Question

Why mycorrhizal roots are not considered as real roots

Answer 1

Functional Genomics

Responses of a mycorrhizal system to specific environmental cues are tightly coordinated by a physiologically appropriate subset of its genes. Nutrient transport from soil to plant via the mycorrhizal fungus has been the hallmark of mycorrhizal studies for more than a century. One gene that may be involved in P transport efficiency of AM is a fungal membrane P transporter, which moves P from the soil into the hyphae. This gene is regulated by the availability of P in the soil environment, but the fungal partner controls the synthesis of polyphosphate (polyP) and thus the amount of P available in the root and shoot. In turn,in vivo 31P nuclear magnetic resonance (NMR) illustrates that mycorrhizal plants typically accumulate P as supramolecular aggregates of polyP (chain length 5–11 units), whereas nonmycorrhizal plants accumulate solely inorganic P (Pi). In mycorrhizae, polyP represents much of the mobile P reservoir.

Mycorrhizal fungi are also capable of utilizing many forms of N and along a variety of pathways. A putative nitrate-transporter gene has been isolated from mycorrhizal Medicago in which the expression is downregulated by some, but not all, AM. NMR reveals that NH4is predominantly incorporated by glutamine synthetase⧸glutamate synthetase pathway (GS⧸GOGAT), with a small proportion by the glutamate dehydrogenase (GDH) pathway. AM may also accelerate the decomposition of organic matter and acquire N directly from these substrates. 15N pulse-chase studies show the incorporation of labeled N into arginine, which is consistent with the ornithine cycle. In turn, the N is stored in relatively immobile complexes with polyP in vacuoles.

Fractionation of N during N uptake by fungi results in considerable shifts in15N abundance such that 15N abundance in fungal tissues is unlikely to reflect those of the original soil N. The patterns of fractionation are also fungus and N source-specific. Intense fractionation against 15N in ericoid systems results in a 15N-enriched plant, whereas EM systems discriminate against 14N (15N-depleted plant). In addition, δ15N values for EM sporocarps in a Quercus woodland range from 2 to 11. Thus, organic and inorganic N compounds can be accessed differentially by fungi. 15N natural abundance also demonstrates that the strict functional boundaries between mycorrhizal and decomposer fungi may be less clear-cut than previously considered (δ15N 5.29% in mycorrhizal Boletus; 6.03% in saprobicBovista).

Spectra from 1H- and 13C-NMR assays in AM Gigaspora demonstrate that fungal spore germ tubes, extraradical hyphae, and intraradical hyphae differ in their lipid (triaglycerol) and carbohydrate (hexose) metabolism. Spore germ tubes are sites of lipid breakdown and gluconeogenesis. In intraradical hyphae, hexose is transported into the cytoplasm, where lipid synthesis occurs. Extraradical hyphae act as sink for lipids, and the lipids are converted to carbohydrates (trehalose) and occasionally glycogen, and stored in spores or vesicles. Fungi convert the simple sugars to trehalose or other complex sugar to prevent or reduce reabsorption. Changes in root phytohormone composition also occur in tandem with the loading and unloading of C.