F. A. Smith

Arbuscular mycorrhizal inhibition of growth in barley cannot be attributed to extent of colonization, fungal phosphorus uptake or effects on expression of plant phosphate transporter genes

Here, we used phosphorus-32 (32P) labelling in compartmented pots combined with quantitative real-time polymerase chain reaction (PCR) analysis of phosphate(Pi) transporter gene expression to investigate regulation of Pi uptake pathways in barley (Hordeum vulgare), an arbuscular mycorrhizal (AM) plant that does not show strong positive growth responses to colonization.Barley was colonized well by Glomus intraradices and poorly by Glomus geosporum,but both fungi induced significant and similar growth depressions compared with non mycorrhizal controls. The lack of correlation between per cent colonization and extent of growth depression suggests that the latter is not related to carbon drain to the fungus. The contribution of the AM Pi uptake pathway for the two fungi was, in general,related to per cent colonization and expression of the AM-inducible Pi transporter gene, HvPT8, but not to plant responsiveness. Glomus intraradices contributed 48%of total plant P whereas G. geosporum contributed very little.The growth depression in plants where the AM uptake pathway was functional suggests that the contribution of the direct Pi uptake pathway via root hairs and epidermis was decreased. This decrease was not correlated with downregulation of the epidermal-expressed Pi transporter genes, HvPT1 and HvPT2. We hypothesize post-transcriptional or post-translational control of this transport process by AM colonization.

Effects of Iron and Manganese Plaques on Arsenic Uptake by Rice Seedlings (Oryza sativa L.) Grown in Solution Culture Supplied with Arsenate and Arsenite

We have shown previously that phosphorus nutrition and iron plaque on the surface of rice roots influence arsenate uptake and translocation by rice in hydroponic culture. We have now investigated the role of iron (Fe) and manganese (Mn) plaque on arsenate and arsenite uptake and translocation in rice seedlings grown hydroponically. Fe and Mn plaques were clearly visible as reddish or brown coatings on the root surface after 12 h induction, and Fe plaque was much more apparent than Mn plaque. Arsenite or arsenate supply did not decrease plant dry weights significantly. There were significant differences in shoot dry weights but little difference in root dry weights between some plaque treatments. Arsenic (As) concentrations in Fe plaque when arsenate was supplied were significantly higher than those in no plaque (control) and Mn plaque treatments, and much higher than those supplied with arsenite. This showed that Fe plaque on the rice root had higher affinity to arsenate than to arsenite. In Fe plaque treatment, the results indicated that most As was sequestered in roots when arsenite was supplied and most As concentrated in Fe plaque when arsenate was supplied. Most As was accumulated in rice roots in Mn plaque and no plaque treatments for both As species.

Growth response of Atriplex nummularia to inoculation with arbuscular mycorrhizal fungi at different salinity levels

Chenopods are generally regarded as non-host plants for mycorrhizal fungi and are believed not to benefit from colonization by mycorrhizal fungi. Perennial Atriplex nummularia Lindl., growing under field conditions, showed a relatively high level of colonization by mycorrhizal fungi (10–30% of root length colonized) in spring and summer. Accordingly, two glasshouse experiments were designed to assess the effects of inoculation with mycorrhizal fungi (with a single species or a mixture of different species) on growth, nutrient uptake, and rhizosphere bacterial community composition of A. nummularia at high and low salinity levels (2.2 and 12 dSm−1). Only low and patchy colonization by mycorrhizal fungi (1–2 of root length colonized) was detected in inoculated plants under glasshouse conditions which was unaffected by salinity. Despite the low colonization, inoculation increased plant growth and affected nutrient uptake at both salinity levels. The effects were higher at an early stage of plant development (6 weeks) than at a later stage (9–10 weeks). Salinity affected the bacterial community composition in the rhizosphere as examined by ribosomal intergenic spacer amplification (RISA) of 16S rDNA, digitization of the band patterns and multivariate analysis. The effects of inoculation with mycorrhizal fungi on growth of A. nummularia may be attributed to (i) direct effects of mycorrhizal fungi on plant nutrient uptake and/or (ii) indirect effects via mycorrhizal-induced changes in the bacterial community composition.

Arbuscular mycorrhizal colonization reduces arsenate uptake in barley via downregulation of transporters in the direct epidermal phosphate uptake pathway

*Here, we used barley (Hordeum vulgare) grown in normal and compartmented pots to investigate sensitivity to arsenic (As) in the absence of a positive growth response to arbuscular mycorrhizas (AM). *We tested the hypothesis that upon inoculation with AM fungi downregulation of HvPht1;1 and HvPht1;2 genes (encoding high-affinity inorganic orthophosphate (P(i))-uptake systems in a direct pathway via root epidermis and root hairs) and upregulation of the AM-induced HvPht1;8 (encoding the P(i)-uptake system responsible for transfer of P(i) from the symbiotic interface to cortical cells) play a role in decreased As uptake and hence reduced As sensitivity in AM plants. *Barley did not respond, or responded negatively to colonization by Glomus intraradices in terms of growth. In terms of specific phosphorus (P) uptake (P uptake per g of root) barley was nonresponsive. There was a significant interaction between As treatment and colonization, resulting in a lower As concentration and uptake in AM compared with nonmycorrhizal (NM) plants. *The decreased uptake of As and higher P : As molar ratios in the AM barley can be explained by the operation of the AM pathway as indicated by induction of HvPht1;8 and by down-regulation of HvPht1;1 and HvPht1;2.

Influence of the mycorrhizal fungus, Glomus coronatum, and soil phosphorus on infection and disease caused by binucleate Rhizoctonia and Rhizoctonia solani on mung bean (Vigna radiata)

Root-infecting fungal pathogens and also parasites, which do not cause major disease symptoms cause problems of contamination in pot cultures of arbuscular mycorrhizal (AM) fungi. We investigated the effect of the AM fungus, Glomus coronatum Giovannetti on disease caused by binucleate Rhizoctonia sp. (BNR) and R. solani in mung bean in the absence (P0) and presence (P1) of added soil phosphorus (P). When G. coronatum and BNR or R. solani were inoculated at the same time, G. coronatum improved the growth of the plants and reduced colonization of roots by BNR, but not by R. solani. R. solani reduced the growth of non-mycorrhizal mung bean in P0 soil 6 weeks after inoculation, whereas BNR had no effect on growth. G. coronatum reduced the severity of disease caused by BNR or R. solani on mung bean in both soil P treatments. When G. coronatum was established in the roots 3 weeks before BNR or R. solani was added to the potting mix, there was no significant effect of BNR or R. solani on growth of mung bean. Prior colonization by G. coronatum slightly reduced indices of disease caused by BNR or R. solani. In both experiments, addition of P stimulated plant growth and reduced the colonization of roots by BNR, but had little effect on disease severity. We conclude that the reduction of the effect of BNR or R. solani on mung bean could not be explained by improved P nutrition, but could be attributed to the presence of G. coronatum within and among the roots.

Influence of arbuscular mycorrhizal (AM) symbiosis on phosphorus leaching through soil cores

Two experiments with soil cores were carried out to investigate the effects of arbuscular mycorrhizal (AM) fungal colonization on mobility of phosphorus (P) during leaching of repacked columns of a soil with a loamy sand texture. Trifolium subterraneum plants inoculated with an AM fungus or not inoculated were grown in cores with low or high P concentrations for 8 or 10 weeks in the glasshouse. Cores were then irrigated with 2500 mL water and the leachate collected. Plant growth and the amounts of P removed by plants, remaining in soil as available P and removed dissolved in leachate were measured. Mycorrhizal fungal colonization and development of external hyphae were also determined. Inoculation and/or P application significantly increased plant growth and plant P removal and decreased P leaching. In low P soils AM fungal colonization significantly increased plant P uptake and decreased soil available P and total dissolved P in leachates. Lower P leaching from cores with AM plants under low P conditions was related to enhancement of plant growth and to scavenging and removal of P from the soil by roots and/or external hyphae. When P was applied AM effects were not observed and available P remaining in the soil after leaching was much higher, regardless of AM fungal colonization.

Arsenic uptake by arbuscular mycorrhizal maize (Zea mays L.) grown in an arsenic-contaminated soil with added phosphorus

The effects of arbuscular mycorrhizal (AM) fungus (Glomus mosseae) and phosphorus (P) addition (100 mg/kg soil) on arsenic (As) uptake by maize plants (Zea mays L.) from an As-contaminated soil were examined in a glasshouse experiment. Non-mycorrhizal and zero-P addition controls were included. Plant biomass and concentrations and uptake of As, P, and other nutrients, AM colonization, root lengths, and hyphal length densities were determined. The results indicated that addition of P significantly inhibited root colonization and development of extraradical mycelium. Root length and dry weight both increased markedly with mycorrhizal colonization under the zero-P treatments, but shoot and root biomass of AM plants was depressed by P application. AM fungal inoculation decreased shoot As concentrations when no P was added, and shoot and root As concentrations of AM plants increased 2.6 and 1.4 times with P addition, respectively. Shoot and root uptake of P, Mn, Cu, and Zn increased, but shoot Fe uptake decreased by 44.6%, with inoculation, when P was added. P addition reduced shoot P, Fe, Mn, Cu, and Zn uptake of AM plants, but increased root Fe and Mn uptake of the nonmycorrhizal ones. AM colonization therefore appeared to enhance plant tolerance to As in low P soil, and have some potential for the phytostabilization of As-contaminated soil, however, P application may introduce additional environmental risk by increasing soil As mobility.

Phosphorus efficiencies and their effects on Zn, Cu, and Mn nutrition of different barley (Hordeum vulgare) cultivars grown in sand culture

A sand-culture experiment was carried out in a growth chamber to investigate the phosphorus (P) efficiencies of 8 barley cultivars that are parents of 4 mapping populations, and the effects of P nutrition on plant uptake of zinc (Zn), copper (Cu), and manganese (Mn). Two sources of phosphate were used, rock phosphate (sparingly soluble) and CaHPO4 (readily available). There were significant differences in P uptake and utilisation efficiencies between the 8 cultivars. Among the cultivars, the Sahara–Clipper pair is of the most interest, because these 2 cultivars had large differences in root/shoot ratios, P allocation between root and shoot, and P uptake/utilisation efficiencies. Higher P availability significantly reduced plant Zn uptake and tissue concentrations in all cultivars. Shoot Zn concentrations were found to decrease significantly with P influx to the xylem (P < 0.01), indicating that genotypic variations in P translocation from roots to shoots may interact with Zn accumulation in shoots. Higher P availability reduced Cu concentrations in shoots, probably due to a dilution effect. P availability (rock phosphate v. CaHPO4) seemed to affect plant uptake of Mn in some cultivars, but further study is needed to elucidate the mechanisms involved and the practical implications of this interaction in Mn-deficient soils.

Ions and Osmoregulation

Various aspects of the involvement of ions in osmoregulation (turgor regulation, volume regulation) have been reviewed recently by Bisson and Gutknecht (1979), Cram (1976), Flowers et al. (1977), Hellebust (1976), Osmond (1979) and by Pitman and Cram (1977). These reviews show that, while we know something of the phenomena of ionic involvement in osmoregulation, little is known of the molecular mechanisms of ion transport, or of the regulation of this transport in response to (e. g.) turgor-related signals. We shall discuss the ways in which ions are involved in osmoregulation in the context of the evolution of phototrophic plants; discussion of the various strategies will, we hope, provide a background for consideration of the genetics of osmoregulation in a particular plant. Our discussion will necessarily deal not only with the constraints on the ionic composition of the cell and its various compartments, but with possible mechanisms of ion transport at the cell and the whole-plant level and the regulation of this transport.

Effects of pH on mycorrhizal colonisation and nutrient uptake in cowpea under conditions that minimise confounding effects of elevated available aluminium

Effects of the arbuscular mycorrhizal (AM) fungi Gigaspora margarita and Glomus etunicatum on the growth of cowpea (Vigna unguiculata L. Walp.) were assessed at low pH by growing plants, with and without AM inoculation, individually in pots containing a mixture of sand and soil adjusted to pH 4.7, 4.9 or 5.2 at the start of the experiment, and with soluble aluminium (Al) concentrations at a sub-toxic level for the plant. Cowpea grew poorly in the absence of AM colonisation, particularly at pH 4.7. Growth was enhanced both by increasing the pH and by inoculating with the AM fungi, with plant responses greater with inoculation. The relative growth improvement by mycorrhizas (mycorrhizal growth response) was highest at pH 4.7, and decreased as the pH increased, although effects were not always significant. Gi. margarita was much more effective than G. etunicatum. There were differential effects of the two fungi on uptake of mineral elements. Plants inoculated with Gi. margarita took up a range of elements, including P and Zn as well as Al, to a much greater extent than those inoculated with G. etunicatum, regardless of medium pH. The effectiveness of Gi. margarita in increasing plant growth was closely correlated with colonised root length.