Differential allocation of seed-loaded fungal ergot alkaloids and de novo synthesis during early ontogeny of morning glories (Convolvulaceae)

Background/Question/Methods

Recent work has shown that certain morning glory species (Convolvulaceae) are infected by symbiotic fungi in the genus Periglandula (Ascomycota:  Clavicipitaceae), which are closely related to fungal endophytes of cool-season grasses.  These fungi produce bioactive ergot alkaloids toxic to a variety of insect and vertebrate herbivores.  The Convolvulaceae-Periglandula association is notable for the high concentrations of ergot alkaloids in seeds of infected plants, which can be 1000x greater than in infected grasses.  These seed-loaded alkaloids may serve to protect the seed from predators but could also be redistributed throughout the seedling to protect vulnerable young tissue from herbivores.  In this study, we investigated the in planta redistribution of seed-loaded alkaloids during early ontogeny of four species, including three species not previously reported as infected.  Specifically, we addressed two questions:  (1) What is the in planta distribution of ergot alkaloids during development and how does it vary among species? And (2) are ergot alkaloids synthesized de novo during seedling development?  Knowing how ergot alkaloids are distributed or synthesized throughout the seedling will inform which parts of the plant are most vulnerable to attack by natural enemies and whether there is interspecific variation in these symbiotic traits. 

Results/Conclusions

Ipomoea amnicola, I. argillicola and I. hildebrandtii showed similar patterns of seed-loaded alkaloid distribution where fungal ergot alkaloids remained most highly concentrated in the cotyledons, with lesser amounts in other above-ground parts of the plant.  By contrast, I. tricolor mobilized alkaloids primarily to below-ground tissues.  Over all species we detected eight unique ergot alkaloids and the total number per species ranged from four to six.  We also observed differential allocation of individual alkaloids to specific tissues.  Finally, we tracked fungal production beyond the seedling stage in I. tricolor.  We treated half of the individuals with fungicide to suppress alkaloid production and measured alkaloid concentrations weekly.  We found greater alkaloid concentrations in leaves of untreated plants by the third week, but there was no difference in either of the first two weeks, suggesting de novo synthesis of alkaloids does not occur immediately after germination.  In conclusion, our study provides evidence that seed-loaded ergot alkaloids are differentially allocated between above- and below-ground tissues among species of Convolvulaceae and that they may play a protective role for the seedling, especially since de novo production of alkaloids does not occur until later in development.