Arbuscular mycorrhiza

An arbuscular mycorrhiza (plural mycorrhizas, a.k.a. endomycorrhiza) is a type of mycorrhiza in which the symbiont fungus (AM fungi, or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. (Not to be confused with ectomycorrhiza or ericoid mycorrhiza.) An arbuscular mycorrhiza (plural mycorrhizas, a.k.a. endomycorrhiza) is a type of mycorrhiza in which the symbiont fungus (AM fungi, or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. (Not to be confused with ectomycorrhiza or ericoid mycorrhiza.) Arbuscular mycorrhizas are characterized by the formation of unique structures, arbuscules and vesicles by fungi of the phylum Glomeromycota. AM fungi help plants to capture nutrients such as phosphorus, sulfur, nitrogen and micronutrients from the soil. It is believed that the development of the arbuscular mycorrhizal symbiosis played a crucial role in the initial colonisation of land by plants and in the evolution of the vascular plants. It has been said that it is quicker to list the plants that do not form endomycorrhizae than those that do. This symbiosis is a highly evolved mutualistic relationship found between fungi and plants, the most prevalent plant symbiosis known, and AMF is found in 80% of vascular plant families in existence today. The tremendous advances in research on mycorrhizal physiology and ecology over the past 40 years have led to a greater understanding of the multiple roles of AMF in the ecosystem. This knowledge is applicable to human endeavors of ecosystem management, ecosystem restoration, and agriculture. Both paleobiological and molecular evidence indicate that AM is an ancient symbiosis that originated at least 460 million years ago. AM symbiosis is ubiquitous among land plants, which suggests that mycorrhizas were present in the early ancestors of extant land plants. This positive association with plants may have facilitated the development of land plants. The Rhynie chert of the lower Devonian has yielded fossils of the earliest land plants in which AM fungi have been observed. The fossilized plants containing mycorrhizal fungi were preserved in silica. The Early Devonian saw the development of terrestrial flora. Plants of the Rhynie chert from the Lower Devonian (400 m.yrs ago) were found to contain structures resembling vesicles and spores of present Glomus species. Colonized fossil roots have been observed in Aglaophyton major and Rhynia, which are ancient plants possessing characteristics of vascular plants and bryophytes with primitive protostelic rhizomes. Intraradical mycelium was observed in root intracellular spaces, and arbuscules were observed in the layer thin wall cells similar to palisade parenchyma. The fossil arbuscules appear very similar to those of existing AMF. The cells containing arbuscules have thickened walls, which are also observed in extant colonized cells. Mycorrhizas from the Miocene exhibit a vesicular morphology closely resembling that of present Glomerales. This conserved morphology may reflect the ready availability of nutrients provided by the plant hosts in both modern and Miocene mutualisms. However, it can be argued that the efficacy of signaling processes is likely to have evolved since the Miocene, and this can not be detected in the fossil record. A finetuning of the signaling processes would improve coordination and nutrient exchange between symbionts while increasing the fitness of both the fungi and the plant symbionts.