No abstract

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.

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.

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.

Below ground and above ground plant-insect-microorganism interactions are complex and regulate most of the developmental responses of important crop plants such as tomato. We investigated the influence of root colonization by a nonmycorrhizal plant-growth-promoting fungus on direct and indirect defenses of tomato plant against aphids. The multitrophic system included the plant Solanum lycopersicum ('San Marzano nano'), the root-associated biocontrol fungus Trichoderma longibrachiatum strain MK1, the aphid Macrosiphum euphorbiae (a tomato pest), the aphid parasitoid Aphidius ervi, and the aphid predator Macrolophus pygmaeus. Laboratory bioassays were performed to assess the effect of T. longibrachiatum MK1, interacting with the tomato plant, on quantity and quality of volatile organic compounds (VOC) released by tomato plant, aphid development and reproduction, parasitoid behavior, and predator behavior and development. When compared with the uncolonized controls, plants whose roots were colonized by T. longibrachiatum MK1 showed quantitative differences in the release of specific VOC, better aphid population growth indices, a higher attractiveness toward the aphid parasitoid and the aphid predator, and a quicker development of aphid predator. These findings support the development of novel strategies of integrated control of aphid pests. The species-specific or strain-specific characteristics of these below ground-above ground interactions remain to be assessed.