Chances of loss of fungal endophytes in agronomic grasses:: A case-study for Lolium rigidum

Natural plant populations have a long history of coevolution with fungal endophytes. Hence, it is plausible that the agronomic conversion of a natural grass population hosting a fungus initiates a change in their symbiotic relationship. Current data comparing the frequencies of fungal infection in commercial cultivars of some agronomic grasses indicate a high variability on the infection incidence, even within the same cultivar, whereas the occurrence of the endophyte is less variable in wild populations of the species. In this manuscript we analyse the status of the fungal-plant interaction in Australian and European populations of Lolium rigidum infected by Neotyphodium spp. and explore the mechanisms that, during the production cycle (initial frequencies of infection, transmission index of the fungus), and after it (loss of fungal viability at different handling and storage scenarios), may confer plasticity to the infection. This annual grass, native to the Mediterranean basin, was introduced to SE Australia a century ago, where it has been profusely grown and commercialized as a forage crop for and and semiarid climates worldwide. Experimental results indicate that the fungal viability within the seed is maintained as far as storage conditions do not differ from those prevailing in natural environments (short periods of time, even in contrasting regimes), and that the drying of the seeds has a significant, negative effect on fungal viability. Results also suggest that other factors come into play long before the harvest. The higher infection frequencies observed in populations experiencing more aridity (63-97% in SE Australian populations vs. 40-71% in NW Mediterranean populations) suggest that the levels of endophyte infection may adjust depending on the prevailing environment. The uncomplete transmission of the symbiont to the spike of the grass (0.68 for the Mediterranean population), which allows the development of endophyte-free individuals from infected ones, is the mechanism of change. According to this, we hypothesize that the frequency of endophyte infection in a given population reproduces the suitability of the symbiotic relationship in a given environment. The adjustment will manifest more rapidly in short-lived species that develop rapid generations, and in monophyte, dense crops that experience high intra-population competitive relationships. (C) 2008 Elsevier B.V. All rights reserved.