A potential role for protein O-fucosylation during pollen-pistil interactions

Protein O-glycosylation regulates diverse developmental and defense processes in plants

Post-translational modifications affect protein functions and play key roles in controlling biological processes. Plants have unique types of O-glycosylation that are different from those of animals and prokaryotes, and they play roles in modulating the functions of secretory proteins and nucleocytoplasmic proteins by regulating transcription and mediating localization and degradation. O-glycosylation is complex because of the dozens of different O-glycan types, the widespread existence of hydroxyproline (Hyp), serine (Ser), and threonine (Thr) residues in proteins attached by O-glycans, and the variable modes of linkages connecting the sugars. O-glycosylation specifically affects development and environmental acclimatization by affecting diverse physiological processes. This review describes recent studies on the detection and functioning of protein O-glycosylation in plants, and provides a framework for the O-glycosylation network that underlies plant development and resistance.

Disruption of pollen tube homogalacturonan synthesis relieves pollen tube penetration defects in the Arabidopsis O-FUCOSYLTRANSFERASE1 mutant

During angiosperm sexual reproduction, pollen tubes must penetrate through multiple cell types in the pistil to mediate successful fertilization. Although this process is highly choreographed and requires complex chemical and mechanical signaling to guide the pollen tube to its destination, aspects of our understanding of pollen tube penetration through the pistil are incomplete. Our previous work demonstrated that disruption of the Arabidopsis thaliana O-FUCOSYLTRANSFERASE1 (OFT1) gene resulted in decreased pollen tube penetration through the stigma-style interface. Here, we demonstrate that second site mutations of Arabidopsis GALACTURONOSYLTRANSFERASE 14 (GAUT14) effectively suppress the phenotype of oft1 mutants, partially restoring silique length, seed set, pollen transmission, and pollen tube penetration deficiencies in navigating the female reproductive tract. These results suggest that disruption of pectic homogalacturonan (HG) synthesis can alleviate the penetrative defects associated with the oft1 mutant and may implicate pectic HG deposition in the process of pollen tube penetration through the stigma-style interface in Arabidopsis. These results also support a model in which OFT1 function directly or indirectly modifies structural features associated with the cell wall, with the loss of oft1 leading to an imbalance in the wall composition that can be compensated for by a reduction in pectic HG deposition.

SPINDLY mediates O-fucosylation of hundreds of proteins and sugar-dependent growth in Arabidopsis

The recent discovery of SPINDLY (SPY)-catalyzed protein O-fucosylation revealed a novel mechanism for regulating nucleocytoplasmic protein functions in plants. Genetic evidence indicates the important roles of SPY in diverse developmental and physiological processes. However, the upstream signal controlling SPY activity and the downstream substrate proteins O-fucosylated by SPY remain largely unknown. Here, we demonstrated that SPY mediates sugar-dependent growth in Arabidopsis (Arabidopsis thaliana). We further identified hundreds of O-fucosylated proteins using lectin affinity chromatography followed by mass spectrometry. All the O-fucosylation events quantified in our proteomic analyses were undetectable or dramatically decreased in the spy mutants, and thus likely catalyzed by SPY. The O-fucosylome includes mostly nuclear and cytosolic proteins. Many O-fucosylated proteins function in essential cellular processes, phytohormone signaling, and developmental programs, consistent with the genetic functions of SPY. The O-fucosylome also includes many proteins modified by O-linked N-acetylglucosamine (O-GlcNAc) and by phosphorylation downstream of the target of rapamycin (TOR) kinase, revealing the convergence of these nutrient signaling pathways on key regulatory functions such as post-transcriptional/translational regulation and phytohormone responses. Our study identified numerous targets of SPY/O-fucosylation and potential nodes of crosstalk among sugar/nutrient signaling pathways, enabling future dissection of the signaling network that mediates sugar regulation of plant growth and development.

Signaling at Physical Barriers during Pollen-Pistil Interactions

In angiosperms, double fertilization requires pollen tubes to transport non-motile sperm to distant egg cells housed in a specialized female structure known as the pistil, mediating the ultimate fusion between male and female gametes. During this journey, the pollen tube encounters numerous physical barriers that must be mechanically circumvented, including the penetration of the stigmatic papillae, style, transmitting tract, and synergid cells as well as the ultimate fusion of sperm cells to the egg or central cell. Additionally, the pollen tube must maintain structural integrity in these compact environments, while responding to positional guidance cues that lead the pollen tube to its destination. Here, we discuss the nature of these physical barriers as well as efforts to genetically and cellularly identify the factors that allow pollen tubes to successfully, specifically, and quickly circumnavigate them.

Reproduction Multitasking: The Male Gametophyte

The gametophyte represents the sexual phase in the alternation of generations in plants; the other, nonsexual phase is the sporophyte. Here, we review the evolutionary origins of the male gametophyte among land plants and, in particular, its ontogenesis in flowering plants. The highly reduced male gametophyte of angiosperm plants is a two- or three-celled pollen grain. Its task is the production of two male gametes and their transport to the female gametophyte, the embryo sac, where double fertilization takes place. We describe two phases of pollen ontogenesis-a developmental phase leading to the differentiation of the male germline and the formation of a mature pollen grain and a functional phase representing the pollen tube growth, beginning with the landing of the pollen grain on the stigma and ending with double fertilization. We highlight recent advances in the complex regulatory mechanisms involved, including posttranscriptional regulation and transcript storage, intracellular metabolic signaling, pollen cell wall structure and synthesis, protein secretion, and phased cell-cell communication within the reproductive tissues.

The morphological and physiological basis of delayed pollination overcoming pre-fertilization cross-incompatibility in Nicotiana

Delayed pollination is widely used to overcome pre-fertilization incompatibility, but its regulatory mechanisms are unclear. When Nicotiana tabacum was cross-pollinated with pollen of N. alata, the incompatibility occurring in the basal 1/4 region of the style (pollinated at anthesis: 0-day-old pistil) was overcome by delayed pollination (of 6-day-old pistil), and the morphological changes and corresponding physiological basis are explored here. The structure and ultrastructure of the pistil were observed under fluorescence microscopy and transmission electron microscopy. Differentially expressed proteins were screened with a monoclonal antibody chip for Nicotiana, and protein expression and distribution were analysed by immunofluorescence. Cellulase and pectinase activities were tested using enzyme-linked immunosorbent assay kits. The style of Nicotiana is solid in the basal region and pollen tubes grow in the extracellular spaces (ECM) of the transmitting tissue (TTS) cells. Seven of the 22 identified proteins were cell wall-associated proteins and were expressed at higher levels during pistil senescence. Cellulase and pectinase activities increased. The TTS cells in the basal 1/4 region of the 0-day-old style were polygonal and tightly arranged, with narrow ECM, but these were oval or partially dissolved in the 6-day-old pistil, leading to wider ECM and richer secretions. The increased expression of cell wall proteins and enhanced enzyme activity during pistil senescence might partially be responsible for the cells becoming oval and the ECM enlarged, providing the morphological basis for delayed pollination overcoming the pre-fertilization incompatibility between N. tabacum and N. alata.

Identification of a role for an E6-like 1 gene in early pollen-stigma interactions in Arabidopsis thaliana

We describe a function for a novel Arabidopsis gene, E6-like 1 (E6L1), that was identified as a highly expressed gene in the stigma and plays a role in early post-pollination stages. In Arabidopsis, successful pollen-stigma interactions are dependent on rapid recognition of compatible pollen by the stigmatic papillae located on the surface of the pistil and the subsequent regulation of pollen hydration and germination, and followed by the growth of pollen tubes through the stigma surface. Here we have described the function of a novel gene, E6-like 1 (E6L1), that was identified through the analysis of transcriptome datasets, as one of highest expressed genes in the stigma, and furthermore, its expression was largely restricted to the stigma and trichomes. The first E6 gene was initially identified as a highly expressed gene during cotton fiber development, and related E6-like predicted proteins are found throughout the Angiosperms. To date, no orthologous genes have been assigned a biological function. Both the Arabidopsis E6L1 and cotton E6 proteins are predicted to be secreted, and this was confirmed using an E6L1:RFP fusion construct. To further investigate E6L1's function, one T-DNA and two independent CRISPR-generated mutants were analyzed for compatible pollen-stigma interactions, and pollen hydration, pollen adhesion, and seed set were mildly impaired for the e6l1 mutants. This work identifies E6L1 as a novel stigmatic factor that plays a role during the early post-pollination stages in Arabidopsis.

A Fruitful Journey: Pollen Tube Navigation from Germination to Fertilization

In flowering plants, pollen tubes undergo tip growth to deliver two nonmotile sperm to the ovule where they fuse with an egg and central cell to achieve double fertilization. This extended journey involves rapid growth and changes in gene activity that manage compatible interactions with at least seven different cell types. Nearly half of the genome is expressed in haploid pollen, which facilitates genetic analysis, even of essential genes. These unique attributes make pollen an ideal system with which to study plant cell-cell interactions, tip growth, cell migration, the modulation of cell wall integrity, and gene expression networks. We highlight the signaling systems required for pollen tube navigation and the potential roles of Ca²⁺ signals. The dynamics of pollen development make sexual reproduction highly sensitive to heat stress. Understanding this vulnerability may generate strategies to improve seed crop yields that are under threat from climate change.

Protein O-fucosylation: structure and function

Fucose is a common terminal modification on protein and lipid glycans. Fucose can also be directly linked to protein via an O-linkage to Serine or Threonine residues located within consensus sequences contained in Epidermal Growth Factor-like (EGF) repeats and Thrombospondin Type 1 Repeats (TSRs). In this context, fucose is added exclusively to properly folded EGF repeats and TSRs by Protein O-fucosyltransferases 1 and 2, respectively. In both cases, the O-linked fucose can also be elongated with other sugars. Here, we describe the biological importance of these O-fucose glycans and molecular mechanisms by which they affect the function of the proteins they modify. O-Fucosylation of EGF repeats modulates the Notch signaling pathway, while O-fucosylation of TSRs is predicted to influence secretion of targets including several extracellular proteases. Recent data show O-fucose glycans mediate their effects by participating in both intermolecular and intramolecular interactions.