Significance: Storage of lipids in oil bodies (OBs) and their subsequent degradation in response to developmental or environmental cues is a common behavior of eukaryotic cells. In Arabidopsis seeds, the major lipase implicated in OB degradation is Sugar-Dependent 1 (SDP1). We report that lipid storage and degradation are severely impaired in seeds of Arabidopsis mutants altered in the retromer function, a multiprotein complex involved in protein trafficking. We show that SDP1 is initially localized on peroxisomes and then migrates to the OB surface during the course of seedling growth. SDP1 translocation occurs through peroxisomal extensions, and a functional retromer allows timely transfer of SDP1 to OBs. Our work provides unanticipated evidence for the role of peroxisome dynamics in interorganelle communication and protein transport.

Early events occurring at the surface of the female organ are critical for plant reproduction, especially in species with a dry stigma. After landing on the stigmatic papilla cells, the pollen hydrates and germinates a tube, which penetrates the cell wall and grows towards the ovules to convey the male gametes to the embryo sac. In self-incompatible species within the Brassicaceae, these processes are blocked when the stigma encounters an incompatible pollen. Based on the generation of self-incompatible Arabidopsis lines and by setting up a live imaging system, we showed that control of pollen hydration has a central role in pollen selectivity. The faster the pollen pumps water from the papilla during an initial period of 10 min, the faster it germinates. Furthermore, we found that the self-incompatibility barriers act to block the proper hydration of incompatible pollen and, when hydration is promoted by high humidity, an additional control prevents pollen tube penetration into the stigmatic wall. In papilla cells, actin bundles focalize at the contact site with the compatible pollen but not with the incompatible pollen, raising the possibility that stigmatic cells react to the mechanical pressure applied by the invading growing tube.

Background: Fertilization in flowering plants depends on the early contact and acceptance of pollen grains by the receptive papilla cells of the stigma. Deciphering the specific transcriptomic response of both pollen and stigmatic cells during their interaction constitutes an important challenge to better our understanding of this cell recognition event.Results: Here we describe a transcriptomic analysis based on single nucleotide polymorphisms (SNPs) present in twoArabidopsis thalianaaccessions, one used as female and the other as male. This strategy allowed us to distinguish 80% of transcripts according to their parental origins. We also developed a tool which predicts male/female specific expression for genes without SNP. We report an unanticipated transcriptional activity triggered in stigma upon incompatible pollination and show that following compatible interaction, components of the pattern-triggered immunity (PTI) pathway are induced on the female side.Conclusions: Our work unveils the molecular signatures of compatible and incompatible pollinations both at the male and female side. We provide invaluable resource and tools to identify potential new molecular players involved in pollen-stigma interaction.

Small non-coding RNAs are central regulators of genome activity and stability. Their regulatory function typically involves sequence similarity with their target sites, but understanding the criteria by which they specifically recognize and regulate their targets across the genome remains a major challenge in the field, especially in the face of the diversity of silencing pathways involved. The dominance hierarchy among self-incompatibility alleles in Brassicaceae is controlled by interactions between a highly diversified set of small non-coding RNAs produced by dominant S-alleles and their corresponding target sites on recessive S-alleles. By controlled crosses, we created numerous heterozygous combinations of S-alleles in Arabidopsis halleri and developed an RT-qPCR assay to compare allele-specific transcript levels for the pollen determinant of self-incompatibility (SCR). This provides the unique opportunity to evaluate the precise base-pairing requirements for effective transcriptional regulation of this target gene. We found strong transcriptional silencing of recessive SCR alleles in all heterozygote combinations examined. A simple threshold model of base-pairing for the sRNA-target interaction captures most of the variation in SCR transcript levels. For a subset of S-alleles, we also measured allele-specific transcript levels of the determinant of pistil specificity (SRK) and found sharply distinct expression dynamics throughout flower development between SCR and SRK. In contrast to SCR, both SRK alleles were expressed at similar levels in the heterozygote genotypes examined, suggesting no transcriptional control of dominance for this gene. We discuss the implications for the evolutionary processes associated with the origin and maintenance of the dominance hierarchy among self-incompatibility alleles.

In Arabidopsis thaliana, successful fertilisation relies on the precise guidance of the pollen tube as it navigates through the female tissues to deliver sperm cells to ovules. While prior research has focused on pistil signals directing pollen tubes towards the ovules, the pollen tube growth within the stigmatic epidermis has received limited attention. Our recent work comparing wild-type pollen tube paths on wild-type and katanin1-5 stigmatic cells, revealed a tight connection between pollen tube directionality and mechanical properties of the invaded stigmatic cell. Given that most mechanical properties of the stigmatic tissue are experimentally challenging to access, we used mathematical modelling to investigate the mechanisms underlying early pollen tube guidance through the papilla cell wall. We found that in ktn1-5, the wild-type pollen tube navigates freely across the curved papilla surface, following curves close to geodesics, whereas the wild-type papilla imposes directional guidance. The order of magnitude analysis of the mechanical forces required for pollen tubes to progress at the papilla surface indicates that both the elongated geometry of the papilla and the difference in rigidity of its cell wall layers combine to efficiently orient the pollen tube towards the papilla base.

Following pollen deposition on the receptive surface of the stigma, pollen germinates a tube that carries male gametes toward the ovule where fertilization occurs. As soon as it emerges from the pollen grain, the pollen tube has to be properly guided through the pistil tissues so as to reach the ovule and ensure double fertilization. Chemical attractants, nutrients as well as receptor kinase-dependent signaling pathways have been implicated in this guidance. Recently, we showed in Arabidopsis that the microtubule severing enzyme KATANIN, by acting both on cortical microtubule (CMT) dynamics and cellulose microfibril (CMF) deposition, conferred particular mechanical properties to the papilla cell wall that act as active guidance factors. Here we confirm the importance of KATANIN and CMT orientation in pollen tube directionality by examining another katanin mutant.