Transcriptomic analysis of pollen-pistil interactions in almond (Prunus dulcis) identifies candidate genes for components of gametophytic self-incompatibility

The cultivated almond exhibits self-incompatibility of the gametophytic type regulated by the S-locus, which is expressed in both the pistil (S-RNase) and pollen (SFB protein). Although almond cultivars are mostly self-incompatible, some cultivars have been found to be self-compatible. For a long time, self-compatibility was unequivocally associated only with the presence of the Sf haplotype. However, recent studies reported the existence of self-incompatible almond cultivars carrying the Sf genotype. This finding suggests the involvement of new, hitherto undiscovered components involved in the almond self-incompatibility system. The aim of this study was to clarify the transcription pattern of the S-genes and to look for additional components of the gametophytic self-incompatibility system in almond. Transcriptome analysis of un-pollinated pistils and incompatible and compatible pollinations of self-compatible and self-incompatible almonds carrying the Sf haplotype was performed using high-throughput RNA sequencing technologies. Among the unigenes, 1357 were shown to be differentially expressed, and gene ontology annotation revealed that they are mostly involved in metabolic processes and binding molecular functions. The expression trend of fourteen representative genes, some of which are putatively involved in the self-(in)compatible response, was confirmed by RT-qPCR. This transcriptomic analysis provides candidate genes for almond components of gametophytic self-incompatibility and could be used as reference for subsequent comparative transcriptomic analyses of pollen and pistil.