Comparing arbuscular mycorrhizal fungi vesicle and hyphal colonization of perennial prairie and annual corn bioenergy systems

Background/Question/Methods

Arbuscular mycorrhizal fungi (AMF) play an important role in plant growth because AMF exchange nutrients for plant derived carbon. AMF produce different structures to obtain nutrients:  hyphae and vesicles. Hyphae are searching structures that extend throughout soil absorbing nutrients. Vesicles are storage structures in which AMF store energy as lipids for use during periods when resources are limited. AMF within perennial plants use lipids from vesicles when emerging from dormancy and when plant photosynthesis is stressed.  In annual plants, AMF cannot rely on a persistent root system to store reserves, so hyphae maximize soil exploration to absorb nutrients and encounter growing roots.  We examined the abundance of vesicle and hyphal structures in two perennial prairie systems (fertilized and unfertilized) and an annual corn system at the Comparison of Biofuel Systems (COBS) site as part of a broader project investigating nutrient cycling and plant production in next generation bioenergy crops. We hypothesized that perennial prairie plants would host more vesicles but fewer hyphae than annual corn plants. Root samples were collected from May through October 2012, cleaned and stained with Trypan blue. Total AMF colonization, vesicle presence, and hyphae presence were determined by the intersect method at 40x magnification.

Results/Conclusions

Across the growing season, annual corn plants had a higher percentage (82%) of total root AMF colonization compared to prairie systems (45%; P<0.0001) on a root specific basis. However, 40% of perennial prairie roots contained vesicles, three times more than annual corn plants (P<0.0001).  Conversely, hyphal colonization rates in corn roots were eight times greater than prairie roots (P<0.0001).  There were no differences in vesicle and hyphal presence between roots sampled in July and October.  Our results support our hypothesis that AMF hosted by perennial and annual plants have different strategies for survival between growing seasons.  Vesicles, and lipids stored within, appear to play an important role in AMF subsistence in perennial systems.  In annual corn roots, which senesce at the end of the growing season, AMF increase hyphal abundance during the growing season, allocating resources to growth into the soil.  Many factors may be contributing to observed differences including different AMF species and communities, different fungal life history strategies, soil nutrient availability, and/or specific plant-fungi interactions.  Future research should explore how each factor influences AMF colonization and growth forms, and how these physiological differences in AMF scale up to system-level nutrient cycling and retention.