Tomato wild relatives and their responsiveness to arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal (AM) symbiosis is one of the most widespread beneficial interactions established between the majority of land plants and a group of soil fungi belonging to Glomeromycotina (1). Among crop plants, tomato (Solanum lycopersicum) is a valuable model species in plant biology and in the field of plant-microbe interactions, including the AM symbiosis. Indeed, a large body of genetic resources, such as monogenic mutants lines, introgression and breeding lines and wild genotypes, are publicly available. As an example, tomato wild relatives (Solanum sect. Lycopersicon) have been used as effective genetic reservoir for cultivated tomato, focusing on the introgression of desirable genes and alleles from wild species (2,3). So far, in tomato, all these resources have been extensively used to unravel many aspects of tomato biology, including fruit ripening or resistance to biotic/abiotic stresses, but rarely used to investigate tomato responsiveness to AM fungi (AMF) (4). Particularly, AMF responsiveness in tomato wild relatives has never been assessed neither at root colonization level nor at plant systemic responses. Our hypothesis is that the ‘domestication syndrome’, occurred during the domestication process which seriously narrowed genetic variation of modern tomato varieties, also influenced plant responses to beneficial microbes such as AMF. Moreover, at the best of our knowledge breeders have rarely considered responsiveness to AMF as a feature to be selected. In this work we analyzed the responses of two tomato wild relatives, Solanum pennellii and Solanum neorickii in parallel to a reference cultivated tomato accession (M82) to inoculation of a monospecific inoculum of Funelliformis mosseae, an AMF very common in natural and agricultural systems. Sterilized seeds were sown in small volume pots with a quartz sand-vermiculite substrate containing or not F. mosseae inoculum, and grown for 60 days under controlled climate conditions watered with a modified Long-Ashton solution containing 3.2 μM phosphate to favor mycorrhization. At the sampling time, biometric parameters and SPAD index were measured. Finally, the mycorrhizal status was monitored: the extent of AMF colonization was quantified on methyl-blue stained roots and the morphology of fungal intraradical structures was observed using confocal microscopy after Wheat Germ Agglutinin (WGA) staining. Preliminary results indicate that, under these experimental conditions, in all tested genotypes, including the tomato reference genotype M82, the AM symbiosis induces a negative response in terms of biomass and growth, as sometimes documented in the literature (5). Morphological analysis of mycorrhizal roots reveals that the fungal intraradical structures (arbuscules and hyphae) are similar among the genotypes with an ‘Arum-type’ colonization model. However, and in contrast to our expectation, mycorrhization is strongly reduced in both wild relatives compared to the cultivated tomato. Further insights on the symbiosis functionality, based on the expression profiles of selected marker genes and measurements of phosphorus concentration, are required to validate or not the working hypothesis that domestication has had an impact on AM responsiveness. 1) P. Bonfante, A. Genre (2010) Nature Communications, 1, 48 2) A. Bolger et al. (2014) Nature Genetics, 46, 1034-1038 3) N.P. Castañeda-Álvarez et al. (2016) Nature Plants, 2, 16022 4) M. Chialva, I. Zouari, A. Salvioli, M. Novero, J. Vrebalov, J.J. Giovannoni, P. Bonfante (2016) Planta, 244(1), 155-165 5) M. Bitterlich, M. Sandmann, J. Graefe (2000) Front Plant Sci., 9: 154.