Synergism, antagonism, and adaptive partner choice in rice-mycorrhizal fungal symbioses

Background/Question/Methods

The relationship between plants and arbuscular mycorrhizal fungi has remained mutually beneficial for over 450 million years, but the forces maintaining this mutualism through evolutionary time are not clear. In natural conditions, both plant and fungal organisms tend to interact multiple partner species, but with a non-random subset of the available partner species. However, it remains unknown whether these non-random associations maximize the fitness of the plant host, the fungal symbiont, both, or neither. We grew elite varieties of Asian rice (Oryza sativa) and landraces of African rice (Oryza glaberrima) in association with single species isolates of seven AMF species as well as a mixed species inocula in order to determine:

1) Do plants provided with a diversity of AMF taxa preferentially associate with the AMF taxa most beneficial to their growth in isolation?

2) Do plants show synergistically positive or antagonistically negative responses to interactions with diverse AMF communities vs. single species?

3) Do plant species vary in their ability to “choose” fungal partners?

Results/Conclusions

We found that the rice species (and cultivars within domesticated species) varied in their response to AMF in general, to specific AMF species, and to diverse vs. single species inoculations of AMF. In particular, Asian rice tended to show neutral to negative responses to AMF, but performed as well or better in mixed compared to single inoculations. On the other hand, African rice tended to respond positively to single species inoculations, but performed antagonistically worse in mixed inoculations. This occurred despite evidence from molecular markers that the most beneficial AMF species in isolation tended to dominate the African rice roots in the mixed inocula treatments (evidence for adaptive partner choice). Interestingly, African (but not Asian) rice roots in the mixed inocula treatments contained a higher abundance of vesicles relative to arbuscules than in any single species inoculation. These results suggest that the ability to control the assembly of AMF communities on a host plant’s roots may vary among plant species, but this ability may not translate into enhanced growth if fungal behavior changes in response to interspecific competitors.