The Brassica mature pollen and stigma proteomes: preparing to meet

Successful pollination in Brassica brings together the mature pollen grain and stigma papilla, initiating an intricate series of molecular processes meant to eventually enable sperm cell delivery for fertilization and reproduction. At maturity, the pollen and stigma cells have acquired proteomes, comprising the primary molecular effectors required upon their meeting. Knowledge of the roles and global composition of these proteomes in Brassica species is largely lacking. To address this gap, gel-free shotgun proteomics was performed on the mature pollen and stigma of Brassica carinata, a representative of the Brassica family and its many crop species (e.g. Brassica napus, Brassica oleracea and Brassica rapa) that holds considerable potential as a bio-industrial crop. A total of 5608 and 7703 B. carinata mature pollen and stigma proteins were identified, respectively. The pollen and stigma proteomes were found to reflect not only their many common functional and developmental objectives, but also the important differences underlying their cellular specialization. Isobaric tag for relative and absolute quantification (iTRAQ) was exploited in the first analysis of a developing Brassicaceae stigma, and revealed 251 B. carinata proteins that were differentially abundant during stigma maturation, providing insight into proteins involved in the initial phases of pollination. Corresponding pollen and stigma transcriptomes were also generated, highlighting functional divergences between the proteome and transcriptome during different stages of pollen-stigma interaction. This study illustrates the investigative potential of combining the most comprehensive Brassicaceae pollen and stigma proteomes to date with iTRAQ and transcriptome data to provide a unique global perspective of pollen and stigma development and interaction.