Landeweert,R., Hoffland,E., Finlay,R.D., Kuyper,T.W. and van Breemen,N.(2001): Linking plants to rocks: ectomycorrhizal fungi mobilize nutrients from minerals. Trends in Ecology & Evolution, 16(5), 248-254.

 Plant nutrients, with the exception of nitrogen, are ultimately derived from weathering of primary minerals. Traditional theories about the role of ectomycorrhizal fungi in plant nutrition have emphasized quantitative effects on uptake and transport of dissolved nutrients. Qualitative effects of the symbiosis on the ability of plants to access organic nitrogen and phosphorus sources have also become increasingly apparent. Recent research suggests that ectomycorrhizal fungi mobilize other essential plant nutrients directly from minerals through excretion of organic acids. This enables ectomycorrhizal plants to utilize essential nutrients from insoluble mineral sources and affects nutrient cycling in forest systems. 』


Organic acids as weathering agents
Nutrient mobilization by symbiotic fungi
 Lichenous fungi
 Ectomycorrhizal fungi
Mineral weathering by ectomycorrhizal fungi
 Experimental evidence
 Ectomycorrhizal fungi and weathering of forest soils
 Mineral weathering by individual ectomycorrhizal hyphae
 Implications of ectomycorrhizal weathering
Box 1. Mineral weathering
Box 2. The structure of ectomycorrhizas
Box 3. Nutrient mobilization by ectomycorrhizal fungi
Box 4. Formation of etch pits

Box 2. The structure of ectomycorrhizas
Ectomycorrhizas typically consist of an intimate association between a living root tip and a basidiomycete or ascomycete fungusa (Fig. I). The fungus forms (a) a mantle of fungal material around the root tip and penetrates the intercellular space between (b) cortical root cells, forming (c) a ‘Hartig net’.
 Transfer of materials between fungus and plant takes place in the Hartig net. External hyphae grow outwards from the fungal mantle into the soil and the fungal mycelium might differentiate into rhizomorphs, which are hyphal aggregates that can transport water and nutrients over several dm to the root. The ectomycorrhizal fungus produces hormones that suppress root-hair growth and might induce typical dichotomous branching of the mycorrhizal root tip. Single mycorrhizal root tips can transform into clusters of root tips colonized by a single fungal species.
Scale bars = 50 μm and 100 μm, respectively.

a Smith, S.E. and Read, D.J. (1997) Mycorrhizal Symbiosis (2nd edn), Academic Press

 外生菌根は典型的には、生きた根の尖端および担子菌類または子嚢菌類の間の密接な共生からなるa (Fig. I)。菌類は、(a)根尖端周囲に菌類物質のマントル(菌套)を形成し、(b)皮層根細胞の間の細胞間げきを貫き、(c)ハルティヒネットを形成する。

〔Landeweert,R., Hoffland,E., Finlay,R.D., Kuyper,T.W. and van Breemen,N.(2001): Linking plants to rocks: ectomycorrhizal fungi mobilize nutrients from minerals. Trends in Ecology & Evolution, 16(5), 248-254.から〕

Box 3. Nutrient mobilization by ectomycorrhizal fungi
As well as the quantitative effects on plant nutrient uptake (increase in uptake surface and exploited soil volume), the ectomycorrhizal fungus influences the uptake of plant nutrients in two qualitative ways:
 (Fig. Ia) Via enzyme production, the ectomycorrhizal fungus can utilize organic nitrogen (N) and phosphorous (P) forms, which would otherwise remain largely unavailable to roots. Nutrient mobilization from amino acids, peptides, proteins, amino sugars, chitin and nucleic acids has been showna, together with transfer of N and P into the host planta,b. Direct hyphal absorption of amino acids and simple peptides can also occura .
( Fig. Ib) The ectomycorrhizal fungus can mobilize P, potassium (K), calcium (Ca) and magnesium (Mg) from solid mineral substrates through organic acid excretionc,d,e. In addition, tunnels in weatherable minerals enable ectomycorrhizal hyphae to reach the interior of the minerals and access P from apatite inclusions. Essential nutrients become available to the host plant via the ectomycorrhizal myceliumc. Analogous to their organic nutrient mobilizing capabilities, the abilities of different ectomycorrhizal fungi to mobilize inorganic nutrients might be species specificf.

 (Fig. Ia) 酵素生産によって、外生菌根菌は有機窒素(N)とリン(P)化学種を利用できるが、これらの大部分はさもなければ根に利用できないままになってしまう。アミノ酸、ペプチド、蛋白質、アミノ糖、キチン、核酸からの栄養流動aが、NおよびPの親植物への輸送a,bとともに、示されてきている。アミノ酸と単純なペプチドの菌糸による直接的な吸収も起こりえるa
 (Fig. Ib) 外生菌根菌は、有機酸の排出によって固体鉱物基質からP・カリウム(K)・カルシウム(Ca)・マグネシウム(Mg)を流動させることができるc,d,e。さらに、風化しやすい鉱物中の穿孔は、外生菌根菌が鉱物内部に到達してアパタイト包有物からPを取得することを可能にする。必須栄養は外生菌根菌糸によって親植物に利用できるようになるc。それらの有機栄養流動能力と類似して、異なる外生菌根菌が無機栄養を流動させる能力は特定の種に限られるであろうf

a Chalot, M. and Brun, A. (1998) Physiology of organic nitrogen acquisition by ectomycorrhizal fungi and ectomycorrhizas. FEMS Microbiol. Rev. 22, 21-44
b Antibus, R.K. et al. (1997) Root surface phosphatase activities and uptake of 32Plabelled inositol phosphate in field-collected gray birch and red maple roots. Mycorrhiza 7, 39-46
c Wallander, H. (2000) Uptake of P from apatite by Pinus sylvestris seedlings colonized by different ectomycorrhizal fungi. Plant Soil 218, 249-256
d Wallander, H. and Wickman, T. (1999) Biotite and microcline as potassium sources in
ectomycorrhizal and non-mycorrhizal Pinus sylvestris seedlings
. Mycorrhiza 9, 25-32
e Wallander, H. (2000) Use of strontium isotopes and foliar K content to estimate
weathering of biotite induced by pine seedlings colonised by ectomycorrhizal fungi from two different soils
. Plant Soil 222, 215-229
f Lapeyrie, F. et al. (1991) Phosphate solubilizing activity of ectomycorrhizal fungi in vitro. Can. J. Bot. 69, 342-346

Fig. 1. Hyphae linking plants to minerals. A thin-section of (a) a
cross-sectioned ectomycorrhizal root tip illustrates how (h)
ectomycorrhizal hyphae, emanating from (fm) the fungal mantle around (r) a root, enclose (m) mineral particles from the adjacent soil. The direct contact between the hyphae and the mineral surface is revealed on (b) a SEM picture of branching hyphae that cover and penetrate a mineral particle. Scale bars = 50 μm and 10 μm respectively.


〔『Landeweert,R., Hoffland,E., Finlay,R.D., Kuyper,T.W. and van Breemen,N.(2001): Linking plants to rocks: ectomycorrhizal fungi mobilize nutrients from minerals. Trends in Ecology & Evolution, 16(5), 248-254.』から〕

Box 4. Formation of etch pits
Chemical weathering of minerals leads to formation of regularly arranged, and regularly shaped, angular cavities, so-called etch pits. Etch pits form because minerals dissolve preferentially at and along crystallographically determined dislocations and planes. Saw-tooth cracks eventually form when sideby-side aligned etch pits coalescea(Fig. Ia, from left to right).
 Thin-section micrographs in cross-polarized light showing (Fig. Ib) a chemically weathered feldspar with partly coalesced etch pits. By contrast, a thin-section micrograph (Fig. Ic) of a tunneled feldspar shows a more or less irregular pattern of open, tubular pores, 3-10 μm in width that criss-cross the interior of the mineral.
Scale bars = 100 μm for Fig. Ib and Ic.

a Berner, R.A. and Holdren, G.R. (1979) Mechanism of feldspars weathering. II. Observations of feldspars from soils. Geochim. Cosmochim. Acta 43, 1173-1185

 鉱物の化学風化は、規則的に並び、規則的な形をした、角張った空孔であるいわゆるエッチピットをもたらす。エッチピットは、結晶学的に決定された転位や面に沿って鉱物が選択的に溶解するために生じる。のこぎりの歯状の割れ目は、並んで配列したエッチピットが合体したときに最終的に形成されるa(図 Ia、 左から右へ)。
 部分的に合体したエッチピットをもつ化学風化した長石を示す、直交ニコル下での薄片顕微鏡写真(図 Ib)。対照的に、穿孔された長石の薄片顕微鏡写真(図 Ic)は、鉱物の内部に縦横に存在する、多少不規則なパターンの開いた管状の幅3〜10μmの形状を示す。

〔Landeweert,R., Hoffland,E., Finlay,R.D., Kuyper,T.W. and van Breemen,N.(2001): Linking plants to rocks: ectomycorrhizal fungi mobilize nutrients from minerals. Trends in Ecology & Evolution, 16(5), 248-254.から〕