Summary• The widespread occurrence of anastomoses and nuclear migration in intact extraradical arbuscular mycorrhizal (AM) networks is reported here.• Visualization and quantification of intact extramatrical hyphae spreading from colonized roots into the surrounding environment was obtained by using a twodimensional experimental model system.• After 7 d the length of extraradical mycelium in the AM symbiont Glomus mosseae ranged from 5169 mm in Thymus vulgaris to 7096 mm in Prunus cerasifera and 7471 mm in Allium porrum , corresponding to 10, 16 and 40 mm mm − 1 root length, respectively. In mycelium spreading from colonized roots of P. cerasifera and T. vulgaris , contacts leading to hyphal fusion were 64% and 78%, with 0.46 and 0.51 anastomoses mm − 1 of hypha, respectively. Histochemical localization of succinate dehydrogenase activity in hyphal bridges demonstrated protoplasmic continuity, while the detection of nuclei in the hyphal bridges confirmed the viability of anastomosed hyphae.• The ability of AM extraradical mycelium to form anastomosis and to exchange nuclei suggests that, beyond the nutritional flow, an information flow might also be active in the network.

We compared root system morphogenesis of micropropogated transplants of Prunus cerasifera L. inoculated with either of the arbuscular mycorrhizal (AM) fungi Glomus mosseae or Glomus intraradices or with the ericoid mycorrhizal species Hymenoscyphus ericae. All plants were grown in sand culture, irrigated with a nutrient solution that included a soluble source of phosphorus, for 75 days after transplanting. Arbuscular mycorrhizal colonization increased both the survival and growth (by over 100%) of transplants compared with either uninoculated controls or transplants inoculated with H. ericae. Arbuscular mycorrhizal colonization increased root, stem and leaf weights, leaf area, root length and specific leaf area, and it decreased root length/leaf area ratio, root/shoot weight ratio and specific root length. Both uptake of phosphorus and its concentration in leaves were increased by AM infection, although the time course of the relationships between intensity of AM infection and P nutrition were complex and suggested a role for factors other than nutrition. The time course for the development of infection varied. It was most rapid with G. mosseae, but it was ultimately higher with G. intraradices. None of the treatments significantly affected the lengths of adventitious roots or the first-, second- or third-order laterals that developed from them. Arbuscular mycorrhizal colonization increased the intensity of branching in all root orders with the effect being most obvious on first-order lateral roots where the number of branches increased from under 100 to over 300 brances m(-1). As a result, although first-order laterals made up 55% of the root systems of control plants, the comparable value was 36% in AM-infected plants. In contrast, second-order laterals represented 25% of control root systems, but 50% of AM-colonized root systems. Glomus intraradices but not G. mosseae increased root diameter. Anatomical studies revealed no changes in the overall form of the root tip, although there were changes in the diameter of the root cap, cell numbers and cell size. Hymenoscyphus ericae increased the duration of the metaphase index. Both AM fungal treatments increased the concentrations of soluble proteins in root extracts and modified the protein profiles by the elimination and addition of protein bands detected by PAGE analysis. We conclude that AM fungal inoculation influenced processes in the root system at different levels, but not all effects were due to improved P nutrition or increased physiological age.

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Plum shoot proliferation was investigated in terms of two distinct processes: axillary bud differentiation and axillary shoot development. Results showed that light quality influenced bud differentiation and interacted with apical dominance in determining shoot outgrowth, resulting in a differentiated structure of shoot clusters and type of branching. Results suggested that blue light, acting through its photoreceptor, increased the number of axillary buds differentiated from apical meristem, but did not remove the apical dominance. Red light removed apical dominance, while reducing the formation of axillary buds; both events appeared to be dependent on the putative amount of phytochrome active form, and independent of light photon fluence rate. On the contrary, blue light action appeared to be dependent on photon fluence rate. In addition, apparent blue±red interactions related to photomorphogenic events fit an antagonistic model for branching regulated by light via cryptochrome and phytochrome photoreceptors. Our results show that the dynamics of shoot cluster development is the product of two events: the formation of new axillary buds and their release from apical dominance.

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