Arbuscular mycorrhizal (AM) fungi, which form symbioses with the roots of the most important crop species, are usually considered biofertilizers, whose exploitation could represent a promising avenue for the development in the future of a more sustainable next-generation agriculture. The best understood function in symbiosis is an improvement in plant mineral nutrient acquisition, as exchange for carbon compounds derived from the photosynthetic process: this can enhance host growth and tolerance to environmental stresses, such as water stress (WS). However, physiological and molecular mechanisms occurring in arbuscular mycorrhiza-colonized plants and directly involved in the mitigation of WS effects need to be further investigated. The main goal of this work is to verify the potential impact of AM symbiosis on the plant response to WS. To this aim, the effect of two AM fungi (Funneliformis mosseae and Rhizophagus intraradices) on tomato (Solanum lycopersicum) under the WS condition was studied. A combined approach, involving ecophysiological, morphometric, biochemical, and molecular analyses, has been used to highlight the mechanisms involved in plant response to WS during AM symbiosis. Gene expression analyses focused on a set of target genes putatively involved in the plant response to drought, and in parallel, we considered the expression changes induced by the imposed stress on a group of fungal genes playing a key role in the water-transport process. Taken together, the results show that AM symbiosis positively affects the tolerance to WS in tomato, with a different plant response depending on the AM fungi species involved.

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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.

Systemin is a signal peptide that promotes the response to wounding and herbivore attack in tomato. This 18-amino acid peptide is released from a larger precursor, prosystemin. To study the role of systemin as a modulator of defense signaling, we generated tomato (Solanum lycopersicum) transgenic plants that overexpress the prosystemin cDNA. We carried out a transcriptomic analysis comparing two different transgenic events with the untransformed control. The Gene Ontology categories of the 503 differentially expressed genes indicated that several biological functions were affected. Systemin promotes the expression of an array of defense genes that are dependent on different signaling pathways and it downregulates genes connected with carbon fixation and carbohydrate metabolism. These alterations present a degree of overlap with the response programs that are classically associated to pathogen defense or abiotic stress protection, implying that end products of the systemin signaling pathway may be more diverse than expected. We show also that the observed transcriptional modifications have a relevant functional outcome, since transgenic lines were more resistant against very different biotic stressors such as aphids (Macrosiphum euphorbiae), phytopathogenic fungi (Botrytis cinerea and Alternaria alternata) and phytophagous larvae (Spodoptera littoralis). Our work demonstrated that in tomato the modulation of a single gene is sufficient to provide a wide resistance against stress by boosting endogenous defense pathways. Overall, the data provided evidence that the systemin peptide might serve as DAMP signal in tomato, acting as a broad indicator of tissue integrity.Electronic supplementary materialThe online version of this article (doi:10.1007/s11105-014-0834-x) contains supplementary material, which is available to authorized users.