iTRAQ-based quantitative proteomic analysis of pistils and anthers from self-incompatible and self-compatible almonds with the S f haplotype

Almond [ Prunus dulcis (Miller) D.A. Webb] exhibits self-incompatibility of the gametophytic type, which is controlled by a locus named S with expression in pistils (S-RNase) and in pollen (SFB protein). Recent studies support the involvement of other unidentified components (named modifier factors) in the incompatibility system of this species. A few proteomic studies have tried to identify modifier factors in Prunus species using comparative proteomics of pollinated pistils. However, there are no studies in which the pollen and un-pollinated pistils from self-incompatible and self-compatible Prunus species are compared. In the present work, IEF and RT-PCR analyses of pistils from two almond selections with the S f haplotype but different incompatibility phenotypes revealed that lack of ribonuclease activity in the self-compatible individual is not due to the absence of S f -RNase transcripts. To identify the proteins differentially expressed in the pistils and anthers of these selections, which could be candidates for modifier factors, isobaric tags for relative and absolute quantitation and 2D-nano-liquid chromatography–electrospray ionisation tandem mass spectrometry were carried out. After a BLAST search against the UniProtKB/SwissProt database, 23 proteins were identified as differentially expressed in pistils and 17 proteins were identified as differentially expressed in anthers. Grouping these proteins into functional categories revealed that most of the pistil proteins had a metabolic function, followed by proteins with a stress resistance and defence function. In the case of anthers, metabolic proteins together with stress resistance and defence proteins were also predominant, although this time in equal proportion. Moreover, the differentially expressed proteins were associated with particular genes, which were located in the Prunus persica Whole Genome version 1.0. Some of the proteins identified here have been linked to pollen development, to pollen dynamics or to pollen–pistil interactions. These results not only provide proteomic profiles of differential expression in mature pistils and anthers, but could also serve as a reference for other comparative proteomic studies of pollinated pistils in almond and in other species with the same incompatibility system.