Interaction between arbuscular mycorrhizal fungus Glomus mosseae and plant growth promoting fungus Phoma sp. on their root colonization and growth promotion of cucumber (Cucumis sativus L.)

Interaction between arbuscular mycorrhizal fungus Glomus mosseae and plant growth promoting fungus Phoma sp. was studied for its effect on their root colonization and plant growth of cucumber. Two isolates of Phoma sp. (GS8-2 and GS8-3) were tested with G. mosseae. The percent root length colonized by G. mosseae was not adversely affected by the presence of Phoma isolates. In contrast, the root colonization of both isolates GS8-2 and GS8-3 in 4-week-old plants was significantly reduced (80.7% and 84.3%, respectively) by added G. mosseae. Inoculating plants with each Phoma isolate significantly increased the shoot dry weight. However, dual inoculation of each Phoma isolate with G. mosseae had no significant effect on growth enhancement.     

Effects of a vesicular-arbuscular mycorrhizal fungus and other soil microorganisms on growth,mineral nutrient acquisition and root exudation of soil-grown maize plants

Maize (Zea mays L. cv. Alize) plants were grown in a calcareous soil in pots divided by 30-m nylon nets into three compartments, the central one for root growth and the outer ones for hyphal growth. Sterle soil was inoculated with either (1) rhizosphere microorganisms other than vesicular-arbuscular mycorrhizal (VAM) fungi, (2) rhizosphere microorganisms together with a VAM fungus [Glomus mosseae (Nicol. and Gerd.) Gerdemann and Trappel], or (3) with a gamma-irradiated inoculum as control. Plants were grown under controlled-climate conditions and harvested after 3 or 6 weeks. VAM plants had higher shootroot ratios than non-VAM plants. After 6 weeks, the concentrations of P, Zn and Cu in roots and shoots had significantly increased with VAM colonization, whereas Mn concentrations had significantly decreased. Root exudates were collected on agar sheets placed on the interface between root and hyphal compartments. Six-week-old VAM and non-VAM plants had similar root exudate compositions of 72–73% reducing sugars, 17–18% phenolics, 7% organic acids and 3% amino acids. In another experiment in which root exudates were collected on agar sheets with or without antibiotics, the amounts of amino acids and carbohydrates recovered were similar in VAM and non-VAM plants. However, threeto sixfold higher amounts of carbohydrates, amino acids and phenolics were recovered when antibiotics were added to the agar sheets. Thus, the high microbial activity in the rhizosphere and on the rhizoplane limits the exudates recovered from roots.     

Effect of root exudation on VA mycorrhizal infection at early stages of plant growth

Summary No relationship between the degree of VA mycorrhizal infection and total sugar content in root exudates of several plant species of different degree of mycorrhizal susceptibility were observed during the early stages of plant growth. Even more, the non host plants tested showed higher sugar exudation ability, when expressed as the amount exuded per g of root, at these early periods of their growth, than plants susceptible to mycorrhizal infection.Root exudates from host and non host plants influenced similarly the percentage of spore germination and number of secondary spores under controlled conditions.     

Growth and arsenic uptake by Chinese brake fern inoculated with an arbuscular mycorrhizal fungus

A split-root experiment investigated the effects of inoculation with the arbuscular mycorrhizal fungus Glomus mosseae and arsenic (As) addition on As uptake by Pteris vittata L. Either part or all of the root system was inoculated with G. mosseae or exposed to As addition (50 ml 1000 μmol L⁻¹ As 1 week before harvest). Mycorrhizal colonization substantially increased frond and root dry weight and P and As contents irrespective of As addition. Frond As contents in mycorrhizal plants were highest when the whole root system was exposed to As. Frond As concentrations and contents were higher when inoculation and As addition were in the same parts of the root system than when spatially separate. There were positive effects of arbuscular mycorrhiza inoculation on plant growth and As uptake, and inoculation of part of the roots seemed to be as effective as inoculation of the whole root system.     

Expression studies of plant genes differentially expressed in leaf and root tissues of tomato colonised by the arbuscular mycorrhizal fungus Glomus mosseae

Arbuscular mycorrhizal (AM) fungi are a multifaceted group of mutualistic symbionts that are common to terrestrial ecosystems. The interaction between AM fungi and plant roots is of environmental and agronomic importance. Understanding the molecular changes within the host plant upon AM fungal colonisation is a pre-requisite to a greater understanding of the mechanisms underlying the interaction. Differential mRNA display was conducted on leaf tissue of tomato plants colonised and non-colonised by the AM fungus Glomus mosseae and five putative differentially regulated cDNAs were identified. All cDNAs isolated shared high sequence similarity to known plant genes. Differential screening was initially used to establish whether the cDNAs were differentially expressed. Semi-quantitative RT-PCR was used to establish gene expression patterns for all five clones within leaf and root tissue of mycorrhizal and non-mycorrhizal colonised tomato plants. Differential regulation was observed for all five cDNAs. Down-regulation within the leaf tissue of mycorrhizal plants was observed for 4 out of the 5 cDNAs with an up-regulation observed only for one. Tissue specific regulation was observed for several cDNAs, with down-regulation observed in mycorrhizal leaf tissue and up-regulation observed within mycorrhizal root tissue as compared to non-mycorrhizal tissue.     

Xyloglucanases in the interaction between saprobe fungi and the arbuscular mycorrhizal fungus Glomus mosseae

We studied the production of xyloglucanase enzymes of pea and lettuce roots in the presence of saprobe and arbuscular mycorrhizal (AM) fungi. The AM fungus Glomus mosseae and the saprobe fungi Fusarium graminearum, Fusarium oxysporum-126, Trichoderma harzianum, Penicillium chrysogenum, Pleurotus ostreatus and Aspergillus niger were used. G. mosseae increased the shoot and root dry weight of pea but not of lettuce. Most of the saprobe fungi increased the level of mycorrhization of pea and lettuce, but only P. chrysogenum and T. harzianum inoculated together with G. mosseae increased the dry weight of pea and lettuce respectively. The AM and saprobe fungi increased the production of xyloglucanases by plant roots. The level of xyloglucanase activities and the number of xyloglucanolytic isozymes in plants inoculated with G. mosseae and most of the saprobe fungi tested were higher than when both microorganisms were inoculated separately. The possible relationship between xylogucanase activities and the ability of AM and saprobe fungi to improve the dry weight and AM root colonization of plants was discussed.     

Interactions between a fluorescent pseudomonad,an arbuscular mycorrhizal fungus and a hypovirulent isolate of Rhizoctonia solani affect plant growth and root architecture of tomato plants

Abstract

Although Rhizoctonia solani is a cosmopolitan soilborne pathogen, the genus includes isolates with different pathogenicity ranging from high virulence to avirulence. The biocontrol strain Pseudomonas fluorescens P190r and the arbuscular mycorrhizal (AM) fungus Glomus mosseae BEG12 were inoculated alone or in combination in tomato plants infested by the mildly virulent pathogen R. solani #235. Plant growth as well as root morphometric and topological parameters were evaluated. The infection of R. solani was significantly reduced by all the combinations of the beneficial microorganisms. Root systems of R. solani‐infected plants were weakly developed but highly branched with a herring‐bone pattern, while those inoculated with the AM fungus, alone or in combination with the bacterial strain, were longer and more developed, and displayed a dichotomous pattern. The interactions among these three microorganisms affected plant growth and root architecture of tomato plants.     

Indigenous and introduced arbuscular mycorrhizal fungi contribute to plant growth in two agricultural soils from south-western Australia

Arbuscular mycorrhizal (AM) fungi occur in all agricultural soils but it is not easy to assess the contribution they make to plant growth under field conditions. Several approaches have been used to investigate this, including the comparison of plant growth in the presence or absence of naturally occurring AM fungi following soil fumigation or application of fungicides. However, treatments such as these may change soil characteristics other than factors directly involving AM fungi and lead to difficulties in identifying the reason for changes in plant growth. In a glasshouse experiment, we assessed the contribution of indigenous AM fungi to growth of subterranean clover in undisturbed cores of soil from two agricultural field sites (a cropped agricultural field at South Carrabin and a low input pasture at Westdale). We used the approach of estimating the benefit of AM fungi by comparing the curvature coefficients ( C) of the Mitscherlich equation for subterranean clover grown in untreated field soil, in field soil into which inoculum of Glomus invermaium was added and in soil fumigated with methyl bromide. It was only possible to estimate the benefit of mycorrhizas using this approach for one soil (Westdale) because it was the only soil for which a Mitscherlich response to the application of a range of P levels was obtained. The mycorrhizal benefit ( C of mycorrhizal vs. non-mycorrhizal plants or C of inoculated vs. uninoculated plants) of the indigenous fungi corresponded with a requirement for phosphate by plants that were colonised by AM fungi already present in the soil equivalent to half that required by non-mycorrhizal plants. This benefit was independent of the plant-available P in the soil. There was no additional benefit of inoculation on plant growth other than that due to increased P uptake. Indigenous AM fungi were present in both soils and colonised a high proportion of roots in both soils. There was a higher diversity of morphotypes of mycorrhizal fungi in roots of plants grown in the Westdale soil than in the South Carrabin soil that had a history of high phosphate fertilizer use in the field. Inoculation with G. invermaium did not increase the level of colonisation of roots by mycorrhizal fungi in either soil, but it replaced approximately 20% of the root length colonised by the indigenous fungi in Westdale soil at all levels of applied P. The proportion of colonised root length replaced by G. invermaium in South Carrabin soil varied with the level of application of P to the soil; it was higher at intermediate levels of recently added soil P.     

Synergistic effects of antagonistic fungi and a plant growth promoting rhizobacterium, an arbuscular mycorrhizal fungus, or composted cow manure on populations of Meloidogyne incognita and growth of tomato

Effects of four antagonistic fungi (Paecilomyces lilacinus, Pochonia chlamydosporia, Trichoderma harzianum and Gliocladium virens) alone and together with a plant growth promoting rhizobacterium Pseudomonas putida, an arbuscular mycorrhizal fungus Glomus intraradices or with composted cow manure (CCM) were assessed on the growth of tomato and on the reproduction of Meloidogyne incognita in glasshouse experiments. Application of all antagonistic fungi (except G. virens), P. putida, G. intraradices or CCM caused a significant increase in the growth of plants without nematodes. However, use of either of these fungi, P. putida, G. intraradices and CCM against plants with nematodes caused a significant increase in tomato growth. Paecilomyces lilacinus caused a 42% increase in the growth of nematode-inoculated plants followed by P. chlamydosporia (36%), T. harzianum (18%) and G. virens (15%). CCM caused about 57% increase in the growth of nematode-inoculated plants followed by P. putida (37%) and G. intraradices (31%). Maximum increase (71%) in the growth of nematode-inoculated plants was observed when CCM was used with P. lilacinus. Moreover, P. lilacinus caused a high reduction (55%) in galling and nematode multiplication, while G. virens the least (25%). Use of P. putida also caused a 39% reduction in galling and nematode multiplication followed by CCM (34%) and G. intraradices (32%). Combined use of CCM with P. lilacinus caused maximum reduction (79%) in galling and nematode multiplication. Re-isolation of antagonistic fungi from nematodes revealed that P. lilacinus parasitised more females and eggs than other antagonistic fungi. Root colonisation by P. putida was increased with P. lilacinus, while colonisation by G. intraradices was reduced in the presence of antagonistic fungi.     

Contribution of an arbuscular mycorrhizal fungus to the uptake of cadmium and nickel in bean and maize plants

Two experiments were carried out in pots with three compartments, a central one for root and hyphal growth and two outer ones which were accessible only for hyphae of the arbuscular mycorrhizal fungus, Glomus mosseae ([Nicol. and Gerd.] Gerdemann and Trappe). In the first experiment, mycorrhizal and nonmycorrhizal bean (Phaseolus vulgaris L.) plants were grown in two soils with high geogenic cadmium (Cd) or nickel (Ni) contents. In the second experiment, mycorrhizal and nonmycorrhizal maize (Zea mays L.) or bean plants were grown in a non-contaminated soil in the central compartment, and either the Cd- or Ni-rich soil in the outer compartments. In additional pots, mycorrhizal plants were grown without hyphal access to the outer compartments. Root and shoot dry weight was not influenced by mycorrhizal inoculation, but plant uptake of metals was significantly different between mycorrhizal and nonmycorrhizal plants. In the first experiment, the contribution of mycorrhizal fungi to plant uptake accounted for up to 37% of the total Cd uptake by bean plants, for up to 33% of the total copper (Cu) uptake and up to 44% of the total zinc (Zn) uptake. In contrast, Ni uptake in shoots and roots was not increased by mycorrhizal inoculation. In the second experiment, up to 24% of the total Cd uptake and also up to 24% of the total Cu uptake by bean could be attributed to mycorrhizal colonisation and delivery by hyphae from the outer compartments. In maize, the mycorrhizal colonisation and delivery by hyphae accounted for up to 41% of the total Cd uptake and 19% of the total Cu uptake. Again, mycorrhizal colonisation did not contribute to Ni uptake by bean or maize. The results demonstrate that the arbuscular mycorrhizal fungus contributed substantially not only to Cu and Zn uptake, but also to uptake of Cd (but not Ni) by plants from soils rich in these metal cations. Deceased 21 September 1996 Deceased 21 September 1996     

Calcareous impact on arbuscular mycorrhizal fungus development and on lipid peroxidation in monoxenic roots

The present work underlined the negative effects of increasing CaCO₃ concentrations (5, 10 and 20 mM) both on the chicory root growth and the arbuscular mycorrhizal fungus (AMF) Glomus irregulare development in monoxenic system. CaCO₃ was found to reduce drastically the main stages of G. irregulare life cycle (spore germination, germinative hyphae elongation, root colonization, extraradical hyphae development and sporulation) but not to inhibit it completely. The root colonization drop was confirmed by the decrease in the arbuscular mycorrhizal fungal marker C16:1ω5 amounts in the mycorrhizal chicory roots grown in the presence of CaCO₃. Oxidative damage evaluated by lipid peroxidation increase measured by (i) malondialdehyde (MDA) production and (ii) the antioxidant enzyme peroxidase (POD) activities, was highlighted in chicory roots grown in the presence of CaCO₃. However, MDA formation was significantly higher in non-mycorrhizal roots as compared to mycorrhizal ones. This study pointed out the ability of arbuscular mycorrhizal symbiosis to enhance plant tolerance to high levels of CaCO₃ by preventing lipid peroxidation and so less cell membrane damage.     

Role of the modification in root exudation induced by arbuscular mycorrhizal colonization on the intraradical growth of Phytophthora nicotianae in tomato

We studied the role of modification in root exudation induced by colonization with Glomus intraradices and Glomus mosseae in the growth of Phytophthora nicotianae in tomato roots. Plants were grown in a compartmentalized plant growth system and were either inoculated with the AM fungi or received exudates from mycorrhizal plants, with the corresponding controls. Three weeks after planting, the plants were inoculated or not with P. nicotianae growing from an adjacent compartment. At harvest, P. nicotianae biomass was significantly reduced in roots colonized with G. intraradices or G. mosseae in comparison to non-colonized roots. Conversely, pathogen biomass was similar in non-colonized roots supplied with exudates collected from mycorrhizal or non-mycorrhizal roots, or with water. We cannot rule out that a mycorrhiza-mediated modification in root exudation may take place, but our results did not support that a change in pathogen chemotactic responses to host root exudates may be involved in the inhibition of P. nicotianae.     

Bacterial effects on arbuscular mycorrhizal fungi and mycorrhiza development as influenced by the bacteria, fungi, and host plant

Bacterial strains from mycorrhizal roots (three belonging to Comamonadaceae and one to Oxalobacteraceae) and from non-mycorrhizal roots (two belonging to Comamonadaceae) of Medicago truncatula and two reference strains (Collimonas fungivorans Ter331 and Pseudomonas fluorescens C7R12) were tested for their effect on the in vitro saprophytic growth of Glomus mosseae BEG12 and on its colonization of M. truncatula roots. Only the Oxalobacteraceae strain, isolated from barrel medic mycorrhizal roots, and the reference strain P. fluorescens C7R12 promoted both the saprophytic growth and root colonization of G. mosseae BEG12, indicating that they acted as mycorrhiza helper bacteria. Greatest effects were achieved by P. fluorescens C7R12 and its influence on the saprophytic growth of G. mosseae was compared to that on Gigaspora rosea BEG9 to determine if the bacterial stimulation was fungal specific. This fungal specificity, together with plant specificity, was finally evaluated by comparing bacterial effects on arbuscular mycorrhizal symbiosis when each of the fungal species was inoculated to two different plant species (M. truncatula and Lycopersicon esculentum). The results obtained showed that promotion of saprophytic growth by P. fluorescens C7R12 was expressed in vitro towards G. mosseae but not towards G. rosea. Bacterial promotion of mycorhization was also expressed towards G. mosseae, but not G. rosea, in roots of M. truncatula and L. esculentum. Taken together, results indicated that enhancement of arbuscular mycorrhiza development was only induced by a limited number of bacteria, promotion by the most efficient bacterial strain being fungal and not plant specific.     

Effects of the root-lesion nematode Pratylenchus vulnus and the mycorrhizal fungus Glomus mosseae on the growth of EMLA-26 apple rootstock

The interaction between Pratylenchus vulnus and the endomycorrhizal fungus Glomus mosseae on the growth of EMLA 26 apple rootstock was studied under shadehouse conditions in the field during the first 6 months of growth. Fresh top weights, fresh root weights, and shoot lengths of mycorrhizal plants with and without P. vulnus were significantly higher than those of nonmycorrhizal plants. Addition of P to non-mycorrhizal controls had little overall effect. Mycorrhizal treatments with the nematode showed a significantly lower amount of nematodes per gram of root than nonmycorrhizal treatments with P. vulnus. Root colonization by G. mosseae was not affected by the presence of the nematode. No nutrient deficiencies were detected in foliar analyses, although low levels of K, A1, and Fe were detected in nematode treatments. The highest levels of S, Mg, Mn and Zn were detected in P. vulnus inoculated plants. Mycorrhizal plants had the highest levels of N, Na, P, K, and Fe. The importance of early mycorrhizal infection of EMLA 26 apple root-stock in the presence of the nematode is discussed.     

Hydroxyproline-rich glycoprotein mRNA accumulation in maize root cells colonized by an arbuscular mycorrhizal fungus as revealed by in situ hybridization

Summary To determine whether the expression of cell wall related genes changes during the establishment of an arbuscular mycorrhizal symbiosis (AM), we studied the expression of a maize hydroxyproline-rich glycoprotein (HRGP) gene. In situ hybridization showed that, in differentiated cells of maize roots, mRNA accumulation corresponding to the gene encoding for HRGP was only found when the cells were colonized by the endomycorrhizal fungusGlomus versiforme.     

Identification of heavy metal-induced genes encoding glutathione S-transferases in the arbuscular mycorrhizal fungus Glomus intraradices

Arbuscular mycorrhizal fungi are able to alleviate the stress for plants caused by heavy metal contamination of soil. To analyze the molecular response of arbuscular mycorrhizal fungi to these pollutants, a subtractive cDNA library was constructed using RNA from Glomus intraradices extraradical hyphae of a root organ culture treated with a mixture of Cd, Zn, and Cu. Screening by reverse Northern blot analysis indicated that, among 308 clones, 17% correspond to genes up-regulated by heavy metals. Sequence analysis of part of the clones resulted, amongst others, in the identification of six genes putatively coding for glutathione S-transferases belonging to two different classes of these enzymes. Expression analyses indicated that the genes are differentially expressed during fungal development and that their RNA accumulation dramatically increases in extraradical hyphae grown in a heavy metal-containing solution.     

Uptake and transport of organic and inorganic nitrogen by arbuscular mycorrhizal fungi

New information on N uptake and transport of inorganic and organic N in arbuscular mycorrhizal fungi is reviewed here. Hyphae of the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. and Gerd.) Gerd. and Trappe (BEG 107) were shown to transport N supplied as ¹⁵N-Gly to wheat plants after a 48 h labelling period in semi-hydroponic (Perlite), non-sterile, compartmentalised pot cultures. Of the ¹⁵N supplied to hyphae in pot cultures over 48 h, 0.2 and 6% was transported to plants supplied with insufficient N or sufficient N, respectively. The increased ¹⁵N uptake at the higher N supply was related to the higher hyphal length density at the higher N supply. These findings were supported by results from in vitro and monoxenic studies. Excised hyphae from four Glomus isolates (BEG 84, 107, 108 and 110) acquired N from both inorganic (¹⁵NH₄ ¹⁵NO₃, ¹⁵NO₃ ⁻ or ¹⁵NH₄ ⁺) and organic (¹⁵N-Gly and ¹⁵N-Glu, except in BEG 84 where amino acid uptake was not tested) sources in vitro during short-term experiments. Confirming these studies under sterile conditions where no bacterial mineralisation of organic N occurred, monoxenic cultures of Glomus intraradices Schenk and Smith were shown to transport N from organic sources (¹⁵N-Gly and ¹⁵N-Glu) to Ri T-DNA transformed, AM-colonised carrot roots in a long-term experiment. The higher N uptake (also from organic N) by isolates from nutrient poor sites (BEG 108 and 110) compared to that from a conventional agricultural field implied that ecotypic differences occur. Although the arbuscular mycorrhizal isolates used contributed to the acquisition of N from both inorganic and organic sources by the host plants/roots used, this was not enough to increase the N nutritional status of the mycorrhizal compared to non-mycorrhizal hosts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Early developmentally regulated genes in the arbuscular mycorrhizal fungus Glomus mosseae: identification of GmGIN1, a novel gene with homology to the C-terminus of metazoan hedgehog proteins

The life cycle of the obligate biotrophic arbuscular mycorrhizal fungi comprises several well-defined developmental stages whose genetic determinants are still unknown. With the aim of understanding the molecular processes governing the early developmental phase of the AM fungal life cycle, a subtractive cDNA library was constructed using a suppressive subtractive hybridization technique. The library contains more than 600 clones with an average size of 500 bp. The isolated cDNAs correspond to genes up-regulated during the early development of the AM fungus Glomus mosseaeversus genes expressed in extraradical hyphae. The expression of several of the isolated genes was further confirmed by RT-PCR analysis. Among the isolated clones, a novel gene named GmGIN1 only expressed during early development in G. mosseae was found. The full-length GmGIN1 cDNA codes for a protein of 429 amino acids. The most interesting feature of the deduced protein is its two-domain structure with a putative self-splicing activity. The N-terminal domain shares sequence similarity with a novel family of GTP binding proteins while the C-terminus has a striking homology to the C-terminal part of the hedgehog protein family from metazoa. The C-terminal part of hedgehog proteins is known to participate in the covalent modification of the N-terminus by cholesterol, and in the self-splicing activity which renders the active form of the protein with signalling function. We speculate that the N-terminal part of GmGIN1, activated through a similar mechanism to the hedgehog proteins, has GTP-binding activity and participates in the signalling events prior to symbiosis formation.     

Effect of Pseudomonas fluorescens on the root exudates of two tomato mutants differently sensitive to Fe chlorosis

Plants with different Fe-mobilization properties are known to differ in the amount and kind of Fe-reducing and Fe-chelating compounds exuded by their roots. Although rhizosphere bacteria are known to affect the exudation of organic compounds by the plant roots, their effect on the root exudates of plants differing in Fe-mobilization properties is not known. We studied the effect of Pseudomonas fluorescens, on the exudation of sugars and organic and amino acids by roots of an iron chlorosis-resistant (T3238FER) and a chlorosis-susceptible (T3238fer) tomato mutant. Under sterile conditions two tomato mutants grew equally well and did not differ in the total amount of sugars and organic acid exuded by their roots. More amino acids, however, were exuded by the roots of T3238FER than T323fer. Mutants differed in the amount of oxalic acid and the amino acids Ala, Asp, Gaba, Gln, Gly, His, Hyl, Ile, Leu, Lys, Phe, Pro, and Val exuded by their roots into sterile rooting media. Addition of P. fluorescens to the rooting medium did not affect the growth of T3238FER but stimulated the root growth of chlorosis-susceptible T3238fer, reduced the amounts of glucose, arabinose and fructose but increased the amount of sucrose, reduced the amounts of fumaric, malic and oxalic acid but increased the amounts of citric and succinic acid in the rooting media of both mutants. P. fluorescens resulted in the following changes in the amino acids in the rooting media: reduced the amounts of Gly, Leu, and Lys in T3238FER, and of Asp, Gln, Hyp, and Ile in T3238fer, and increased the amounts of Cys, Glu, His, Hyp, Ile, Phe and Tyr in T3238FER and of Ala, Glu, His, Phe, and Ser in T323fer—in cases more than 40-fold. These differential effects of P. fluorescens in altering the pattern of organic and amino acids compounds with some Fe-chelating properties detected in the rooting medium of these two mutants may indicate that the differences in Fe-chlorosis susceptibility of these tomato mutants may be the result of, or modified by, the interactions between plant roots and rhizosphere microorganisms. We postulate that the Fe-chlorosis susceptibility in plants may be the product of the interactions between soil microorganisms and plant roots, and may not be solely related to the plant per se.