The endophytic fungus Serendipita indica affects auxin distribution in Arabidopsis thaliana roots through alteration of auxin transport and conjugation to promote plant growth.

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.

Do Opposites Attract? Auxin-Abscisic Acid Crosstalk: New Perspectives.

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.

The calcium sensor CBL7 is required for Serendipita indica-induced growth stimulation in Arabidopsis thaliana, controlling defense against the endophyte and K+ homoeostasis in the symbiosis.

Calcium is an important second messenger in plants. The activation of Ca2+ signalling cascades is critical in the activation of adaptive processes in response to environmental stimuli. Root colonization by the growth promoting endophyte Serendipita indica involves the increase of cytosolic Ca2+ levels in Arabidopsis thaliana. Here, we investigated transcriptional changes in Arabidopsis roots during symbiosis with S. indica. RNA-seq profiling disclosed the induction of Calcineurin B-like 7 (CBL7) during early and later phases of the interaction. Consistently, reverse genetic evidence highlighted the functional relevance of CBL7 and tested the involvement of a CBL7-CBL-interacting protein kinase 13 signalling pathway. The loss-of-function of CBL7 abolished the growth promoting effect and affected root colonization. The transcriptomics analysis of cbl7 revealed the involvement of this Ca2+ sensor in activating plant defense responses. Furthermore, we report on the contribution of CBL7 to potassium transport in Arabidopsis. We analysed K+ contents in wild-type and cbl7 plants and observed a significant increase of K+ in roots of cbl7 plants, while shoot tissues demonstrated K+ depletion. Taken together, our work associates CBL7 with an important role in the mutual interaction between Arabidopsis and S. indica and links CBL7 to K+ transport.

The Indole-3-Acetamide-Induced Arabidopsis Transcription Factor MYB74 Decreases Plant Growth and Contributes to the Control of Osmotic Stress Responses.

The accumulation of the auxin precursor indole-3-acetamide (IAM) in the ami1 mutant has recently been reported to reduce plant growth and to trigger abiotic stress responses in Arabidopsis thaliana. The observed response includes the induction of abscisic acid (ABA) biosynthesis through the promotion of NCED3 expression. The mechanism by which plant growth is limited, however, remained largely unclear. Here, we investigated the transcriptional responses evoked by the exogenous application of IAM using comprehensive RNA-sequencing (RNA-seq) and reverse genetics approaches. The RNA-seq results highlighted the induction of a small number of genes, including the R2R3 MYB transcription factor genes MYB74 and MYB102. The two MYB factors are known to respond to various stress cues and to ABA. Consistent with a role as negative plant growth regulator, conditional MYB74 overexpressor lines showed a considerable growth reduction. RNA-seq analysis of MYB74 mutants indicated an association of MYB74 with responses to osmotic stress, water deprivation, and seed development, which further linked MYB74 with the observed ami1 osmotic stress and seed phenotype. Collectively, our findings point toward a role for MYB74 in plant growth control and in responses to abiotic stress stimuli.