Deciphering species-specific pollen tube guidance in Solanum

Abstract

Small, secreted cysteine-rich proteins (CRPs)combine a highly stable cysteine spacing,ensuring conservation of their 3D structure andfunction, and hypervariable inter-cysteine blocks, allowing quick evolution of specific recognition domains. Interestingly, several CRPs were shown to control key pollen-pistil interactions in aspecies-specific way. The most emblematicexample is perhaps the LURE defensin-likefamily, controlling directional guidance of pollentubes (PTs) in Torenia and Arabidopsis.We chose wild potatoes (Solanum sect. Petota) asa case study to investigate the impact of rapidCRP divergence in plant speciation. Gathering ~200 close species with overlapping distribution areas, this taxon indeed exhibits strong reproductive isolation. Lab-on-a-chipmicrofluidic experiments carried out on 4 species show that species-preferential PT attraction is a key factor in this isolation. We suspect polymorphic CRPs to control this attraction. High-throughput sequencing technologies were applied to profile the ovule secretome as well as the reproductive transcriptomes of our 4 speciesof interest. To screen out candidate genes, we developped KAPPA, a sequence search algorithm specifically dedicated to CRPs, and obtained a set of 32 defensin-like groups expressed in ovules. Five promising chemoattractant candidates exhibiting (i) ovule-specific expression, (ii) down-regulation in guidance-defective ovules, and (iii) interspecific divergence were selectedfor further characterization. They are currently being investigated with on-gel assays and specific microfluidic devices tailored for Solanum PTs. This study will lead to a better understanding of CRP-mediated PT chemoattraction as one of the major species-specificity checkpoints that mustbe unlocked by pollen tubes in the pistil.