First Cryo-Scanning Electron Microscopy Images and X-Ray Microanalyses of Mucoromycotinian Fine Root Endophytes in Vascular Plants

AIMS

Arbuscule-producing fine root endophytes (FRE) (previously incorrectly Glomus tenue) were recently placed within subphylum Mucoromycotina; the first report of arbuscules outside subphylum Glomeromycotina. Here, we aimed to estimate nutrient concentrations in plant and fungal structures of FRE and to test the utility of cryo-scanning electron microscopy (cryoSEM) for studying these fungi.

METHODS

We used replicated cryoSEM and X-ray microanalysis of heavily colonized roots of Trifolium subterraneum.

RESULTS

Intercellular hyphae and hyphae in developed arbuscules were consistently very thin; 1.35 ± 0.03 μm and 0.99 ± 0.03 μm in diameter, respectively (mean ± SE). Several intercellular hyphae were often adjacent to each other forming "hyphal ropes." Developed arbuscules showed higher phosphorus concentrations than senesced arbuscules and non-colonized structures. Senesced arbuscules showed greatly elevated concentrations of calcium and magnesium.

CONCLUSION

While uniformly thin hyphae and hyphal ropes are distinct features of FRE, the morphology of fully developed arbuscules, elevated phosphorus in fungal structures, and accumulation of calcium with loss of structural integrity in senesced arbuscules are similar to glomeromycotinian fungi. Thus, we provide evidence that FRE may respond to similar host-plant signals or that the host plant may employ a similar mechanism of association with FRE and AMF.

Large Area Cryo-Planing of Vitrified Samples Using Broad-Beam Ion Milling

On account of its excellent resolution and high throughput, cryoSEM imaging has recently seen resurgence. In this work, we report on the development of cryogenic triple ion gun milling (CryoTIGM™), a broad ion beam milling technique for cryo-planing of vitrified, "frozen-hydrated" specimens. We find that sections prepared with CryoTIGM™ are smooth over exceptionally large areas (~700,000 µm2), and reveal ultrastructural details in similar or better quality than freeze-fractured samples.