Abstract
Self-incompatibility (SI) protects plants from inbreeding depression due to self-pollination and promotes the outcrossing process to maintain a high degree of heterozygosity during evolution. Corylus is an important woody oil and nut species that shows sporophytic SI (SSI). Yet the molecular mechanism of SI in Corylus remains largely unknown. Here we conducted self- (“Dawei” × “Dawei”) and cross-pollination (“Dawei” × “Liaozhen No. 7”) experiments and then performed an RNA-Seq analysis to investigate the mechanism of pollen–stigma interactions and identify those genes that may be responsible for SSI in Corylus. We uncovered 19,163 up- and 13,314 downregulated genes from the comparison of different pollination treatments. These differentially expressed genes (DEGs) were significantly enriched in plant–pathogen interaction, plant hormone signal transduction, and MAPK signaling pathway–plant. We found many notable genes potentially involved in pollen–stigma interactions and SSI mechanisms, including genes encoding receptor-like protein kinases (RLK), calcium-related genes, disease-resistance genes, and WRKY transcription factors. Four upregulated and five downregulated DEGs were consistently identified in those comparison groups involving self-incompatible pollination, suggesting they had important roles in pollen–pistil interactions. We further identified the S-locus region of the Corylus heterophylla genome based on molecular marker location. This predicted S-locus contains 38 genes, of which 8 share the same functional annotation as the S-locus genes of Corylus avellana: two PIX7 homologous genes (EVM0002129 and EVM0025536), three MIK2 homologous genes (EVM0002422, EVM0005666, and EVM0009820), one aldose 1-epimerase (EVM0002095), one 3-dehydroquinate synthase II (EVM0021283), and one At3g28850 homologous gene (EVM0016149). By characterizing the pistil process during the early postpollination phase via transcriptomic analysis, this study provides new knowledge and lays the foundation for subsequent analyses of pollen-pistil interactions.
Highlights
To maintain genetic diversity, species of flowering plants have evolved diverse sexual reproduction modes, such as dichogamy, monoecy, heterostyly, and self-incompatibility (SI) (Nettancourt, 1997)
A specific Gametophytic SI (GSI) system is found in the Papaveraceae, whereby the programmed cell death of incompatible pollen tubes can be induced by Ca2+-based signaling cascade when the pollen S-receptors match and bind the S-products secreted in the pistil (Franklin-Tong and Franklin, 2003; Thomas and Franklin-Tong, 2004)
We focused on the term “recognition of pollen” (0048544, Figure 4C) containing 71 differentially expressed genes (DEGs), of which 59 are homologous with the G-type lectin S-receptor-like serine/threonine-protein kinase, another 7 DEGs are receptorlike serine/threonine-protein kinase, 3 DEGs are homologous to a putative receptor protein kinase (Zmpk1), 1 DEG is a hypothetical protein FH972_006764 from Carpinus fangiana, and 1 DEG is a DETOXIFICATION 34 (AtDTX34)
Summary
Species of flowering plants have evolved diverse sexual reproduction modes, such as dichogamy, monoecy, heterostyly, and self-incompatibility (SI) (Nettancourt, 1997). Sporophytic SI (SSI) in the Brassicaceae is determined by the interaction between stigma-localized S-receptor kinase (SRK) and pollen coat-specific S-locus cysteine-rich protein (SCR/SP11). They mediate the recognition and rejection of incompatible pollen and lead to failed pollen germination or pollen tubes’ penetration on the stigma surface (Hiscock and McInnis, 2003). The Amaryllidaceae (Sage et al, 1999) and Theacea (Chen et al, 2012) plants exhibit lateacting SI (LSI), in which incompatible-pollinated species fail to bear seeds even though their pollen tubes could reach the ovary (Gibbs, 2014)
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