Arbuscular mycorrhiza-induced growth promotion and disease resistance are fine-tuned by growth-defense tradeoffs in Lotus japonicus and tomato.

Arbuscular mycorrhizal fungi (AMF) are representative symbiotic partners of plants, and trade nutrients with them. This symbiotic association confers plants with the agronomically beneficial traits such as plant growth promotion and stress tolerance. Arbuscular mycorrhizae (AM) are divided into two morphotypes, the Arum-type and the Paris-type, based on fungal structures within the host plant cells. Although the phylogeny of host plants typically determines the AM morphotype, the AMF, Rhizophagus irregularis and Gigaspora margarita, can form Arum-type AM and Paris-type AM, respectively, in tomato (Solanum lycopersicum). In this study, the traits resulting from the AM symbiosis and root transcriptomes between Lotus japonicus and tomato inoculated with these two phylogenetically distal AMF were compared. In L. japonicus, Arum-type AMs formed when colonized by both AMF, as expected. Shoot growth in both plants was significantly promoted when inoculated by these AMF, although the impact of G. margarita was greater than that by R. irregularis colonization. A transcriptome analysis of both plants inoculated by the two AMF strongly suggested changes in the expression levels of genes associated with defense response. AMF inoculation induced resistance against Fusarium diseases in both plants, but the level of disease resistance in Rhizophagus-colonized plants was higher than in Gigaspora-colonized plants. Thus, the colonized AMF identity, and not the AM morphotype, determines the level of AM-induced traits, plant growth promotion and disease resistance. Negative relationships between these two traits would exist as a growth-defense tradeoff to fine-tune the balance in response to limited resources, and to optimize fitness.