Paola Fortuna

At the Root of the Wood Wide Web: Self Recognition and Nonself Incompatibility in Mycorrhizal Networks

Arbuscular mycorrhizal (AM) fungi are mutualistic symbionts living in the roots of 80% of land plant species, and developing extensive, belowground extraradical hyphae fundamental for the uptake of soil nutrients and their transfer to host plants. Since AM fungi have a wide host range, they are able to colonize and interconnect contiguous plants by means of hyphae extending from one root system to another. Such hyphae may fuse due to the widespread occurrence of anastomoses, whose formation depends on a highly regulated mechanism of self recognition. Here, we examine evidences of self recognition and nonself incompatibility in hyphal networks formed by AM fungi and discuss recent results showing that the root systems of plants belonging to different species, genera and families may be connected by means of anastomosis formation between extraradical mycorrhizal networks, which can create indefinitely large numbers of belowground fungal linkages within plant communities.

Plugging into the network: belowground connections between germlings and extraradical mycelium of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs; nevertheless their spores can germinate in the absence of host plants. Such inconsistent behavior is balanced by diverse survival strategies. The ability of AM fungal hyphae to fuse might represent a fundamental survival strategy because germlings could plug into compatible mycorrhizal networks, thus gaining access to plant-derived carbon before asymbiotic growth arrest. An in vivo experimental system was used to grow extraradical mycelium produced by Glomus mosseae colonizing three different plant species and germlings of the same isolate. After symbiotic and asymbiotic mycelia came into contact we showed that germling hyphae fused with symbiotic network hyphae and established protoplasm connections with nuclei occurring in fusion bridges. The frequency of anastomoses between germling and symbiotic hyphae was 4.9–23.9%. Prefusion and postfusion incompatible responses, with protoplasm withdrawal in interacting hyphae, were evident in some hyphal contacts. Given the multigenomic nature of AMF, the mingling of germling nuclei with those of the mycorrhizal network through perfect fusions might represent a means for the maintenance of genetic diversity in the absence of sexual recombination.

In vitro growth response of Prunus cerasifera shoots as influenced by different light-dark cycles and sucrose concentrations

Trials were carried out to test if the higher growth response shown by shoot clusters of Mr. S. 2/5, a clonal selection of Prunus cerasifera, submitted to short and frequent light-dark regimes could be related to the amount of sucrose added to growth medium.The reduction of sucrose from 30 gl-1 (control) to 22.5 gl-1, 15 gl-1 and 7.5 gl-1 caused a progressive and remarkable inhibition of shoot tip growth. With 15 gl-1 the value of some growth parameters was reduced by more than half. Under 16-h daylength, the best sucrose concentration was 30 gl-1, while with 4-h light-2-h dark no statistical differences appeared between 30 gl-1 and 22.5 gl-1 sucrose. Compared to 16-h light-8-h dark, the 4-h light-2-h dark cycle at the three highest sucrose concentrations gave rise to higher values of fresh and dry weight as well as increasing the number of axillary shoots produced.The increment in growth response induced by the shorter light-dark regime decreased with diminishing growth capacity in the cultures when sucrose concentration was lowered, but it was still appreciable even with 7.5 gl-1. Since the 4-h light-2-h dark cycle induced a favourable effect in culture growth with all sucrose concentrations, we conclude that the greater growth response observed with this light regime was not triggered by carbohydrate availability but by some other unknown factors.

Effects of arbuscular mycorrhizal fungi on in vivo root initiation and development of micropropagated plum shoots

SummaryThe influence of arbuscular mycorrhizal fungi on in vivo root initiation and development of Mr.S. 2/5 microcuttings was investigated. Micropropagated shoots of the Prunus cerasifera L. rootstock, clone Mr.S. 2/5, were placed in a sterile rooting mixture or inoculated with arbuscular mycorrhizal (AM) fungi. A part of the uninoculated microcuttings received auxin treatment (IBA 200 mg F1) or were supplied with a non-sterile sievate of mycorrhizal inoculum. Rooting percentage, root system morphology and growth increments of in vivo rooted microcuttings were evaluated. In vivo rooting of Mr.S. 2/5 shoots was obtained successfully but mycorrhizal inoculation did not show a positive influence on rooting ability of microcuttings. First harvest of plantlets was carried out very early, when mycorrhizal and non-mycorrhizal plants showed no significant growth differences. By this time, AM fungi had greatly increased the proportion of the root system present as higher-order laterals and branching intensity of ...

In Vivo Model Systems for Visualisation, Quantification and Experimental Studies of Intact Arbuscular Mycorrhizal Networks

In this chapter, the methods devised for the visualisation, in vivo monitoring and quantification of anastomosis formation in mycorrhizal networks formed by arbuscular mycorrhizal fungi are described. Anastomoses can be detected in hyphae originating from the same or different germlings and scored for compatible or incompatible responses. Successful anastomoses occur between hyphae belonging to the same or different individuals of the same isolate, whereas hyphae of individuals belonging to different genera, species and geographically different co-specific isolates are unable to fuse. The structure of extraradical networks growing from mycorrhizal roots can be studied by means of a bidimensional model system, allowing the assessment of growth rate, extent, viability and anastomosing ability of ex-novo-produced extraradical mycelium. This experimental model system has been also modified to show interconnections between mycorrhizal networks originating from different host plant species, genera and families, which can create immeasurably large numbers of fungal linkages within communities.