Programmed Cell Death in Stigmatic Papilla Cells Is Associated With Senescence-Induced Self-Incompatibility Breakdown in Chinese Cabbage and Radish

Self-incompatibility (SI) is a genetic mechanism flowering plants adopted to reject self-pollen and promote outcrossing. In the Brassicaceae family plants, the stigma tissue plays a key role in self-pollen recognition and rejection. We reported earlier in Chinese cabbage (Brassica rapa) that stigma tissue showed upregulated ethylene responses and programmed cell death (PCD) upon compatible pollination, but not in SI responses. Here, we show that SI is significantly compromised or completely lost in senescent flowers and young flowers of senescent plants. Senescence upregulates senescence-associated genes in B. rapa. Suppressing their expression in young stigmas by antisense oligodeoxyribonucleotide abolishes compatible pollination-triggered PCD and inhibits the growth of compatible pollen tubes. Furthermore, ethylene biosynthesis genes and response genes are upregulated in senescent stigmas, and increasing the level of ethylene or inhibiting its response increases or decreases the expression of senescence-associated genes, respectively. Our results show that senescence causes PCD in stigmatic papilla cells and is associated with the breakdown of SI in Chinese cabbage and in radish.

The molecular signatures of compatible and incompatible pollination

Fertilization in flowering plants depends on the early contact and recognition of pollen grains by the receptive papilla cells of the stigma. To identify the associated molecular pathways, we developed a transcriptomic analysis based on single nucleotide polymorphisms (SNPs) present in twoArabidopsis thalianaaccessions, one used as female and the other as male. We succeeded in distinguishing 80 % of transcripts according to their parental origins and drew up a catalog of genes whose expression is modified after pollen-stigma interaction. Global analysis of our data reveals that pattern-triggered immunity (PTI)-associated transcripts are upregulated after compatible pollination. From our analysis, we predicted the activation of the Mitogen-activated Protein Kinase 3 on the female side after compatible pollination, which we confirmed through expression and mutant analysis. Our work defines the molecular signatures of compatible and incompatible pollination, highlights the active status of incompatible stigmas, and unravels a new MPK3-dependent cell wall feature associated with stigma-pollen interaction.