Mycorrhiza-induced protection against pathogens is both genotype-specific and graft-transmissible

In addition to the nutrient exchange that is promoted by the arbuscular mycorrhiza symbiosis (AMS) between plants and fungi, AMS triggers mycorrhiza-induced protection against plant pathogens. Although the induction of this protection against diverse plant pathogens has been described for several plant species, it is not clear if its onset differs among genotypes within a species. To address this, we have examined if and how this defense response is triggered by AMS in common bean and tomato. Leaflets from three different genotypes of mycorrhizal common beans and two genotypes of tomato were challenged with the pathogens Sclerotinia sclerotiorum and Xanthomonas campestris pv. vesicatoria, respectively, to determine if disease protection induced by mycorrhiza is genotype-specific. We have found that one tomato and two common bean genotypes display this type of protection, although this was not observed in Az Hig common bean and Micro-Tom tomato. These findings indicate that mycorrhiza-induced disease protection is genotype-specific for the species and genotypes included in this study. Previous work has shown that defense induced by mycorrhiza colonization is effective against foliar pathogens, suggesting the existence of a signal that must move from colonized roots to shoots. We examined the possibility that this defense response can be triggered in scions from non-mycorrhizal plants when they were grafted onto mycorrhizal rootstock. Pathogen infection assays were then performed on leaflets of both scions and rootstock, and infection damage was compared to non-grafted plants. Our results indicate that in genotypes displaying mycorrhiza-induced disease protection, scions originating from non-mycorrhizal plants acquired the ability to decrease disease symptoms when grafted onto mycorrhizal rootstocks, indicating that they are responsive to the putative signal that moves from mycorrhizal roots to the upper part of the plant to trigger disease protection. This grafting experimental system may be useful in elucidating the molecular mechanisms involved in the systemic signaling of mycorrhiza-induced defense response.