Nitrogen metabolism in ectomycorrhizal fungi: fHANT-AC gene regulation in Laccaria bicolor.

KEMPPAINEN M & PARDO AG

Microbiology Book Series # 2: “Current Research, Technology and Education Topics in Applied Microbiology and Microbial Biotechnology” (Méndez Vilas A, ed).

Formatex

Lugar: Bardajoz; Año: 2010; p. 306 - 318

The mycorrhizal symbiosis is an ancient mutualistic association between fungi and the roots of the vast majority of terrestrial plant species. In natural ecosystems the plant nutrient uptake (N, P and several micronutrients) from soil happens via the extraradical mycelia of these symbiotic fungi. This association also improves plant water acquisition, heavy metal tolerance and resistance to pathogens. While most herbaceous plants and tropical trees are engaged in endomycorrhiza-type interactions, forest trees of boreal and temperate zones are typically ectomycorrhizal (ECM)  plants. These species include the majority of economically important trees and the fungal symbionts are predominantly filamentous basidiomycetes. In order to understand how an ECM fungus exploits soil N resources the expression profile of Laccaria fHANT-AC, a gene cluster responsible for growth on nitrate and containing Lbnrt, Lbnr and Lbnir, was analyzed on variable N regimensand using the RNA silencing technology. As a result a novel regulatory mechanism, not previously described for fungal nitrate acquisition, was discovered. The repression of Laccaria fHANT-AC on ammonium seems not to be mediated via L-glutamine as in ascomycete fungi. Growth on different organic N sources, these including also L-glutamine, results in high transcript levels of Laccaria fHANT-AC and suggest a direct ammonium repression effect. Also uptake of nitratecould be observed when growing on organic N. This finding indicates that the symbiotic fungus, differing from its saprotrophic competitors in soil, has the capacity for efficient simultaneous utilization of both inorganic and organic soil N resources. This also suggests that Laccaria can maintain active nitrate uptake from soil despite the known high hyphal free L-glutamine concentration in ECM fungi destined for N translocation towards the host plant. This novel fHANT-AC regulation pattern reveals an elegant adaptive response of an ECM fungus for maximizing its N acquisition especially in soil-horizons rich in organic N and with spatial and temporal fluctuation of nitrate.