ACTIVITY OF GLUTAMINE SYNTHETASE AND GLUTAMATE DEHYDROGENASE IN TRIFOLIUM SUBTERRANEUM L. AND ALLIUM CEPA L: EFFECTS OF MYCORRHIZAL INFECTION AND PHOSPHATE NUTRITION

Abstract

Summary Activities of glutamate dehydrogenase and glutamine synthetase were determined using crude extracts of roots and shoots of mycorrhizal and non‐mycorrhizal plants of Trifolium subterraneum L. and Allium cepa L., grown at different levels of fertilizer phosphate. Glutamate dehydrogenase activity was low in all tissues [0.1 to 1.6 μmol NAD(P)H oxidized min−1 gFW−1 tissue] and there was no consistent effect of mycorrhizal infection or phosphate nutrition on this activity. Glutamine synthetase (GS) activity (assayed by the transferase method) was in the range 1 to 40/iimol γ‐glutamyl hydroxamate produced min−1 gFW−1. In general, activity of this enzyme was low in phosphate‐deficient plants and was increased both by mycorrhizal infection and by improved phosphate supply. In T. subterraneum routine assays of GS were done on roots only. The effects of mycorrhizal infection in increasing enzyme activity in roots were similar whether natural soil inoculum (containing a mixture of several mycorrhizal fungi) or inoculum of Glomus mosseae Nichol. & Gerd. was used. Both increased phosphate supply and mycorrhizal infection increased nodulation of clover plants as well as GS activity, so that it was difficult to relate changes in GS activity to the interacting effects of mycorrhizal infection and phosphate nutrition. Onions had low GS activity in uninfected roots, compared with shoots. Again improved phosphate supply resulted in increased enzyme activity in both roots and shoots. However, the patterns of interaction between phosphate supply, P concentration in tissues, mycorrhizal infection and enzyme activity were different in the two tissues. In shoots, as expected, the effects were consistent with an indirect effect of mycorrhizal infection on enzyme activity, via improved P nutrition. In roots there appeared to be a ‘fungal effect’ superimposed on the phosphate effect. This was investigated by manipulating the amount of fungal tissue in mycorrhizal roots via differences in propagule density of G. mosseae in soil. Results were again consistent with the hypothesis that the mycorrhizal fungi contributed GS activity to the symbiotic root system. Fungal structures were separated from roots following digestion in cellulase and pectinase. GS activity was high in fungal tissue from young roots (29 to 31 d), but low in older infections (55 d). The high activity could not have been caused by contamination of fungal tissue by root cells. The digestion technique reduced GS activity in uninfected and infected root segments, so that results obtained with separated fungi are not quantitatively comparable with those obtained from extracts of fresh tissues. We conclude that vesicular‐arbuscular mycorrhizal fungi are able to assimilate ammonium via GS. This ability would be important in increased uptake of nitrogen which is an inevitable prerequisite for increased growth following relief of phosphate stress. It is also consistent with the recent findings by others that hyphae of G. mosseae can absorb and translocate 15NH+4

Effects of mycorrhizal colonization on biomass production and nitrogen fixation of black locust (Robinia pseudoacacia) seedlings grown under elevated atmospheric carbon dioxide

“…Thomas, unpublished). Further, Smith et al (1985) showed that AM fungi are able to assimilate NH % + via glutamine synthetase. The NH % + is adsorbed onto soil particles and is relatively non-mobile, so that even in moist soils depletion zones develop readily and, as with P, diffusion rather than absorption limits the rate of uptake.…”

EFFECTS OF MYCORRHIZAL INFECTION ON PLANT GROWTH, NITROGEN AND PHOSPHORUS NUTRITION IN GLASSHOUSE‐GROWN ALLIUM CEPAL.

“…There were clear interactions between phosphate nutrition and enzyme activity. In general, activities of nitrate reductase and of glutamine synthetase were higher in both roots and shoots of P-sufficient plants, regardless of whether the improved P status was brought about by mycorrhizal infection or by fertilization with P. In addition, mycorrhizas have a direct effect on the glutamine synthetase activity of infected roots and glutamine synthetase has been shown to be active in fungal tissue separated from the roots (see Smith et al, 1985). This means that assimilation of ammonium as well as translocation of N compounds could occur in the fungal component of a mycorrhiza and could contribute to a mechanism of ammonium uptake similar to that envisaged for P. Nevertheless, as Ames et al (1984) emphasize, data based on total plant N, on tissue N concentrations and on percentage usage of N fertilizer failed to show that VA mycorrhizal infection increased the rate of uptake of N by plants.…”

Detection of symbiosis‐related polypeptides during the early stages of the establishment of arbuscular mycorrhiza between Glomus mosseae and Pisum sativum roots

“…Until now, considering the fact that the AM fungi are strict symbionts with very limited growth without the host roots, the origin of additional polypeptides in iVM roots was very uncertain. Approaches have been developed to study fungal polypeptides corresponding to the symbiotic stage of the AM fungi by using enzymatic digestion of host root (Capaccio & Callow, 1982;GianinazziPearson & Gianinazzi, 1983;Smith et al, 1985;Smith & Gianinazzi-Pearson, 1987;McGee & Smith, 1990;Simoneau et al, 1994). However, this strategy could lead to artefactual alterations in polypeptides of AM fungi (Smith and GianinazziPearson, 1987;McGee & Smith, 1990).…”