Constitutive Overexpression of RAM1 Leads to an Increase in Arbuscule Density in Brachypodium distachyon

Susceptibility and plant immune control-a case of mycorrhizal strategy for plant colonization, symbiosis, and plant immune suppression

Plants and microbes (mycorrhizal fungi to be precise) have evolved together over the past millions of years into an association that is mutualist. The plants supply the fungi with photosynthates and shelter, while the fungi reciprocate by enhancing nutrient and water uptake by the plants as well as, in some cases, control of soil-borne pathogens, but this fungi-plant association is not always beneficial. We argue that mycorrhizal fungi, despite contributing to plant nutrition, equally increase plant susceptibility to pathogens and herbivorous pests' infestation. Understanding of mycorrhizal fungi strategies for suppressing plant immunity, the phytohormones involved and the signaling pathways that aid them will enable the harnessing of tripartite (consisting of three biological systems)-plant-mycorrhizal fungi-microbe interactions for promoting sustainable production of crops.

Brachypodium: 20 years as a grass biology model system; the way forward?

It has been 20 years since Brachypodium distachyon was suggested as a model grass species, but ongoing research now encompasses the entire genus. Extensive Brachypodium genome sequencing programmes have provided resources to explore the determinants and drivers of population diversity. This has been accompanied by cytomolecular studies to make Brachypodium a platform to investigate speciation, polyploidisation, perenniality, and various aspects of chromosome and interphase nucleus organisation. The value of Brachypodium as a functional genomic platform has been underscored by the identification of key genes for development, biotic and abiotic stress, and cell wall structure and function. While Brachypodium is relevant to the biofuel industry, its impact goes far beyond that as an intriguing model to study climate change and combinatorial stress.

Molecular Regulation of Arbuscular Mycorrhizal Symbiosis

Plant-microorganism interactions at the rhizosphere level have a major impact on plant growth and plant tolerance and/or resistance to biotic and abiotic stresses. Of particular importance for forestry and agricultural systems is the cooperative and mutualistic interaction between plant roots and arbuscular mycorrhizal (AM) fungi from the phylum Glomeromycotina, since about 80% of terrestrial plant species can form AM symbiosis. The interaction is tightly regulated by both partners at the cellular, molecular and genetic levels, and it is highly dependent on environmental and biological variables. Recent studies have shown how fungal signals and their corresponding host plant receptor-mediated signalling regulate AM symbiosis. Host-generated symbiotic responses have been characterized and the molecular mechanisms enabling the regulation of fungal colonization and symbiosis functionality have been investigated. This review summarizes these and other recent relevant findings focusing on the molecular players and the signalling that regulate AM symbiosis. Future progress and knowledge about the underlying mechanisms for AM symbiosis regulation will be useful to facilitate agro-biotechnological procedures to improve AM colonization and/or efficiency.

Multifarious and Interactive Roles of GRAS Transcription Factors During Arbuscular Mycorrhiza Development

Arbuscular mycorrhiza (AM) is a mutualistic symbiotic interaction between plant roots and AM fungi (AMF). This interaction is highly beneficial for plant growth, development and fitness, which has made AM symbiosis the focus of basic and applied research aimed at increasing plant productivity through sustainable agricultural practices. The creation of AM requires host root cells to undergo significant structural and functional modifications. Numerous studies of mycorrhizal plants have shown that extensive transcriptional changes are induced in the host during all stages of colonization. Advances have recently been made in identifying several plant transcription factors (TFs) that play a pivotal role in the transcriptional regulation of AM development, particularly those belonging to the GRAS TF family. There is now sufficient experimental evidence to suggest that GRAS TFs are capable to establish intra and interspecific interactions, forming a transcriptional regulatory complex that controls essential processes in the AM symbiosis. In this minireview, we discuss the integrative role of GRAS TFs in the regulation of the complex genetic re-programming determining AM symbiotic interactions. Particularly, research being done shows the relevance of GRAS TFs in the morphological and developmental changes required for the formation and turnover of arbuscules, the fungal structures where the bidirectional nutrient translocation occurs.

The wild grass Brachypodium distachyon as a developmental model system

The arrival of cheap and high-throughput sequencing paired with efficient gene editing technologies allows us to use non-traditional model systems and mechanistically approach biological phenomena beyond what was conceivable just a decade ago. Venturing into different model systems enables us to explore for example clade-specific environmental responses to changing climates or the genetics and development of clade-specific organs, tissues and cell types. We-both early career researchers working with the wild grass model Brachypodium distachyon-want to use this review to (1) highlight why we think B. distachyon is a fantastic grass developmental model system, (2) summarize the tools and resources that have enabled discoveries made in B. distachyon, and (3) discuss a handful of developmental biology vignettes made possible by using B. distachyon as a model system. Finally, we want to conclude by (4) relating our personal stories with this emerging model system and (5) share what we think is important to consider before starting work with an emerging model system.

Colonization of Mutualistic Mycorrhizal and Parasitic Blast Fungi Requires OsRAM2-Regulated Fatty Acid Biosynthesis in Rice

Arbuscular mycorrhizal (AM) fungi form a mutual association with the majority of land plants, including most angiosperms of the dicotyledon and monocotyledon lineages. The symbiosis is based upon bidirectional nutrient exchange between the host and symbiont that occurs between inner cortical cells of the root and branched AM hyphae called arbuscules that develop within these cells. Lipid transport and its regulation during the symbiosis have been intensively investigated in dicotyledon plants, especially legumes. Here, we characterize OsRAM2 and OsRAM2L, homologs of Medicago truncatula RAM2, and found that plants defective in OsRAM2 were unable to be colonized by AM fungi and showed impaired colonization by Magnaporthe oryzae. The induction of OsRAM2 and OsRAM2L is dependent on OsRAM1 and the common symbiosis signaling pathway pathway genes CCaMK and CYCLOPS, while overexpression of OsRAM1 results in increased expression of OsRAM2 and OsRAM2L. Collectively, our data show that the function and regulation of OsRAM2 is conserved in monocot and dicot plants and reveals that, similar to mutualistic fungi, pathogenic fungi have recruited RAM2-mediated fatty acid biosynthesis to facilitate invasion.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Conservation and Diversity in Gibberellin-Mediated Transcriptional Responses Among Host Plants Forming Distinct Arbuscular Mycorrhizal Morphotypes

Morphotypes of arbuscular mycorrhizal (AM) symbiosis, Arum, Paris, and Intermediate types, are mainly determined by host plant lineages. It was reported that the phytohormone gibberellin (GA) inhibits the establishment of Arum-type AM symbiosis in legume plants. In contrast, we previously reported that GA promotes the establishment of Paris-type AM symbiosis in Eustoma grandiflorum, while suppressing Arum-type AM symbiosis in a legume model plant, Lotus japonicus. This raises a hitherto unexplored possibility that GA-mediated transcriptional reprogramming during AM symbiosis is different among plant lineages as the AM morphotypes are distinct. Here, our comparative transcriptomics revealed that several symbiosis-related genes were commonly upregulated upon AM fungal colonization in L. japonicus (Arum-type), Daucus carota (Intermediate-type), and E. grandiflorum (Paris-type). Despite of the similarities, the fungal colonization levels and the expression of symbiosis-related genes were suppressed in L. japonicus and D. carota but were promoted in E. grandiflorum in the presence of GA. Moreover, exogenous GA inhibited the expression of genes involved in biosynthetic process of the pre-symbiotic signal component, strigolactone, which resulted in the reduction of its endogenous accumulation in L. japonicus and E. grandiflorum. Additionally, differential regulation of genes involved in sugar metabolism suggested that disaccharides metabolized in AM roots would be different between L. japonicus and D. carota/E. grandiflorum. Therefore, this study uncovered the conserved transcriptional responses during mycorrhization regardless of the distinct AM morphotype. Meanwhile, we also found diverse responses to GA among phylogenetically distant AM host plants.