Plant performance is largely driven by belowground processes involving fungi and bacteria. The bioactive region surrounding plant roots known as the rhizosphere is a place teaming with diversity. We are only really beginning to grasp the mechanisms and feedbacks associated with the rhizosphere. A new paper by Botir Khaitov and his team use the common bean plant, Phaseolus vulgaris, to infer aboveground performance while manipulating the plants fungal and bacterial community. Herbivore selection was a main factor in this study, with the spider mite, Tetranychus urticae showing a preference between some rhizosphere treatments. This papers results help us understand another dimension of plant fitness, and yet again show the importance of fungi and bacteria living amongst plant roots.
The common bean plant, Phaseolus vulgaris was utilized in this manipulative study because the plant forms symbiotic relationship with both fungi and nitrogen fixing bacteria. Strong interactions are more easily deciphered with a plant species that maintains both mutualisms. For this experiment, sterilized seedlings grown without any soil mutualists were transplanted to one liter growth pots and inoculated with arbuscular mycorrhizal fungi (AMF), nitrogen fixing bacteria (NFB), or both. Control pots had no mutualists introduced.
Spider mites where reared on room temperature bean plants without soil mutualists. The herbivore experiment was carried out by placing 20-30 nymphs (spider mite juveniles) on leaf arenas delineated by moist paper towel strips that kept mites from escaping. To measure egg production as a function of rhizosphere treatment, females where identified and placed on separate leaf arenas. To analyze spider mite plant host preference, leaves from each treatment (+AMF/+NFB, -AMF/+NFB, -AMF/-NFB, +AMF/-NFB) where arranged in a cross pattern. Then mites were placed in the middle of the four leaves, and their feeding preference was recorded.
Not surprisingly, plants grown with both mutualists grew larger. Even though these plants had to send more sugars down to the rhizosphere to maintain these communities, the services provided by AMF and NFB outweigh their cost. The thought-provoking part of this study is seen through the interaction between bean plant soil treatment and the subsequent fitness of the feeding spider mites.
Spider mites were more fit when the ate plant tissues with intact rhizosphere mutualists. Females laid the most eggs when feeding on +AMF/+NFB plants, closely followed by -AMF/+NFB and +AMF/-NFB treatments. Quite clearly, -AMF/-NFB treatments yielded the least fit spider mites, producing the fewest number of eggs. This spider mite fitness experiment mirrored the mite host plant choice experiment, with mites having a much stronger preference for plant tissues that grew with both mutualists intact.
This new research changes the way we look at mutualisms within the rhizosphere. Without a doubt, AMF and NFB enhance plant growth, but because plants with these intact mutualists attract more herbivores, it begs the question, what does symbiosis actually entail? If fungal and bacterial mutualists entice more herbivores to feed on that specific plant, and if those herbivores can reproduce more efficiently on these rhizosphere rich plants, would these plants actually do better without these mutualists? This study indicates that plant fitness may be indirectly reduced by its own mutualists. Soil ecology for reasons like this are so extremely difficult to untangle. One thing is for sure though. Belowground species interactions alter aboveground species interactions. Whether or not these mutualisms enhance or reduce plant fitness is a whole other question that will take some careful, long-term, studies across a soil nutrient gradients.