Root Hair Colonization by Mycorrhizal Fungi
Mycorrhizal fungi, i.e., the soil fungi that form mutualistic associations with many land plants, are provided by the host with carbon sources required to complete their life cycle, whereas they assist the plant in nutrient uptake from soil. Such acquisition is also considered to be one of the primary functions of root hairs. The aim of this chapter is to investigate the importance of root hairs in the establishment of mycorrhizal interactions, to verify whether plant (root hairs) and fungal (extraradical hyphae) structures work synergistically to provide efficient mineral nutrition. Evidence from morphological studies, where the mycorrhizal typologies have been compared, point to the direct involvement of root hairs in ecto- and arbuscular mycorrhizas (AM). Root hairs probably play a role during the first stages of ectomycorrhizal development, being sensitive to diffusible factors released by the symbiotic fungi and acting as a preferential anchorage site. During AM establishment, a variety of interactions are reported. In liverworts, AM fungi often penetrate the rhizoids, but the colonization process in higher plants does not usually involve root hairs. The analysis of mutant plants with impaired root hair development has not demonstrated any discernible impact on their mycorrhizal capacities. Confocal microscopy has recently provided important insight into understanding the plant responses upon encountering mycorrhizal fungi. Root hairs respond to the fungal presence with nuclear movements, although fungal penetration most often occurs through atrichoblasts or, occasionally, at the base of trichoblasts. Taken together, experimental evidence points to a strong difference in root hair involvement during AM development and nodulation. Nitrogen-fixing bacteria may have found in root hairs a specific anatomical niche, often neglected by AM fungi, to achieve tissue colonization.
Mineral nutrient acquisition from soil is considered one of the primary functions of root hairs, together with the anchorage of the plant (Gilroy and Jones (2000). However, these crucial structures are not alone in the acquisition of nutrients, since mycorrhizal fungi, i.e., the soil fungi that form mutualistic associations with many land plants, also assist their hosts in this way. The mycorrhizal symbiosis is in fact characterized by reciprocal nutrient exchanges between the symbiotic partners: while the fungus obtains photosynthetically derived carbon compounds, the plant receives mineral nutrients. The fungus receives up to 20% of the photoassimilated carbon allocated by the plant to the root (Smith and Read 1997). In exchange, the fungus improves the mineral supply to the plant (mainly phosphate) through the external mycelium, extending through and beyond the nutrient depletion area that surrounds the root (Jakobsen 1995). In fact, mineral nutrients such as phosphorus have very limited mobility in soil, and depletion zones – where the entire available nutrient has been scavenged – quickly appear around roots (Marschner 1995). To obtain phosphorus, plants have to extend their root surface area, and they are helped in this task by mycorrhizal fungal hyphae, which are thinner and more extensive than the root hairs themselves.
The biological meaning of the symbiotic association between the plant roots and some soil fungi, defined for the first time as “mycorrhiza” by Frank 1885), has become more and more apparent in recent years, especially since the corresponding research covers important aspects of ecology, evolutionary biology, genetics, and developmental biology.
Despite their enormous ecological relevance, knowledge of the cellular and molecular mechanisms that control the success of plant–fungal symbiotic associations is still limited, as highlighted by recent reviews and books (Martin et al. (2007; Gianinazzi-Pearson et al. 2007; Pühler and Strack 2007). However, the development of technological platforms in plant genomics is greatly facilitating the comprehensive identification of genes that are activated during mycorrhizal symbiosis. For these reasons, research on mycorrhizas has entered the mainstream of biology, since new tools to uncover symbiont communication and associated developmental mechanisms, diversity, and contributions of symbiotic partners to functioning of mycorrhizal associations are now available. All of these new aspects have been documented in the above-mentioned reviews, as well as in Paszkowski 2006), Bucher (2007), and Genre and Bonfante (2007).
The aim of this chapter is to investigate the importance of root hairs in the establishment of mycorrhizal interactions, and to verify whether plant (root hairs) and fungal (extraradical hyphae) structures work synergistically to provide efficient mineral uptake. For these reasons, a short summary of the main mycorrhizal typologies and morphological features is provided to address the question “what is the involvement of root hairs in mycorrhizal associations?”
KeywordsRoot Hair Mycorrhizal Fungus Arbuscular Mycorrhiza Arbuscular Mycorrhiza Symbiotic Fungus
This research was funded by the Italian Project Prin 2006, the EU Marie Curie–Integral project (MRTN-CT-2003-505227) and the local 60% project of Torino University. The results on root hair cell responses to AM colonization were obtained in collaboration with David Barker in LIPM–INRA/CNRS, Castanet-Tolosan, France.
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