Role of mycorrhizal infection in the growth and reproduction of wild vs. cultivated plants

Summary We tested the hypothesis that mycorrhizal infection benefits wild plants to a lesser extent than cultivated plants. This hypothesis stems from two observations: (1) mycorrhizal infection improves plant growth primarily by increasing nutrient uptake, and (2) wild plants often possess special adaptations to soil infertility which are less pronounced in modern cultivated plants. In the first experiment, wild (Avena fatua L.) and cultivated (A. sativa L.) oats were grown hydroponically at four different phosphorus levels. Wild oat was less responsive (in shoot dry weight) to increasing phosphorus availability than cultivated oat. In addition, the root: shoot ratio was much more plastic in wild oat (varying from 0.90 in the low phosphorus solution to 0.25 in the high phosphorus solution) than in cultivated oat (varying from 0.44 to 0.17). In the second experiment, mycorrhizal and non-mycorrhizal wild and cultivated oats were grown in a phosphorus-deficient soil. Mycorrhizal infection generally improved the vegetative growth of both wild and cultivated oats. However, infection significantly increased plant lifespan, number of panicles per plant, shoot phosphorus concentration, shoot phosphorus content, duration of flowering, and the mean weight of individual seeds in cultivated oat, while it had a significantly reduced effect, no effect, or a negative effect on these characters for wild oat. Poor positive responsiveness of wild oat in these characters was thus associated with what might be considered to be inherent adaptations to nutrient deficiency: high root: shoot ratio and inherently low growth rate. Infection also increased seed phosphorus content and reproductive allocation.     

Regulation of reproduction in wild and cultivatedLycopersicon esculentum Mill. by vesicular-arbuscular mycorrhizal infection

Summary We examined how mycorrhizal infection byGlomus etunicatum Becker and Gerd. affected flowering phenology and components of reproduction in eight wild accessions and two cultivars ofLycopersicon esculentum Mill. We did this by performing a detailed demographic study of flower, fruit and seed production. Mycorrhizal infection had variable effects on the ten accessions. Infection significantly decreased the time taken to initiate flowering in some accessions. In addition, infection increased flowering duration in some accessions. In many accessions, infection significantly increased seed production, primarily by increasing the number of inflorescences and infructescences. In some accessions, mycorrhizal infection also increased the proportion of flowers that produced mature fruits or the number of seeds per fruit. Among accessions, shoot phosphorus content was correlated with seed productivity for both mycorrhizal and non-mycorrhizal plants. However, non-mycorrhizal plants produced more seed biomass per mg of shoot phosphorus than mycorrhizal plants.     

Localization of proton-ATPase genes expressed in arbuscular mycorrhizal tomato plants

In arbuscular mycorrhizal symbioses, solutes such as phosphate are transferred to the plant in return for photoassimilates. The uptake mechanism is probably facilitated by a proton gradient generated by proton H⁺-ATPases. We investigated expression of Lycopersicon esculentum Mill. H⁺-ATPases in mycorrhizal and non-mycorrhizal plants to determine if any are specifically regulated in response to colonization. Tissue expression and cellular localization of H⁺-ATPases were determined by RNA gel blot analysis and in situ hybridization of mycorrhizal and non-mycorrhizal roots. LHA1, LHA2, and LHA4 had high levels of expression in roots and were expressed predominantly in epidermal cells. LHA1 and LHA4 were also expressed in cortical cells containing arbuscules. The presence of arbuscules in root sections was correlated with lower levels of expression of these two isoforms in the epidermis. These results suggest that LHA1 and LHA4 expression is decreased in epidermal cells located in regions of the root that contain arbuscules. This provides evidence of differential regulation between molecular mechanisms involved in proton-coupled nutrient transfer either from the soil or fungus to the plant.     

Tolerance ofFestuca rubra L. to zinc in relation to mycorrhizal infection

Summary Plant yield of mycorrhizal and non-mycorrhizalFestuca rubra L. was linearly decreased with increasing zinc concentrations in nutrient solution. In all cases, non-mycorrhizal plant growth was significantly greater than that of mycorrhizal plants. Zinc and phosphorus concentrations of root and shoot of mycorrhizal plants were greater in all zinc treatments while mycorrhizal plants showed equal or lower tolerance indices to zinc than non-mycorrhizal plants. Yield depressions of mycorrhizal plants may be the result of enhanced zinc and phosphorus concentrations combined with the cost for growth and maintenance of the mycorrhizal fungi.     

Interactive effects of salinity and nitrogen on growth and yield of tomato plants

Summary Tomato (Lycopersicon esculentum var. VF 145) plants were grown with Typic Xerofluvents soil in a greenhouse irrigated with recycled nutrient solutions having increasing levels of N and salinity. Positive response of plants to increasing levels of N was obtained at the lowest initial salinity level of 1 dS/m (dS/m=mmho/cm, referenced at 25°C). At the higher initial salinity levels of 5 and 9 dS/m, increasing N was ineffective in counteracting adverse effects on growth and yield caused by the presence of enhanced salt concentrations of the nutrient solution. Total N uptake was linearly correlated with the total water uptake and was severely suppressed by impaired growth associated with the two higher initial salinity levels, irrespective of N levels. The effect of salinity on leaf N concentrations changed over time. Leaf Cl and P concentrations indicated a possible suppressing effect of Cl on P uptake into plant tops.Based on portions of the thesis submitted by the senior author in partial satisfaction of the requirements for the Ph.D. degree in Soil Science. Supported in part by a grant from the Kearney Foundation of Soil Science.     

Differential influence of native and introduced arbuscular mycorrhizal fungi on growth of dominant and subordinate plants

The impact of arbuscular mycorrhizal fungi (AMF) on plant ecosystems has been intensively reported. In this research, we explored the difference between native and introduced AMF in promoting the growth of dominant and subordinate plant species. In glasshouse experiments, dominants and subordinates from subtropical grasslands were colonized by native AMF or introduced AMF, Glomus versiforme. The biomass revealed that mycorrhizal dependencies (MD) on the native AMF of the dominants were much higher than those of the subordinates, while MD on the introduced AMF changed following the replacement of native AMF with introduced AMF. A close relationship between biomass promotion and increase in phosphorus uptake was observed, indicating the important role of AMF-enhanced nutrient acquisition by roots. Our results show that plant community structures are partly determined by MD on native AMF, and could be modified by introducing exogenous AMF species.     

Density-dependent response to mycorrhizal infection in Abutilon theophrasti Medic

Summary One purpose of this study was to determine whether an increase in plant density would result in a decrease in response to mycorrhizal infection (particularly as measured by phosphorus content). Increases in plant density generally result in increases in root density in the volume of soil occupied by the plants. Root density, in turn, largely determines phosphorus uptake. If mycorrhizal plants had significantly higher effective root densities than non-mycorrhizal plants due to the fungal hyphae and thus were more thorough in exploiting a given volume of soil for phosphorus, then a given increase in root density might result in a greater proportional increase in phosphorus uptake for non-mycorrhizal plants than for mycorrhizal plants. Two experiments were performed in which mycorrhizal infection and available soil volume per plant were manipulated; one in which the number of plants within a given pot size was varied (experiment 1), and another in which single plants were grown in pots of differing volume (experiment 2). The two experiments yielded similar results but for apparently different reasons. In the first experiment, for a given increase in root density, non-mycorrhizal plants had a greater proportional increase in phosphorus uptake than mycorrhizal plants. Thus, as predicted, response to mycorrhizal infection was greatest at the lowest planting density (highest available soil volume per plant, lowest root density). In experiment 2, response to infection was also greatest at the highest available soil volume per plant (largest pot), but pot size did not influence root density. These results show that the benefit from mycorrhizal infection may be partly determined by root density and they suggest that plants either occurring in patches of contrasting root density in a given community, or occurring in different communities with inherently different root densities may differ in their reliance upon mycorrhizal fungi for phosphorus uptake.     

Vesicular-arbuscular mycorrhizal infection in lettuce (Lactuca sativa) in relation to calcium supply

Summary Infection of lettuce roots (Lactuca sativa L.) by the vesicular-arbuscular mycorrhizal fungiGlomus caledonium andGlomus mosseae was dependent on the amount of calcium (supplied as CaCl₂·2H₂O or CaSO₄·2H₂O) in the nutrient solution; those plants growing at low calcium concentrations being poorly infected.     

Intensity of mycorrhizal infection and response of onion at different stages of growth

Summary A time course experiment was conducted to study the intensity of root infection and the response of onion. Mycorrhizal infection with internal hyphae and arbuscules was observed in onion roots 15 days after sowing and infection percentage progressively increased up to 35 days. Plants inoculated with the mycorrhizal fungus weighed less than non-mycorrhizal plants during initial stages up to 35 days but grew faster later after 38 days.     

Impaired growth in transgenic plants over-expressing an expansin isoform

Expansins are cell wall proteins characterised by their ability to stimulate wall loosening during cell expansion. The expression of some expansin isoforms is clearly correlated with growth and the external application of expansins can stimulate cell expansion in vivo in several systems. We report here the expression of a heterologous expansin coding sequence in transgenic tomato plants (Lycopersicon esculentum Mill.) under the control of a constitutive promoter. In some transgenic lines with high levels of expansin activity extractable from cell walls, we observed alterations of growth: mature plants were stunted, with shorter leaves and internodes, and dark-grown seedlings had shorter and wider hypocotyls than their wild-type counterparts. Examination of hypocotyl sections revealed similar differences at the cellular level: cortical and epidermal cells were shorter and wider than those from wild-type seedlings. The observed stimulation of radial expansion did not compensate for the decreased elongation, and overall growth was reduced in the transgenics. As this observation can seem paradoxical given the known effect of expansins on isolated cell walls, we examined the mechanical behaviour of transgenic tissue. We measured a decrease in hypocotyl elongation in response to acidic pH in the transformants. This result may account for the alterations in cell expansion, and could itself be explained by a reduced susceptibility of transgenic cell walls to expansin action.     

The role of the external mycelial network of vesicular-arbuscular mycorrhizal fungi: a study of carbon transfer between plants interconnected by a common mycelium

The transfer of ¹⁴C from Lolium perenne (the donor) to Plantago lanceolata (the receiver), mediated by vesicular-arbuscular (VA) mycorrhizal fungi, was examined when the two species were grown together or separately. The VA mycorrhizal infection led to a significant increase, relative to that in uninfected plants, in the ¹⁴C transferred from donor to receiver plants, not only when the roots of the two plants were growing in intimate mixture, but also when they were separated by a root-free zone of 2.33 cm. The majority of isotope transfer between the two plant species was along the direct pathway via VA mycelium.     

Effect of ionic Al in culture solutions on the growth ofArnica montana L. andDeschampsia flexuosa (L.) Trin.

Summary Plants of five tomato strains were grown under low-K stress at three Na levels. These plants were harvested at three time intervals, and Na accumulation and distribution were measured in their tissues. Strain differences were observed for the ability to substitute Na for K under low-K stress. In two strains with high Na-substitution capacity, efficiency in substitution was associated with the accumulation of more Na and the maintenance of higher Na concentrations in shoot tissues than in other strains. In a third strain which also had a relatively high Na-substitution capacity at the highest solution Na level, an unusual efficiency in Na substitution was indicated, because the strain neither accumulated Na nor maintained high tissue Na levels.     

Influence of vesicular-arbuscular mycorrhizal infection and P addition on growth and P nutrition of Anthyllis cytisoides L. and Brachypodium retusum (Pers.) Beauv.

The effect of P applications and mycorrhizal inoculation on the growth and P nutrition of Anthyllis cytisoides L. (Fabaceae) and Brachypodium retusum (Pers.) Beauv. (Poaceae) was studied. Both plants are widely distributed and well adapted to semi-arid habitats in southern Spain. In all treatments, even with high P doses, mycorrhizal plants showed a higher concentration of phosphorus in their tissues than non-mycorrhizal plants. Mycorrhizal inoculation enhanced the growth of the plants when no P was applied. At high P addition, non-mycorrhizal plants showed higher growth than mycorrhizal plants. The response of each plant type to P application was somewhat different.     

Influence of substrate on the interaction ofGlomus intraradices andFusarium oxysporum f.sp.radicis-lycopersici on tomatoes

Summary The influence of five substrates on the interaction betweenGlomus intraradices andFusarium oxysporum f.sp.radicis-lycopersici and its effect on tomato plants development was investigated. The presence ofG. intraradices decreased root necrosis in all substrates and affected the Fusarium population with different intensity depending on the substrate used. Substrates were found to influence disease development, Fusarium population in the substrate, root colonization by the endomycorrhizal fungus and growth of the host plant. In addition to providing good experimental conditions, the use of calcined montmorillonite clay also facilitated washing, recuperation, necrosis evaluation and staining of roots. Its use is proposed as a standard medium for experimental work on the interactions between endomycorrhizal fungi, root pathogens and host plants.Contribution no J 981 of the Saint-Jean Research Station and no 268 of the Sainte-Roy Research Station     

Sequences and interactions of mycorrhizal fungi on birch

Summary Soil cores collected under a birch tree (Betula pubescens) on an experimental plot showed a progressive change in types of sheathing mycorrhiza with distance from the tree base. Seedlings grown in cores in a glasshouse also developed different mycorrhizal types depending on distance from the tree at which the cores were taken, but the types on seedlings were often different from those in the parent cores. When cores were taken directly beneath fruitbodies and sown to birch in a glasshouse, seedlings developed mycorrhizas of Laccaria, Inocybe and Hebeloma in cores from beneath these fruitbodies, but they seldom developed Lactarius mycorrhizas and never developed Leccinum mycorrhizas in cores taken beneath these fruitbodies. Similarly, when seedlings were grown in soils supplemented with vermiculite-peat inocula in a glasshouse, Laccaria and Hebeloma readily formed mycorrhizas, butLactarius pubescens seldom did so and Leccinum andAmanita muscaria never dit so. Yet all these fungi form mycorrhizas on birch seedlings in aseptic conditions.The results suggest a distinction between early stage and late stage mycorrhizal fungi of birch. Early stage fungi readily infect seedlings from resident or introduced inoculum in normal, unsterile soil, whereas late stage fungi do not readily form mycorrhizas in these conditions.     

Initial and residual toxicity of soil-applied thiram on the vesicular-arbuscular mycorrhizal symbiosis

The effect of the non-systemic fungicide thiram on the vesicular-arbuscular mycorrhizal (VAM) symbiosis and on Leucaena leucocephala was evaluated in a greenhouse experiment. In the uninoculated soil treated with P at a level optimal for mycorrhizal activity, mycorrhizal colonization of roots was low, and did not change as the concentration of thiram in the soil increased with the from 0 to 1000 mg/kg. When this soil was inoculated VAM fungus Glomus aggregatum, with VAM colonization was enhanced significantly, but decreased increase in thiram concentration until it coincided with the level observed in the uninoculated soil. Similarly, symbiotic effectiveness was reduced, its expression delayed or completely eliminated with increase in the concentration of thiram. Amending soil to a P level sufficient for non-mycorrhizal host growth fully compensated for thiram-induced loss of VAM activity if the thiram levels did not exceed 125 mg/kg. In soil treated with 50 mg thiram/kg, the toxicity of the fungicide dissipated within 66 days of application. At higher concentrations, the toxicity of the chemical on the mycorrhizal symbiosis appeared to be enhanced.Contribution from the Hawaii Institute of Tropical Agriculture and Human Resources Journal Series No. 3716     

Compatibility studies in three wild species of Lycopersicon

A study was made on the self-compatibility in L. hirsutum (LH), L. pennellii (LP) and L. chilense (LC) and on cross-compatibility or preferable unilateral compatibility between these wild species and L. esculentum (LE). From the results of various test crosses and selfings and of cytological research and fruit and seed setting as well it was concluded that the material used is compatible but it differed in the degree of the expression. The highest self- and cross-compatibility was found in LP where number of seed, their viability and also pollen tube growth was similar to LE. Differences between LC and LH in self-compatibility and the congruity with LE were very small on the basis of pollen tube elongation but relatively fewer number of fruit and seed were obtained in the cross LE × LC than from the combination of LE × LH. In the reciprocal crosses when wild species were used as the pistillate parent, no seed were obtained as fertilization was prevented.     

Interactions between the soilborne root pathogenPhytophthora nicotianae var.parasitica and the arbuscular mycorrhizal fungusGlomus mosseae in tomato plants

In order to study the influence of Arbuscular Mycorrhiza (AM) on the development of root rot infection, tomato plants were raised with or withoutGlomus mosseae and/orPhytophthora nicotianae var.parasitica in a sand culture system. All plants were fed with a nutrient solution containing one of two phosphorus (P) levels, 32µM (I P) or 96µM (II P), to test the consequence of enhanced P nutrition by the AM fungus on disease dynamics. Mycorrhizal plants had a similar development to that of control plants. Treatment withPhytophthora nicotianae var.parasitica resulted in a visible reduction in plant weight and in a widespread root necrosis in plants without mycorrhiza. The presence of the AM fungus decreased both weight reduction and root necrosis. The percentage reduction of adventitious root necrosis and of necrotic root apices ranged between 63 and 89% The enhancement of P nutrition increased plant development, but did not appreciably decrease disease spread. In our system, mycorrhiza increased plant resistance toP. nicotianae var.parasitica infection. Although a contribution of P nutrition by mycorrhiza cannot be excluded, other mechanisms appear to play a crucial role.     

Assessment of in vitro growth of apical stem sections and adventitious organogenesis to evaluate salinity tolerance in cultivated tomato

The possible use of in vitro shoot morphogenesis and shoot apex culture to evaluate salt tolerance in cultivated tomato (Lycopersicon esculentum Mill.) has been analyzed, using two cultivars with similar salt tolerance, Pera and Hellfrucht frühstamm (HF). The effect of salt on shoot regeneration was studied by culturing leaf explants on media supplemented with 0, 43, 86, 129 and 172 mM NaCl. The presence of NaCl in the regeneration media at 86 mM strongly inhibited shoot regeneration in the cultivar HF, but not in Pera. However, the substitution of NaCl by mannitol, maintaining the same water potential in the culture media, decreased the regeneration percentage in Pera but did not affect HF. Shoot apices of both cultivars were also subcultured at 6-week intervals, for 4 subcultures, at the same NaCl concentrations as used in the previous experiment, and the shoot growth, leaf and root number, rooted shoot and shoot necrosis were recorded at the end of each subculture. Root formation was the parameter most affected by salt in both cultivars, Pera being more sensitive than HF. The substitution of NaCl by mannitol significantly increased the percentage of rooted shoots in Pera after four subcultures, and slightly decreased this percentage in HF. Shoot necrosis was only observed in the last subculture at NaCl higher than 86 mM, the percentage of necrotic shoots being higher in Pera than in HF (75% and 45%, respectively). The lack of agreement between the results obtained with the in vitro tests, e.g., adventitious shoot formation and growth of apical stem sections, suggests that this approach may not be a reliable tool to evaluate salt tolerance in cultivated tomato. This revised version was published online in June 2006 with corrections to the Cover Date.