Biolistic transformation of arbuscular mycorrhizal fungi. Progress and perspectives

Monitoring nutrients in plants with genetically encoded sensors: achievements and perspectives

Understanding mechanisms of nutrient allocation in organisms requires precise knowledge of the spatiotemporal dynamics of small molecules in vivo. Genetically encoded sensors are powerful tools for studying nutrient distribution and dynamics, as they enable minimally invasive monitoring of nutrient steady-state levels in situ. Numerous types of genetically encoded sensors for nutrients have been designed and applied in mammalian cells and fungi. However, to date, their application for visualizing changing nutrient levels in planta remains limited. Systematic sensor-based approaches could provide the quantitative, kinetic information on tissue-specific, cellular, and subcellular distributions and dynamics of nutrients in situ that is needed for the development of theoretical nutrient flux models that form the basis for future crop engineering. Here, we review various approaches that can be used to measure nutrients in planta with an overview over conventional techniques, as well as genetically encoded sensors currently available for nutrient monitoring, and discuss their strengths and limitations. We provide a list of currently available sensors and summarize approaches for their application at the level of cellular compartments and organelles. When used in combination with bioassays on intact organisms and precise, yet destructive analytical methods, the spatiotemporal resolution of sensors offers the prospect of a holistic understanding of nutrient flux in plants.

The intracellular delivery of TAT-aequorin reveals calcium-mediated sensing of environmental and symbiotic signals by the arbuscular mycorrhizal fungus Gigaspora margarita

Arbuscular mycorrhiza (AM) is an ecologically relevant symbiosis between most land plants and Glomeromycota fungi. The peculiar traits of AM fungi have so far limited traditional approaches such as genetic transformation. The aim of this work was to investigate whether the protein transduction domain of the HIV-1 transactivator of transcription (TAT) protein, previously shown to act as a potent nanocarrier for macromolecule delivery in both animal and plant cells, may translocate protein cargoes into AM fungi. We evaluated the internalization into germinated spores of Gigaspora margarita of two recombinant TAT fusion proteins consisting of either a fluorescent (GFP) or a luminescent (aequorin) reporter linked to the TAT peptide. Both TAT-fused proteins were found to enter AM fungal mycelia after a short incubation period (5-10 min). Ca2+ measurements in G. margarita mycelia pre-incubated with TAT-aequorin demonstrated the occurrence of changes in the intracellular free Ca2+ concentration in response to relevant stimuli, such as touch, cold, salinity, and strigolactones, symbiosis-related plant signals. These data indicate that the cell-penetrating properties of the TAT peptide can be used as an effective strategy for intracellularly delivering proteins of interest and shed new light on Ca2+ homeostasis and signalling in AM fungi.

An STE12 gene identified in the mycorrhizal fungus Glomus intraradices restores infectivity of a hemibiotrophic plant pathogen

Mechanisms of root penetration by arbuscular mycorrhizal (AM) fungi are unknown and investigations are hampered by the lack of transformation systems for these unculturable obligate biotrophs. Early steps of host infection by hemibiotrophic fungal phytopathogens, sharing common features with those of AM fungal colonization, depend on the transcription factor STE12. Using degenerated primers and rapid amplification of cDNA ends, we isolated the full-length cDNA of an STE12-like gene, GintSTE, from Glomus intraradices and profiled GintSTE expression by real-time and in situ RT-PCR. GintSTE activity and function were investigated by heterologous complementation of a yeast ste12Delta mutant and a Colletotrichum lindemuthianum clste12Delta mutant. * Sequence data indicate that GintSTE is similar to STE12 from hemibiotrophic plant pathogens, especially Colletotrichum spp. Introduction of GintSTE into a noninvasive mutant of C. lindemuthianum restored fungal infectivity of plant tissues. GintSTE expression was specifically localized in extraradicular fungal structures and was up-regulated when G. intraradices penetrated roots of wild-type Medicago truncatula as compared with an incompatible mutant. Results suggest a possible role for GintSTE in early steps of root penetration by AM fungi, and that pathogenic and symbiotic fungi may share common regulatory mechanisms for invasion of plant tissues.

Calcium opens the dialogue between plants and arbuscular mycorrhizal fungi

Calcium ion is considered a ubiquitous second messenger in all eukaryotic cells. Analysis of intracellular Ca(2+) concentration dynamics has demonstrated its signalling role in plant cells in response to a wide array of environmental cues. The implication of Ca(2+) in the early steps of the arbuscular mycorrhizal symbiosis has been frequently claimed, mainly by analogy with what firmly demonstrated in the rhizobium-legume symbiosis. We recently documented transient Ca(2+) changes in plant cells challenged with diffusible molecules released by arbuscular mycorrhizal fungi. Ca(2+) measurements by the recombinant aequorin method provided new insights into the molecular communications between plants and these beneficial fungi.

Imaging mycorrhizal fungal transformants that express EGFP during ericoid endosymbiosis

Ericoid endomycorrhizal fungi form intracellular associations with the epidermal root cells of plants belonging to Ericales. In natural environments, these fungi increase the ability of their host plants to colonise soils polluted with toxic metals, although the underlying mechanisms are not clearly understood. Genetic transformation is a powerful tool to study the function of specific genes involved in the interaction of symbiotic fungi with the host plants and with the environment. Here, we investigated the possibility to genetically transform an ericoid endomycorrhizal strain. A metal tolerant mycorrhizal Oidiodendron maius strain isolated from a contaminated area was chosen to develop the transformation system. Two different protocols were used: protoplasts and Agrobacterium-mediated transformation. Stable transformants were obtained with both techniques. They remained competent for mycorrhizal formation and GFP-transformed fungi were visualised in planta. This is the first report of stable transformation of an ericoid endomycorrhizal fungus. The protocol set up could represent a good starting point for the identification of genes important in the ericoid mycorrhiza formation and in the understanding of how this symbiosis is established and functions. The success in the genetic transformation of this strain will allow us to better define its potential use in bioremediation strategies.

The mycorrhizal fungus Gigaspora margarita possesses a CuZn superoxide dismutase that is up-regulated during symbiosis with legume hosts

A full-length cDNA showing high similarity to previously described CuZn superoxide dismutases (SODs) was identified in an expressed sequence tag collection from germinated spores of the arbuscular mycorrhizal fungus Gigaspora margarita (BEG 34). The corresponding gene sequence, named GmarCuZnSOD, is composed of four exons. As revealed by heterologous complementation assays in a yeast mutant, GmarCuZnSOD encodes a functional polypeptide able to confer increased tolerance to oxidative stress. The GmarCuZnSOD RNA was differentially expressed during the fungal life cycle; highest transcript levels were found in fungal structures inside the roots as observed on two host plants, Lotus japonicus and Medicago truncatula. These structures also reacted positively to 3,3'-diaminobenzidine, used to localize H2O2 accumulation. This H2O2 is likely to be produced by CuZnSOD activity since treatment with a chelator of copper ions, generally used to inhibit CuZnSODs, strongly reduced the 3,3'-diaminobenzidine deposits. A slight induction of GmarCuZnSOD gene expression was also observed in germinated spores exposed to L. japonicus root exudates, although the response showed variation in independent samples. These results provide evidence of the occurrence, in an arbuscular mycorrhizal fungus, of a functional SOD gene that is modulated during the life cycle and may offer protection as a reactive oxygen species-inactivating system against localized host defense responses raised in arbuscule-containing cells.

Molecular genetics of the arbuscular mycorrhizal symbiosis

During arbuscular mycorrhiza (AM) development, fungal hyphae grow throughout root epidermal, exodermal and cortical cell layers to reach the inner cortex where the symbiosis' functional units, the arbuscles, develop. Three essential components of a plant signalling network, a receptor-like kinase, a predicted ion-channel and a calmodulin-dependent protein kinase have been identified. A detailed morphological study of symbiotic plant mutants revealed that different subsets of plant genes support the progress of fungal infection in successive root cell layers. Moreover, evidence of a diffusible fungal signalling factor that triggers gene activation in the root has recently been obtained.

Differential expression of a metallothionein gene during the presymbiotic versus the symbiotic phase of an arbuscular mycorrhizal fungus

A full-length cDNA encoding a metallothionein (MT)-like polypeptide, designated GmarMT1, was identified in an expressed sequence tag collection from germinated spores of the arbuscular mycorrhizal fungus Gigaspora margarita (BEG34). The GmarMT1 gene is composed of two exons separated by an 81-bp intron. It codes for a 65-amino acid polypeptide comprising a plant type 1 MT-like N-terminal domain and a C-terminal domain that is most closely related to an as-yet-uncharacterized fungal MT. As revealed by heterologous complementation assays in yeast, GmarMT1 encodes a functional polypeptide capable of conferring increased tolerance against Cd and Cu. The GmarMT1 RNA is expressed in both presymbiotic spores and symbiotic mycelia, even in the absence of metal exposure, but is significantly less abundant in the latter stage. An opposite pattern was observed upon Cu exposure, which up-regulated GmarMT1 expression in symbiotic mycelia but not in germinated spores. Together, these data provide the first evidence, to our knowledge, for the occurrence in an arbuscular mycorrhizal fungus of a structurally novel MT that is modulated in a metal and life cycle stage-dependent manner and may afford protection against heavy metals (and other types of stress) to both partners of the endomycorrhizal symbiosis.