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The concept of symbiosis can be described as a continuum of interactions between organisms ranging from mutualism to parasitism that can also change over time. Arbuscular mycorrhizal fungi (AMF) are among the most important obligate plant symbionts. Once the symbiosis is well established, mycorrhizal plants are more tolerant to biotic or abiotic stresses, so the AMF relationship with the host plant is generally described as mutualistic. However, little is known about AMF effects on the plant during the early stages of root colonization. The aim of this work was to assess the type of interaction (mutualistic or parasitic) between the arbuscular mycorrhizal (AM) fungus Funelliformis mosseae and Solanum lycopersicum cv. Rio Grande plants, at 7, 14, 21, and 28 days after inoculation (DAI), considering that in the adopted experimental design (one plant per pot), the seedling was the only carbon source for fungus development in the absence of common mycorrhizal networks with other plants. At each harvest, mycorrhizal colonization, shoot and root weights, morphometric parameters, and photosynthetic efficiency were evaluated. The presence of the AM fungus in the tomato root system was observed starting from the 14th DAI, and its level increased over time. Few effects of the fungus presence on the considered parameters were observed, and no stress symptoms ever appeared; so, we can state that the fungus behaved as a mutualistic symbiont during the early stages of plant growth. Moreover, a trend towards a positive effect on plant growth was observed at 28 DAI in mycorrhizal plants.

The aim of this work was to assess the effects of plant-beneficial microorganisms (two Pseudomonas strains and a mixed mycorrhizal inoculum, alone or in combination) on the quality of tomato fruits of plants grown in the field and subjected to reduced fertilization. Pseudomonas strain 19Fv1T was newly characterized during this study. The size and quality of the fruits (concentration of sugars, organic acids and vitamin C) were assessed. The microorganisms positively affected the flower and fruit production and the concentrations of sugars and vitamins in the tomato fruits. In particular, the most important effect induced by arbuscular mycorrhizal (AM) fungi was an improvement of citric acid concentration, while bacteria positively modulated sugar production and the sweetness of the tomatoes. The novelty of the present work is the application of soil microorganisms in the field, in a real industrial tomato farm. This approach provided direct information about the application of inocula, allowed the reduction of chemical inputs and positively influenced tomato quality.

There is increasing interest in the quality of crops because of the implications concerning health, economic revenue, and food quality. Here we tested if inoculation with a mixture of arbuscular mycorrhizal fungi (AMF) and/or two strains of plant growth-promoting bacteria (PGPB), in conditions of reduced chemical inputs, affects the quality and yield of strawberry fruits. Fruit quality was measured by concentrations of soluble sugars, various organic acids, and two vitamins (ascorbic and folic acid). Co-inoculation with the AMF and each of the two PGPB resulted in increased flower and fruit production, larger fruit size, and higher concentrations of sugars and ascorbic and folic acid in comparison with fruits of uninoculated plants. These results provide further evidence that rhizospheric microorganisms affect fruit crop quality and show that they do so even under conditions of reduced chemical fertilization and can thus be exploited for sustainable agriculture.

Plants possess a broad range of strategies to cope with cadmium (Cd) stress, including the arbuscular mycorrhizal (AM) symbiosis. In cell responses towards Cd, the contribution of changes in ploidy levels is still unclear. We used flow cytometry to investigate if nuclear ploidy changes are involved in response mechanisms toward Cd and to analyze the effect of the symbiotic status on populations of nuclei. The impact of Cd was investigated in roots of two pea (Pisum sativum L.) genotypes differing in their Cd-sensitivity (Cd-sensitive VIR4788 and Cd-tolerant VIR7128). In pea seedlings grown under hydropony, 25 and 250 microM Cd concentrations lead to an increase in 4 C together with a decrease in 2 C nuclei. The same genotypes, grown in soil/sand substrate, were inoculated or not with the AM fungus Glomus mosseae BEG12 and treated or not with Cd at transplanting (Cd1) or 2 weeks after (Cd2). The Cd2 increased the proportion of 6 and 8 C nuclei in the mycorrhizal VIR4788 and in the non-mycorrhizal VIR7128 genotypes. Thus, changes in ploidy levels reflect pea responses towards Cd, which are modulated by the symbiotic interaction. The Cd-induced increase in ploidy may account for changes in DNA transcription and/or translation.

The ability of fluorescent pseudomonads and arbuscular mycorrhizal fungi (AMF) to promote plant growth is well documented but knowledge of the impact of pseudomonad-mycorrhiza mixed inocula on root architecture is scanty. In the present work, growth and root architecture of tomato plants (Lycopersicon esculentum Mill. cv. Guadalete), inoculated or not with Pseudomonas fluorescens 92rk and P190r and/or the AMF Glomus mosseae BEG12, were evaluated by measuring shoot and root fresh weight and by analysing morphometric parameters of the root system. The influence of the microorganisms on phosphorus (P) acquisition was assayed as total P accumulated in leaves of plants inoculated or not with the three microorganisms. The two bacterial strains and the AMF, alone or in combination, promoted plant growth. P. fluorescens 92rk and G. mosseae BEG12 when co-inoculated had a synergistic effect on root fresh weight. Moreover, co-inoculation of the three microorganisms synergistically increased plant growth compared with singly inoculated plants. Both the fluorescent pseudomonads and the myco-symbiont, depending on the inoculum combination, strongly affected root architecture. P. fluorescens 92rk increased mycorrhizal colonization, suggesting that this strain is a mycorrhization helper bacterium. Finally, the bacterial strains and the AMF, alone or in combination, improved plant mineral nutrition by increasing leaf P content. These results support the potential use of fluorescent pseudomonads and AMF as mixed inoculants for tomato and suggest that improved tomato growth could be related to the increase in P acquisition.

The protective effects induced by arbuscular mycorrhizal (AM) fungi against a phytoplasma of the Stolbur group have been investigated in tomato by morphometry and flow cytometry. Symptoms induced by the phytoplasma were less severe when the plants also harboured AM fungi. Morphological parameters such as shoot and root fresh weight, shoot height, internode length, leaf number and adventitious root diameter were closer to those of healthy plants when arbuscular mycorrhiza were present. Reduced nuclear senescence was observed in AM plants infected with phytoplasmas; the percentages of nuclear populations with different ploidy levels were intermediate between AM and phytoplasma-infected plants. The mechanisms underlying these interactions are discussed and a direct action of the AM fungus is hypothesized.