Protein Profiling of Pollen-Pistil Interactions in Almond (Prunus dulcis) and Identification of a Transcription Regulator Presumably Involved in Self-Incompatibility

The cultivated almond displays a gametophytic self-incompatibility system, which avoids self-fertilization, and it is controlled by a multi-allelic locus (S-locus) containing two genes specifically expressed in pistil (S-RNase) and pollen (SFB). Studies on almonds with the same S-haplotype but different phenotype pointed to the existence of unknown components in this system to explain its functioning. The increase of knowledge on this reproductive barrier would allow better management of fruit production and germplasm selection. This work proposes candidates to components of the almond gametophytic self-incompatibility system, by identifying differentially expressed proteins (DEPs) after compatible and incompatible pollen-pistil interactions in almonds with the same S-haplotype but a different incompatibility phenotype using iTRAQ and 2D-nano-LC ESI/MSMS analyses. The protein quantitation analysis revealed 895 DEPs, which were grouped into different functional categories. The largest functional group was "metabolic proteins", followed by "stress resistance and defense proteins", with higher up-regulation after pollination. The identity of certain DEPs, such as Thaumatin, LRR receptors, such as kinase and pathogenesis related protein PR-4, indicated that some pollen-pistil interactions in almond could have the same bases as host-parasite interactions. Furthermore, additional RT-qPCR analysis revealed the differentially expressed transcription regulator GLABROUS1 enhancer-binding protein-like (GEBPL) could be involved in the gametophytic self-incompatibility system in almond.