Colonization of seeds by soilborne fungi: Linking seed dormancy-defense syndromes, evolutionary constraints, and fungal traits

The diverse soilborne fungi that recruit to seeds after dispersal include some of the most important agents of seed mortality, as well as strains that enhance germination or inhabit seeds without detriment. Ecological factors that influence seed colonization are not well understood yet are fundamental to the interactions between soilborne fungi and seeds that ultimately influence plant demography and community structure. Here we present current perspectives on seed defense syndromes and related frameworks for predicting colonization success of fungi, with a focus on seeds of tropical pioneer trees. We present a case study that tests whether fungal host range can be predicted by field observations of host use, seed defense syndromes, or phylogenetic relatedness of fungi or hosts. We show that phylogenetic relatedness of hosts, but not fungi, is a strong predictor of fungal colonization of seeds. We posit that the impacts of individual fungi and microbial consortia on seed viability and germination may in turn reflect fungal interactions with the suites of plant defenses codified recently under the broad framework of seed dormancy-defense syndromes. Our findings set the stage for experiments that track colonization, germination, and seedling establishment in the field, important for understanding impacts of fungi on the recruitment of tropical trees.