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The dual-specificity tyrosine-phosphorylation-regulated kinase, DYRK1B, is expressed de novo during myogenesis, amplified or mutated in certain cancers and mutated in familial cases of metabolic syndrome. DYRK1B is activated by cis auto-phosphorylation on tyrosine-273 (Y273) within the activation loop during translation but few other DYRK1B phosphorylation sites have been characterised to date. Here, we demonstrate that DYRK1B also undergoes trans-autophosphorylation on serine-421 (S421) in vitro and in cells and that this site contributes to DYRK1B kinase activity. Whilst a DYRK1BS⁴²¹ᴬ mutant was completely defective for p-S421 in cells, DYRK1B inhibitors caused only a partial loss of p-S421 suggesting the existence of an additional kinase that could also phosphorylate DYRK1B S421. Indeed, a catalytically inactive DYRK1Bᴰ²³⁹ᴬ mutant exhibited very low levels of p-S421 in cells but this was increased by KRASᴳ¹²ⱽ. In addition, selective activation of the RAF-MEK1/2-ERK1/2 signalling pathway rapidly increased p-S421 in cells whereas activation of the stress kinases JNK or p38 could not. S421 resides within a Ser-Pro phosphoacceptor motif that is typical for ERK1/2 and recombinant ERK2 phosphorylated DYRK1B at S421 in vitro. Our results show that DYRK1B is a novel ERK2 substrate, uncovering new links between two kinases involved in cell fate decisions. Finally, we show that DYRK1B mutants that have recently been described in cancer and metabolic syndrome exhibit normal or reduced intrinsic kinase activity.

The influence of external phosphorus (P) on carbon (C) allocation and metabolism as well as processes related to P metabolism was studied in monoxenic arbuscular mycorrhiza cultures of carrot (Daucus carota). Fungal hyphae of Glomus intraradices proliferated from the solid minimal medium containing the colonized roots into C-free liquid minimal medium with different P treatments. The fungus formed around three times higher biomass in P-free liquid medium than in medium with 2.5 mM inorganic P (high-P). Mycelium in the second experiment was harvested at an earlier growth stage to study metabolic processes when the mycelium was actively growing. P treatment influenced the root P content and [13C]glucose administered to the roots 7 d before harvest gave a negative correlation between root P content and 13C enrichment in arbuscular mycorrhiza fungal storage lipids in the extraradical hyphae. Eighteen percent of the enriched 13C in extraradical hyphae was recovered in the fatty acid 16:1ω5 from neutral lipids. Polyphosphate accumulated in hyphae even in P-free medium. No influence of P treatment on fungal acid phosphatase activity was observed, whereas the proportion of alkaline-phosphatase-active hyphae was highest in high-P medium. We demonstrated the presence of a motile tubular vacuolar system in G. intraradices. This system was rarely seen in hyphae subjected to the highest P treatment. We concluded that the direct responses of the extraradical hyphae to the P concentration in the medium are limited. The effects found in hyphae seemed instead to be related to increased availability of P to the host root.

Summary Epacrids, a group of southern hemisphere plants formerly considered members of the separate family Epacridaceae, are in fact most closely allied to the Vaccinioid tribe (Ericaceae). Epacrids and other extant ericoid mycorrhiza‐forming plants appear to have a monophyletic origin. In common with many Ericaceae they form ericoid mycorrhizas. ITS sequence data indicate that the fungi forming ericoid mycorrhizas with epacrids and other extant Ericaceae are broadly similar, belonging to a poorly defined group of ascomycetes with phylogenetic affinities to Helotiales. The basic development and structure of ericoid mycorrhizal infections in epacrids is similar to other Ericaceae. However, data are limited on the structure and physiology of both hair roots and ericoid mycorrhizas for all Ericaceae. Relatively little is known about the functional significance of ericoid mycorrhizas in epacrids in southern hemisphere habitats that are often poor in organic matter accumulation. However the abilities of fungal endophytes of epacrids to utilize organic N and P substrates equal those of endophytes from northern hemisphere heathland plant hosts. Investigations using 15N/13C‐labelled organic N substrates suggest that mycorrhizal endophytes are important, at least, to the N nutrition of their epacrid hosts in some habitats.

Mycorrhizal fungi, to be effective for the plant, must be able to transfer mineral nutrient elements from sites of uptake at hyphal tips across various distances to the exchange region in the mycorrhiza. Vacuoles are likely to be important in this transport, since they contain elements of nutritional significance in abundance. In tip cells of hyphae of most fungi – known to include three ectomycorrhizal basidiomycetes, an ericoid mycobiont, and two arbuscular mycorrhizal fungi – the vacuoles form a motile tubular reticulum. The vacuoles are most active in hyphal tips, but non-motile vacuoles at a distance from the tip can be induced to become motile by environmental changes. Neither the tubular vacuolar reticulum nor its contents are properly preserved by conventional fixation and embedding. Vacuolar tubules are readily shown in vivo with fluorescent tracers, throughout the extramatrical mycelium and in outer hyphae of the sheath in eucalypt mycorrhizas synthesised with Pisolithus sp., but they have proved harder to label in field-collected ectomycorrhizas and ericoid mycorrhizas. Freeze-substitution does preserve the structure of vacuoles and vacuolar tubules, and careful anhydrous techniques allow them to be microanalysed, indicating high content of K and P in vacuoles of hyphal tips, and also in sheath and Hartig net of ectomycorrhizas. Vacuoles contain polyphosphate in diffuse, non-granular form. Polyphosphate is present right up to the tip region of hyphae as well as in sheath and Hartig net: thus important mineral nutrient elements are present at both ends of the long hyphal transport pathway. Exactly what happens in between, however, remains to be elucidated.

The proportion of the root system comprising hair roots and their ericoid mycorrhizal colonization have been estimated in Woollsia pungens (Cav.) F. Muell. (Epacridaceae) at a site in New South Wales, Australia over a 12 month period. The technique used was a modification of the grid-line intercept method. Hair roots persisted and comprised at least about 50% of the root system all year round. The percentage of root length that was hair root varied with the season, being lowest in April/May (50%) and highest in October (70%). Hair root colonization differed significantly over the 12 month period, being highest over the winter to spring period (June–Oct) but there were infected hair roots present at all times of the year. There was a significant negative relationship between the percentage hair root length infected and log10 transformations of both mean daily maximum and minimum temperatures, but no correlation with rainfall or soil moisture content. These findings contrast with those for south-west Australia where hair roots (and mycorrhizal colonization) are reported to disappear in summer.

Intact mature sclerotia of Sclerotium rolfsii Sacc. and Sclerotium cepivorum Berk. produced in culture are impermeable to the apoplastic tracer sulphorhodamine G. Both of these species produce sclerotia with rinds. Some movement of sulphorhodamine into sclerotia of Rhizoctonia solani Kuhn, which have no rind, occurred but the fluorochrome was arrested after permeation of at most the outer five layers of cells. In all cases, low permeability depended on an intact outer layer, and when sclerotia of each species were bisected to provide direct access of sulphorhodamine to all tissue layers, the fluorochrome permeated the cell walls and extracellular matrix (where present) of many cells within the sclerotium. A marked reduction in permeability of intact sclerotia occurs at maturity in a number of species and might be important in long-term survival.

Epacrids, a group of southern hemisphere plants formerly considered members of the separate family Epacridaceae, are in fact most closely allied to the Vaccinioid tribe (Ericaceae). Epacrids and other extant ericoid mycorrhiza-forming plants appear to have a monophyletic origin. In common with many Ericaceae they form ericoid mycorrhizas. ITS sequence data indicate that the fungi forming ericoid mycorrhizas with epacrids and other extant Ericaceae are broadly similar, belonging to a poorly defined group of ascomycetes with phylogenetic affinities to Helotiales. The basic development and structure of ericoid mycorrhizal infections in epacrids is similar to other Ericaceae. However, data are limited on the structure and physiology of both hair roots and ericoid mycorrhizas for all Ericaceae. Relatively little is known about the functional significance of ericoid mycorrhizas in epacrids in southern hemisphere habitats that are often poor in organic matter accumulation. However the abilities of fungal endophytes of epacrids to utilize organic N and P substrates equal those of endophytes from northern hemisphere heathland plant hosts. Investigations using 15 N/13 C-labelled organic N substrates suggest that mycorrhizal endophytes are important, at least, to the N nutrition of their epacrid hosts in some habitats.

The apoplasmic permeability of ectomycorrhizal roots of intact Eucalyptus pilularis seedlings infected with Pisolithus tinctorius on aseptic agar plates was examined using the nonbinding fluorochrome 8-hydroxypyrene-1,3,6-trisulphonate and lanthanum ions in conjunction with anhydrous freeze substitution and dry sectioning. Most mycorrhizas formed in the air above the agar surface, and in these the sheath rapidly became nonwettable and impermeable to the fluorochrome but was nevertheless permeable to lanthanum ions. In a few mycorrhizas which developed in contact with the agar the sheath remained permeable to both tracers when fully developed. This increased hydrophobicity of the sheath in mycorrhizas in the air above the agar surface might be explained by deposition of hydrophobins, but nevertheless it still allows an apoplasmic pathway for radial movement of ions. Regardless of their sheath permeation both apoplasmic tracers were always found throughout the Hartig net and were arrested at the Casparian bands and suberin lamellae of the exodermis. It is concluded that the fluorochrome must have moved longitudinally along the Hartig net which is a region of higher permeability than the sheath. Casparian bands in the exodermis of ectomycorrhizal roots have similar properties to those in nonmycorrhizal roots in excluding solutes and their exclusion of lanthanum ions indicates that they are not permeable to ions. The data do not support the concept of a totally sealed apoplasmic exchange compartment, but the differential permeability suggests that the sheath might allow radial transfer of ions but block loss of sugars and organic molecules of similar size.