TPLATE Recruitment Reveals Endocytic Dynamics at Sites of Symbiotic Interface Assembly in Arbuscular Mycorrhizal Interactions

Unraveling the rhizobial infection thread

Most legumes can form an endosymbiotic association with soil bacteria called rhizobia, which colonize specialized root structures called nodules where they fix nitrogen. To colonize nodule cells, rhizobia must first traverse the epidermis and outer cortical cell layers of the root. In most legumes, this involves formation of the infection thread, an intracellular structure that becomes colonized by rhizobia, guiding their passage through the outer cell layers of the root and into the newly formed nodule cells. In this brief review, we recount the early research milestones relating to the rhizobial infection thread and highlight two relatively recent advances in the symbiotic infection mechanism, the eukaryotically conserved 'MYB-AUR1-MAP' mitotic module, which links cytokinesis mechanisms to intracellular infection, and the discovery of the 'infectosome' complex, which guides infection thread growth. We also discuss the potential intertwining of the two modules and the hypothesis that cytokinesis served as a foundation for intracellular infection of symbiotic microbes.

Extracellular Vesicles in the Arbuscular Mycorrhizal Symbiosis: Current Understanding and Future Perspectives

The arbuscular mycorrhizal (AM) symbiosis is an ancient and highly conserved mutualism between plant and fungal symbionts, in which a highly specialized membrane-delimited fungal arbuscule acts as the symbiotic interface for nutrient exchange and signaling. As a ubiquitous means of biomolecule transport and intercellular communication, extracellular vesicles (EVs) are likely to play a role in this intimate cross-kingdom symbiosis, yet, there is a lack of research investigating the importance of EVs in AM symbiosis despite known roles in microbial interactions in both animal and plant pathosystems. Clarifying the current understanding of EVs in this symbiosis in light of recent ultrastructural observations is paramount to guiding future investigations in the field, and, to this end, this review summarizes recent research investigating these areas. Namely, this review discusses the available knowledge regarding biogenesis pathways and marker proteins associated with the various plant EV subclasses, EV trafficking pathways during symbiosis, and the endocytic mechanisms implicated in the uptake of these EVs. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Long-lasting impact of chitooligosaccharide application on strigolactone biosynthesis and fungal accommodation promotes arbuscular mycorrhiza in Medicago truncatula

The establishment of arbuscular mycorrhiza (AM) between plants and Glomeromycotina fungi is preceded by the exchange of chemical signals: fungal released Myc-factors, including chitooligosaccharides (CO) and lipo-chitooligosaccharides (LCO), activate plant symbiotic responses, while root-exuded strigolactones stimulate hyphal branching and boost CO release. Furthermore, fungal signaling reinforcement through CO application was shown to promote AM development in Medicago truncatula, but the cellular and molecular bases of this effect remained unclear. Here, we focused on long-term M. truncatula responses to CO treatment, demonstrating its impact on the transcriptome of both mycorrhizal and nonmycorrhizal roots over several weeks and providing an insight into the mechanistic bases of the CO-dependent promotion of AM colonization. CO treatment caused the long-lasting regulation of strigolactone biosynthesis and fungal accommodation-related genes. This was mirrored by an increase in root didehydro-orobanchol content, and the promotion of accommodation responses to AM fungi in root epidermal cells. Lastly, an advanced downregulation of AM symbiosis marker genes was observed at the latest time point in CO-treated plants, in line with an increased number of senescent arbuscules. Overall, CO treatment triggered molecular, metabolic, and cellular responses underpinning a protracted acceleration of AM development.

A transcriptional activator from Rhizophagus irregularis regulates phosphate uptake and homeostasis in AM symbiosis during phosphorous starvation

Introduction

Phosphorus (P) is one of the most important nutrient elements for plant growth and development. Under P starvation, arbuscular mycorrhizal (AM) fungi can promote phosphate (Pi) uptake and homeostasis within host plants. However, the underlying mechanisms by which AM fungal symbiont regulates the AM symbiotic Pi acquisition from soil under P starvation are largely unknown. Here, we identify a HLH domain containing transcription factor RiPho4 from Rhizophagus irregularis.

Methods

To investigate the biological functions of the RiPho4, we combined the subcellular localization and Yeast One-Hybrid (Y1H) experiments in yeasts with gene expression and virus-induced gene silencing approach during AM symbiosis.

Results

The approach during AM symbiosis. The results indicated that RiPho4 encodes a conserved transcription factor among different fungi and is induced during the in planta phase. The transcription of RiPho4 is significantly up-regulated by P starvation. The subcellular localization analysis revealed that RiPho4 is located in the nuclei of yeast cells during P starvation. Moreover, knock-down of RiPho4 inhibits the arbuscule development and mycorrhizal Pi uptake under low Pi conditions. Importantly, RiPho4 can positively regulate the downstream components of the phosphate (PHO) pathway in R. irregularis.

Discussion

In summary, these new findings reveal that RiPho4 acts as a transcriptional activator in AM fungus to maintain arbuscule development and regulate Pi uptake and homeostasis in the AM symbiosis during Pi starvation.

Mechanisms in Growth-Promoting of Cucumber by the Endophytic Fungus Chaetomium globosum Strain ND35

Endophytic fungi are effective in plant growth and development by secreting various kinds of plant hormones and nutrients. However, the cellular and molecular interactions between the endophytic fungi and plant growth-promoting have remained less explored. The present study was designed to explore the effects of the infection and colonization events of Chaetomium globosum strain ND35 on cucumber growth and the expression pattern of some metabolically important genes in development of the cucumber radicle. The results demonstrated that strain ND35 can infect and colonize the outer layers (cortical cells) of cucumber root and form a symbiotic structure with the host cell, similar to a periarbuscular membrane and establish chemical communication with the plant. Through transcriptome analysis, we found the differentially expressed genes (DEGs) caused by strain ND35 were mainly enriched in phenylpropanoid biosynthesis, plant hormone signal transduction, plant-pathogen interaction and photosynthesis. Correspondingly, the contents of reactive oxygen species (ROS), hydrogen peroxide (H₂O₂), indole-3-acetic acid (IAA), gibberellin (GA), zeatin (ZT), salicylic acid (SA), jasmonic acid (JA) and the activity of phenylalanine ammonia lyase (PAL), 4-coumarate-CoA ligase (4CL), cinnamyl alcohol dehydrogenase (CAD), and peroxidase (POD) in ND35-colonized seedlings were generally higher than those of non-inoculated seedlings. Overall, the infection and colonization events of C. globosum strain ND35 increased cucumber growth through complex regulation of plant hormones biosynthesis and metabolism. Furthermore, although the endophytic fungus strain ND35 produced IAA, GA, ZT, and ergosterol in the fermentation broth, and there are enabled to promote growth of cucumber, it is uncertain whether there are ND35-derived microbial hormones in plants. This study of the interaction between cucumber and strain ND35 contributes to a better understanding of the plant-endophytic fungi interactions, and may help to develop new strategies for crop production.

Root cortex development is fine-tuned by the interplay of MIGs, SCL3 and DELLAs during arbuscular mycorrhizal symbiosis

Root development is a crucial process that determines the ability of plants to acquire nutrients, adapt to the substrate and withstand changing environmental conditions. Root plasticity is controlled by a plethora of transcriptional regulators that allow, in contrast to tissue development in animals, post-embryonic changes that give rise to new tissue and specialized cells. One of these changes is the accommodation in the cortex of hyperbranched hyphae of symbiotic arbuscular mycorrhizal (AM) fungi, called arbuscules. Arbuscule-containing cells undergo massive reprogramming to coordinate developmental changes with transport processes. Here we describe a novel negative regulator of arbuscule development, MIG3. MIG3 induces and interacts with SCL3, both of which modulate the activity of the central regulator DELLA, restraining cortical cell growth. As in a tug-of-war, MIG3-SCL3 antagonizes the function of the complex MIG1-DELLA, which promotes the cell expansion required for arbuscule development, adjusting cell size during the dynamic processes of the arbuscule life cycle. Our results in the legume plant Medicago truncatula advance the knowledge of root development in dicot plants, showing the existence of additional regulatory elements not present in Arabidopsis that fine-tune the activity of conserved central modules.

Home sweet home: how mutualistic microbes modify root development to promote symbiosis

Post-embryonic organogenesis has uniquely equipped plants to become developmentally responsive to their environment, affording opportunities to remodel organism growth and architecture to an extent not possible in other higher order eukaryotes. It is this developmental plasticity that makes the field of plant-microbe interactions an exceptionally fascinating venue in which to study symbiosis. This review article describes the various ways in which mutualistic microbes alter the growth, development, and architecture of the roots of their plant hosts. We first summarize general knowledge of root development, and then examine how association of plants with beneficial microbes affects these processes. Working our way inwards from the epidermis to the pericycle, this review dissects the cell biology and molecular mechanisms underlying plant-microbe interactions in a tissue-specific manner. We examine the ways in which microbes gain entry into the root, and modify this specialized organ for symbiont accommodation, with a particular emphasis on the colonization of root cortical cells. We present significant advances in our understanding of root-microbe interactions, and conclude our discussion by identifying questions pertinent to root endosymbiosis that at present remain unresolved.