‘Designer’ mycorrhizas?: Using natural genetic variation in AM fungi to increase plant growth

The arbuscular mycorrhizal symbiosis formed between arbuscular mycorrhizal fungi (AMF; Glomeromycota) and plant roots is probably the most abundant symbiosis in the world. This symbiosis is formed by the majority of plant species and contributes to improving plant growth and promotes plant diversity ( Smith and Read, 2008 ). Mycorrhizal fungi increase plant growth by improving acquisition of phosphate, an essential nutrient for plant growth. With the rapidly expanding human population, world phosphate reserves are at a critical level, and improved strategies of phosphate use and cycling in agro-ecosytems are essential ( Gilbert, 2009 ; Gross, 2010 ). Furthermore, producing superphosphate fertilizer is energy intensive and expensive. Clearly, AMF have the potential to improve the sustainable use of phosphate in agro-ecosystems, especially in tropical acidic soils. The fungi can be applied to the soil in the form of spores. The spores are normally applied with a substrate carrier, most often soil, and the fungus with the carrier is known as inoculum. However, for several reasons they have had limited use in agriculture. First, most crops are naturally colonized by AMF in the soil and so adding more AMF may not seem necessary. Second, a given AMF may improve the growth of some plant species but not others. So, there is unlikely to be any universal inoculum that is effective for all crops. Third, and economically the most significant, is that most AMF species have to be grown in unsterile conditions with plants. It is labour intensive, expensive and does not guarantee that the inoculum is free of other soil microorganisms that could be potential pathogens. However, by coupling recent advances in understanding the ecology, natural genetic diversity and genetics of AMF with technological advances in inoculum production, it may be possible in the future to create ‘designer’ mycorrhiza; using inoculum that have been manipulated for a desired growth effect with a given crop and that can be produced in an economically viable way.