Priority effects of wheat seed endophytes on a rhizosphere symbiosis

Seed endophytes, as primary symbionts, may influence formation of secondary symbioses in maturing hosts. We hypothesize that such priority effects are a function of the presence and the identity of the primary symbiont, and are simulated via inoculation with primary symbionts. We sampled primary symbionts in winter wheat using culture-based techniques and next generation sequencing. Ninety-one percent of 6300 seeds had either one or no isolates. NGS data supported this with over 99% fewer reads detected in seeds having no isolates. Alternaria and Cladosporium spp., accounted for almost 80% of the primary symbionts isolated from seeds but only 36% of OTUs. Bacteria accounted for 13% of isolates but 27% of OTUs. We grew seedlings with or without primary symbiont isolates aseptically in vitro with roots in contact with a suspension of the rhizosphere symbiont Phialocephala fortinii. Seedlings having Cladosporium spp. or bacteria as primary symbionts increased the P. fortinii root colonization in 3 weeks by 30% and 50%, respectively, compared to seedlings lacking symbionts. Alternaria primary symbionts had no effect on root colonization by P. fortinii. However, Cladosporium and Alternaria inocula reduced P. fortinii colonization by 75% and 42%, respectively, at 3 weeks, so natural priority effects were not simulated by inoculation with fungi. Bacillus inoculum doubled P. fortinii colonization, but colonization was delayed. Primary symbionts of wheat seed can have strong, natural priority effects dependent on their presence and identity, but inoculation does not always simulate natural effects.