Despite the promise (Wood and Cummings 1992; Podila and Douds 2000), there has been remarkably little real breakthrough in using mycorrhizae for applied activities, beyond the early work of inoculation for nutrient enhancement. Technologies for inoculation are available (Brundrett et al. 1996), but selection for desirable characteristics and matching plant and fungal genotypes may prove to be crucial. In many cases, if not most, agricultural technologies actually select for populations of fungi that may be detrimental for the crops grown (Johnson et al. 1997). Many of the EM and virtually all of AM fungi are generalists, with the ability to invade a wide range of host plants, but with highly variable response variables (Johnson et al. 1997). Bever et al. (2001) have shown how less-than-optimal taxa can persist in a population and even come to dominate.
Large investments are being made in developing trans-genic crops. However, no efforts are underway to even determine if compatibilities or response matches are even important! Kaldorf et al. (2001) have shown that genetically-modified hybrid aspen showed no differences from non-transgenic races in AM or EM infection, and only marginal changes in the fungal community composition were observed. We have also found that transgenic corn had no effect on the species composition or AM infection (Snyder and Allen, unpublished data). Hiremath and Podila (2000) reviewed efforts that demonstrate that genetic transformations of mycorrhizal fungi are possible. Certainly, genes are rapidly being identified and studied (Maldonado-Mendoza et al. 2001) and genes can be added to individual fungi. However, to our knowledge, no transgenic mycorrhizal fungi have been successfully tested and no effects of differing fungi on host responses, either positive or negative, have been evaluated.
This remains an important area of study. Experiences with fungal virus' have shown that extranuclear genetic material can dramatically affect fungal-plant interactions (Roane et al.
1986) and mycorrhizal fungi are no different from others in this trait (Douhan et al. 2003).
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