The genetics behind AM symbiosis: the case of Tomato wild relatives

Arbuscular mycorrhiza (AM) is the most widespread mutualistic symbiosis established between land plants and Glomeromycotina, a group of soil fungi [1]. Among mycorrhizal crops, tomato has been extensively investigated for AM interactions being a valuable plant model. However, genetics behind AM symbiosis responsiveness in tomato has been mostly faced using functional genomics approaches (RNAseq and microarray). However, precious genetic resources are available to afford this topic, such as wild relatives, introgression lines and mutants. In particular, tomato wild relatives, offer an effective genetic reservoir for cultivated tomato [2,3], but have rarely been investigated for their susceptibility and responsiveness to the AM symbiosis at root and systemic level. We analyzed the responses of two species, Solanum pennellii and Solanum neorickii and a cultivated tomato (M82), to the colonization by Funneliformis mosseae. Plants were grown for 60 days in an alveolar tray with a substrate containing or not the AM fungal inoculum and watered with low phosphate fertilizer. Plant growth traits, SPAD and mycorrhizal status were assessed. Arbuscule morphology was obtained using Wheat Germ Agglutinin, while symbiosis functionality was tested by evaluating AM-marker gene's expression. Results indicate that, although in all genotypes arbuscule morphology was maintained, in S. pennellii mycorrhization was reduced while in S. neorickii was similar to M82. Interestingly, under these growth conditions, in both wild relatives a negative growth response was observed. In conclusion, tomato wild relatives are a powerful tool to understand AM genetics suggesting that mycorrhiza responsiveness is a crucial trait to be considered in breeding programs. [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, 2:6022