1. Understanding the Arbuscule at the Heart of Endomycorrhizal Symbioses in Plants 
Leonie H. Luginbuehl, Giles E.D. Oldroyd 
Current Biology 
Volume 27, Issue 17, Pages R952-R963 (September 2017)
DOI: /j.cub
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2. Figure 1 Arbuscules in maize roots.
(A) Fungal structures in roots of rice are stained with Alexa Fluor 488 wheat germ agglutinin. (B) A graphic rendition of the image in (A) showing the intercellular fungal hyphae growing through the root cortex with intracellular arbuscules indicated. A degenerating arbuscule is at the top of the image, with degraded and septated fungal hyphae.
Current Biology  , R952-R963DOI: ( /j.cub )
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3. Figure 2 Regulation of arbuscule formation.
Both fungal signals (through the symbiosis-signalling pathway) and hormonal signals (through gibberellic-acid signalling) are thought to regulate transcription factors that control gene expression during arbuscule development. The symbiosis-signalling pathway is activated upon recognition of fungal signals (chitooligosaccharides and lipochitooligosaccharides) by unknown plasma-membrane localised receptors. SYMBIOSIS RECEPTOR KINASE (SYMRK) [133,134] is thought to act as a co-receptor in the recognition of fungal signals. The potassium channel DOESN’T MAKE INFECTIONS 1 (DMI1) and the calcium channel CNGC15 are located on the nuclear membranes and coordinate the induction of perinuclear calcium oscillations [18,119]. The calcium ATPase MCA8 is responsible for the reuptake of released calcium into the nuclear envelope [135]. CCaMK is activated by calcium oscillations and phosphorylates the transcription factor CYCLOPS. In a complex with DELLA proteins, CYCLOPS regulates the expression of RAM1, a GRAS-domain transcription factor that is able to interact with several other GRAS-domain proteins, such as RAD1, and regulates the expression of genes involved in arbuscule development and nutrient exchange between the plant and the fungus. The GRAS-domain protein MIG1 interacts with DELLAs and has been proposed to regulate genes involved in radial cell expansion. MYB1 is required for the transcriptional regulation of genes involved in arbuscule degeneration and interacts with both DELLAs and the GRAS-domain protein NSP1. Other GRAS-domain proteins are likely to be involved in regulating the transcriptional programme underlying arbuscule formation and function.
Current Biology  , R952-R963DOI: ( /j.cub )
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4. Figure 3 Arbuscule development and function.
Arbuscules consist of a broad arbuscule trunk and fine arbuscule branches. The fungal hyphae are surrounded by the periarbuscular membrane (PAM). Several components of the secretion pathway were shown to be required for periarbuscular membrane formation, including SNARE proteins and the Exo70I subunit of the exocyst. The deposition of periarbuscular membrane-localised proteins depends on their time of expression as well as the redirection of the secretion pathway to the periarbuscular membrane. The area between the periarbuscular membrane and the fungal hyphae, the periarbuscular space (PAS), contains amorphously structured plant cell-wall material. During the AM symbiosis, the fungus delivers mineral nutrients such as phosphate (Pi) and ammonium (NH4+) to the plant, and periarbuscular membrane-localised proteins, such as PT4 and AMT2, are required for phosphate and ammonium transport, respectively. The proton (H+)-ATPase HA1 creates a proton gradient across the periarbuscular membrane to energise the nutrient transporters. In return for receiving mineral nutrients, plants provide the fungus with fixed carbon in the form of sugars and lipids. Plant fatty acyl synthase (FAS), FatM, and RAM2 are involved in the biosynthesis of lipids delivered to the fungus. The ABCG transporters STR and STR2 have been proposed to export lipids, likely in the form of 2-monopalmitin, across the periarbuscular membrane.
Current Biology  , R952-R963DOI: ( /j.cub )
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