When phosphorus levels are low, mycorrhizae outcompete root hairs in maize

Phosphorus (P) is a key plant macronutrient that is a limiting factor for crop growth in many parts of the world. Plant adaptations to P limitation include root morphological adaptation, physiological changes such as root exudates to improve P mobility, and cooperation with arbuscular mycorrhizal fungi (AMF). An important morphological adaptation for P uptake is the growth of root hairs. These small projections from root epidermal cells increase the root’s surface area and can lead to significantly increased biomass and shoot P content. Similarly, mycorrhizal hyphae effectively increase root surface area as well as reach, leading to increased P uptake.

Both solutions come at a cost to the plant, though the price of AM colonisation is much higher, at up to 15-20% of the plant’s photosynthetic carbon. So does the higher cost of supporting mycorrhizal fungi versus root hairs pay off under P deficient conditions?

In a new article published in Annals of Botany, lead author Xiaomin Ma and colleagues set out to address this question. They used two genetically highly similar maize lines – a wild type with root hairs and a mutant without – to quantify the relative contributions of AMF colonization and root hairs in aiding P uptake under deficient conditions. The researchers also investigated the extent to which AMF infection affected various morphological and physiological factors in the maize plants, including the expression of symbiosis-related genes.

The authors found that AMF colonization led to significant improvements in both plant growth and P uptake, and that the improvements were greater than those provided by the presence of root hairs. The shoot P concentration was also massively improved with AMF colonization. This led to greater growth, which in turn allowed further P uptake via both hyphae and roots. The root hairless mutant experienced both greater colonisation and greater expression levels of mycorrhizae-specific genes compared to plants with root hairs. Overall, the root hairless mutant benefitted more from the presence of AMF than did the wild type plants, suggesting the two strategies are somewhat antagonistic. Indeed, root hair length and AMF colonisation have been shown to be inversely correlated in a number of plant species. “Our data are consistent with the general assumption that root hairs and AMF inoculation are two alternative ways to increase [phosphorus] acquisition under P-deficiency, but these two strategies compete with each other and root hairs cannot compensate for lack of AMF in maize,” write the authors.