ABSTRACT
Arbuscular mycorrhizal (AM) fungi form a root endosymbiosis with many agronomically important crop species and both enhance the ability of their host to obtain nutrients from the soil and increase host tolerance to biotic and abiotic stressors. However, AM fungal species differ in the benefits they provide to their host plants. Here, we examined the putative molecular mechanisms involved in the regulation of the physiological response of Medicago truncatula to either Rhizophagus irregularis or Glomus aggregatum, a high or a low benefit AM fungus, respectively. Colonization with R. irregularis led to higher growth and nutrient uptake benefits than the colonization with G. aggregatum. These benefits were linked to an elevated expression in the roots of genes involved in strigolactone biosynthesis (NSP1, NSP2, CCD7, and MAX1a), mycorrhiza-induced phosphate (PT8), ammonium (AMT2;3), and nitrate (NPF4.12) transporters and the putative ammonium transporter NIP1;5. R. irregularis also stimulated the expression of photosynthesis-related genes in the shoot and the upregulation of the sugar transporters SWEET1.2, SWEET3.3 and SWEET 12 and the lipid biosynthesis gene RAM2 in the roots. In contrast, G. aggregatum induced the expression of biotic stress defense response genes in the shoots and several genes associated with abiotic stress in the roots. This suggests that either the host perceives colonization by G. aggregatum as a pathogen attack or that G. aggregatum can prime host defense responses. Our findings reveal novel insights into the molecular mechanisms that control the host plant response to colonization with high- and low-benefit arbuscular mycorrhizal fungal symbionts.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Funding. This work was funded by the USDA (2017-67014-26530), the SD Soybean Research and Promotion Council, and the Agricultural Experiment Station at SDSU to HB and SS. KG acknowledges support of the North Carolina Agriculture Research Service (NCARS), the North Carolina Soybean Producers Association (2019-1656), and the USDA (2020-67013-31800).
A new figure was added (Figure 1 in the revised pre-print) and data from a short-term experiment was removed (Fig. 8). Also, all graphs were updated to be more transparent with the data.
