Differential gene expression analysis identifies a group of defensin-like peptides from Solanum chacoense ovules with in vitro pollen tube attraction activity.

Solanum chacoense is a wild potato species with superior genetic resistance to diseases and pests that has been extensively used for introgression into cultivated potato. One determinant of crossing success between wild and cultivated potato species is the effective ploidy of the parents. However, little is known about whether other, prezygotic level, breeding barriers exist. We hypothesize ovular pollen tube guidance may serve as such a checkpoint. Tests for species-specific pollen tube guidance using semi-in vivo assays suggested a positive correlation between species-specificity and taxonomic distance. RNA-seq of ovules dissected from wild type plants at anthesis (mature ovules) and two days before anthesis (immature ovules), as well as from a frk1 (fertilization-related kinase 1) mutant lacking an embryo sac (ES) identified a list of 284 ES-dependent transcripts highly expressed in mature ovules and poorly expressed in all other samples. Among these are 17 Solanum chacoensecysteine-rich proteins (ScCRPs), considered to be candidates for pollen tube attractants since identified attractants in other species are also CRPs. A group of three cloned and purified ScCRP2 peptides belonging to the DEFL protein family showed moderate levels of in vitro pollen tube attraction activity in functional assays. We conclude that ScCRP2s are good candidates for ovular pollen tube guidance in S. chacoense.