of this strain and comparative analysis with Penicillium rubens Wisconsin 54-1255 (formerly named P. chrysogenum) revealed differences in genome size and structure. In addition, variations in carbohydrate metabolism were identified and experimentally confirmed, highlighting possible differences at the genomic level that could explain the differences in lifestyle between the two strains. RESUMEN El aumento de la población mundial junto con el contexto del cambio climático está amenazando la seguridad alimentaria. Los avances en el incremento de la productividad agrícola no son suficientes para satisfacer la creciente demanda de alimento, lo que exige enfoques integrales que optimicen el rendimiento de los cultivos. Las plantas coexisten con una amplia variedad de microorganismos, algunos beneficiosos y otros patógenos. Comprender las interacciones beneficiosas dentro del microbioma de las plantas es clave para identificar mecanismos moleculares que puedan aumentar la capacidad productiva de los cultivos y afrontar los desafíos alimentarios. La interacción mutualista entre Serendipita indica y Arabidopsis thaliana es un modelo relevante para el estudio de las interacciones planta microorganismo. No obstante, los mecanismos moleculares por los cuáles S. indica aumenta el crecimiento radicular en plántulas de A. thaliana se desconocen. En este trabajo se han empleado análisis transcriptómicos, análisis metabolómicos, genética reversa y microscopía confocal para descifrar los detalles de los procesos relacionados con la auxina en el crecimiento radicular de plántulas colonizadas por S. indica. Los resultados mostraron un aumento de la señalización de auxina en raíces inoculadas con el hongo y destacaron el papel esencial que desempeña la conjugación de auxina en la homeostasis hormonal de las plántulas colonizadas. En particular, los genes GRETCHEN HAGEN 3 (GH3), GH3.5 y GH3.17 son fundamentales para el efecto promotor del crecimiento mediado por el hongo. Además, se observó una alteración transcripcional de PIN2 en raíces colonizadas por el hongo, lo que favoreció la creación de máximos locales de auxina en las puntas de las raíces. Este proceso, combinado con la conjugación de auxina mediada por GH3.5 y GH3.17, contribuyó a la promoción del crecimiento radicular. Además de promover el crecimiento vegetal, S. indica aumenta la tolerancia a estrés abiótico en sus huéspedes. A pesar de no haber co-evolucionado con la mayoría de las plantas que coloniza, su capacidad para inducir tolerancia sugiere que los mecanismos moleculares que...