Effects of Genetic Differences among Crop Species and Cultivars Upon the Arbuscular Mycorrhizal Symbiosis

Abstract

Crop species and cultivars within species can differ markedly in their ability to respond to colonization by arbuscular mycorrhizal (AM) fungi. The variation in response to mycorrhizas has largely been overlooked by plant breeders, even when germplasm is screened for efficiency of nutrient uptake and efficiency of nutrient use, processes influenced greatly by mycorrhizal fungi. Host germplasm is frequently selected under conditions that would not favor, and indeed might prevent, a response to mycorrhizal fungi, i.e. high nutrient availability. Continued breeding of crop plants without regard to mycorrhiza responsiveness is irresponsible, as it could result not only in increased requirement for nutrient inputs in the short term, but also the inadvertent, permanent loss of these genes in crop germplasm. Loss of colonization by AM fungi could also result in the loss of other important benefits provided by mycorrhizal fungi (drought tolerance, resistance to disease, uptake of other nutrients, maintaining soil structure) and reduce the population of AM fungi able to colonize other hosts in the cropping system. Plant breeders should determine the contribution of mycorrhizal fungi to nutrient uptake by evaluating plant growth with and without mycorrhizas over a full range of nutrient levels; i. e. develop response curves for the variable of interest. Ultimately the goal should be to map genes responsible for mycorrhizal colonization and responsiveness, and to utilize them in developing cultivars which can more effectively acquire nutrients from input low input agricultural systems and take advantage of the other benefits provided by the mycorrhizal symbiosis.