Role of adhesion in the colonization of barley leaves by the yeast Rhodosporidium toruloides

To investigate whether leaf-surface yeasts adhere to the phylloplane in a t wo-phase manner, with weak, nonspecific attachment followed by stronger, ti me-dependent adhesion, we observed adhesion kinetics of the basidiomycete y east Rhodosporidium toruloides to barley. While 50-60% of the cells adhered in short-term assays (up to 3 h), fewer than 10% were adherent after 10 da ys. Ten attachment-minus (Atr) mutants, deficient in phase I attachment, di d not adhere after 5-day incubations, further suggesting a lack of stronger , independent phase II adhesion. Long-term (5 day) adhesion was similar for two isolates of R. toruloides and the ubiquitous leaf-surface fungus Aureo basidium pullulans. Long-term adhesion of R. toruloides to leaves of a waxl ess barley mutant was significantly greater than to leaves of the wild-type cv. Bonus. Application of exogenous nutrients (dilute yeast carbon base) t o resident, wild-type R. toruloides populations resulted in both a rapid re colonization to the apparent carrying capacity of the leaves and an increas e in the total adherent populations. Atr mutants recolonized barley segment s, when supplied with nutrients, after more than 99% of the cells had been removed by agitation. Therefore, adhesion of R. toruloides to leaves was no t required for subsequent colonization of the phylloplane. Overall, these d ata suggest that the frequency of yeast emigration from leaf surfaces, micr obial growth rates, and leaf surface characteristics are major factors infl uencing colonization of leaf surfaces.