Arabidopsis DOK1 encodes a functional dolichol kinase involved in reproduction

Distinctly localized lipid phosphate phosphatases mediate endoplasmic reticulum glycerolipid metabolism in Arabidopsis

Inter-organelle communication is an integral subcellular process in cellular homeostasis. In plants, cellular membrane lipids are synthesized in the plastids and endoplasmic reticulum (ER). However, the crosstalk between these organelles in lipid biosynthesis remains largely unknown. Here, we show that a pair of lipid phosphate phosphatases (LPPs) with differential subcellular localizations is required for ER glycerolipid metabolism in Arabidopsis (Arabidopsis thaliana). LPPα2 and LPPε1, which function as phosphatidic acid phosphatases and thus catalyze the core reaction in glycerolipid metabolism, were differentially localized at ER and chloroplast outer envelopes despite their similar tissue expression pattern. No mutant phenotype was observed in single knockout mutants; however, genetic suppression of these LPPs affected pollen growth and ER phospholipid biosynthesis in mature siliques and seeds with compromised triacylglycerol biosynthesis. Although chloroplast-localized, LPPε1 was localized close to the ER and ER-localized LPPα2. This proximal localization is functionally relevant, because overexpression of chloroplastic LPPε1 enhanced ER phospholipid and triacylglycerol biosynthesis similar to the effect of LPPα2 overexpression in mature siliques and seeds. Thus, ER glycerolipid metabolism requires a chloroplast-localized enzyme in Arabidopsis, representing the importance of inter-organelle communication in membrane lipid homeostasis.

The SEEL motif and members of the MYB-related REVEILLE transcription factor family are important for the expression of LORELEI in the synergid cells of the Arabidopsis female gametophyte

Synergid cells in the micropylar end of the female gametophyte are required for critical cell-cell signaling interactions between the pollen tube and the ovule that precede double fertilization and seed formation in flowering plants. LORELEI (LRE) encodes a putative GPI-anchored protein that is expressed primarily in the synergid cells, and together with FERONIA, a receptor-like kinase, it controls pollen tube reception by the receptive synergid cell. Still, how LRE expression is controlled in synergid cells remains poorly characterized. We identified candidate cis-regulatory elements enriched in LRE and other synergid cell-expressed genes. One of the candidate motifs ('TAATATCT') in the LRE promoter was an uncharacterized variant of the Evening Element motif that we named as the Short Evening Element-like (SEEL) motif. Deletion or point mutations in the SEEL motif of the LRE promoter resulted in decreased reporter expression in synergid cells, demonstrating that the SEEL motif is important for expression of LRE in synergid cells. Additionally, we found that LRE expression is decreased in the loss of function mutants of REVEILLE (RVE) transcription factors, which are clock genes known to bind the SEEL and other closely related motifs. We propose that RVE transcription factors regulate LRE expression in synergid cells by binding to the SEEL motif in the LRE promoter. Identification of cis-regulatory elements and transcription factors involved in the expression of LRE will serve as a foundation to characterize the gene regulatory networks in synergid cells.

Mutations of Kluyveromyces lactis dolichol kinase enhances secretion of recombinant proteins

Although there are similarities in the core steps of the secretion pathway from yeast to higher eukaryotes, significant functional differences exist even among diverse yeast species. Here, we used next-generation sequencing to identify two mutations in the Kluyveromyces lactis KlSEC59 gene, encoding dolichol kinase (DK), which are responsible for an enhanced secretion phenotype in a previously isolated mutant, MD2/1-9. Compared with the temperature-sensitive Saccharomyces cerevisiae sec59-1 mutant, which exhibits reduced N-glycosylation and decreased secretory efficacy, the identified K. lactis DK mutations had fewer effects on glycosylation, as well as on survival at high temperature and cell wall integrity. Moreover, despite some glycosylation defects, double DK mutations (G405S and I419S) in the K. lactis mutant strain demonstrated three times the level of recombinant α-amylase secretion as the wild-type strain. Overexpression of potential suppressors KlMNN10, KlSEL1, KlERG20, KlSRT1, KlRER2, KlCAX4, KlLPP1 and KlDPP1 in the DK-mutant strain restored carboxypeptidase Y glycosylation to different extents and, with the exception of KISRT1, reduced α-amylase secretion to levels observed in wild-type cells. Our results suggest that enhanced secretion related to reduced activity of mutant DK in K. lactis results from mild glycosylation changes that affect activity of other proteins in the secretory pathway.

A pair of nonspecific phospholipases C, NPC2 and NPC6, are involved in gametophyte development and glycerolipid metabolism in Arabidopsis

Phospholipases play crucial roles in plant membrane lipid homeostasis. Nonspecific phospholipase C (NPCs) establish a unique class of phospholipases found only in plants and certain bacteria. Here, we show that two previously uncharacterized NPC isoforms, NPC2 and NPC6, are required for male and female gametophyte development in Arabidopsis. Double mutant plants of npc2-1 npc6-2 could not be retrieved because npc2-1 npc6-2 ovule and pollen development is affected. Genetic complementation, reciprocal crossing and microscope observation of npc2-1/- npc6-2/+ and npc2-1/+ npc6-2/- plants suggest that NPC2 and NPC6 are redundant and are required for normal gametophyte development. Both NPC2 and NPC6 proteins are localized to the plastids. Promoter-GUS assays in transgenic Arabidopsis revealed that NPC2 and NPC6 are preferentially expressed in floral organs rather than in leaves. In vitro enzyme assays showed that NPC2 and NPC6 hydrolyze phosphatidylcholine and phosphatidylethanolamine, but not phosphatidate, being consistent with the reported substrate selectivity of NPCs. The amounts of phosphatidylcholine, phosphatidylethanolamine and phosphatidylglycerol were increased in buds but not in flowers of npc2-1/- npc6-2/+ and npc2-1/+ npc6-2/- plants, presumably due to reduced phospholipid hydrolysis activity in developing flowers. Our results demonstrate that NPC2 and NPC6 play crucial roles in gametogenesis during flower development.

Arabidopsis dolichol kinase AtDOK1 is involved in flowering time control

Dolichols are a class of isoprenoids that consist of highly polymerized and unsaturated long-chain isoprenes. They play crucial roles in protein glycosylation including N-glycosylation, because the oligosaccharide is assembled on a lipid carrier, dolichyl diphosphate. Arabidopsis DOLICHOL KINASE 1, AtDOK1 (At3g45040), encodes a functional dolichol kinase that is involved in plant reproductive processes. The expression of AtDOK1 is limited to highly pluripotent cells although protein glycosylation is thought to be required ubiquitously in the entire plant body. In this study, we further explored AtDOK1 functions by creating leaky knockdown mutants of DOK1. We used a microRNA-mediated gene suppression technique because knockout of DOK1 causes lethality. The DOK1 knockdown mutants showed an early flowering phenotype without any remarkable growth defect in vegetative tissues. Indeed, AtDOK1 was highly expressed in emerging shoot apical meristems as well as inflorescence and floral meristems. A subcellular localization study of DOK1 revealed that DOK1 was localized at the endoplasmic reticulum. Our findings suggest that the endoplasmic reticulum-localized catalytically active DOK1 is highly expressed in the meristems and is involved in the control of flowering time, possibly by post-transcriptional regulation including protein glycosylation.

Endoplasmic Reticulum Stress Response in Arabidopsis Roots

Roots are the frontier of plant body to perceive underground environmental change. Endoplasmic reticulum (ER) stress response represents circumvention of cellular stress caused by various environmental changes; however, a limited number of studies are available on the ER stress responses in roots. Here, we report the tunicamycin (TM) -induced ER stress response in Arabidopsis roots by monitoring expression patterns of immunoglobulin-binding protein 3 (BiP3), a representative marker for the response. Roots promptly responded to the TM-induced ER stress through the induction of similar sets of ER stress-responsive genes. However, not all cells responded uniformly to the TM-induced ER stress in roots, as BiP3 was highly expressed in root tips, an outer layer in elongation zone, and an inner layer in mature zone of roots. We suggest that ER stress response in roots has tissue specificity.

Global DNA methylation variations after short-term heat shock treatment in cultured microspores of Brassica napus cv. Topas

Heat stress can induce the cultured microspores into embryogenesis. In this study, whole genome bisulphite sequencing was employed to study global DNA methylation variations after short-term heat shock (STHS) treatments in cultured microspores of Brassica napus cv. Topas. Our results indicated that treatment on cultured Topas microspores at 32 °C for 6 h triggered DNA hypomethylation, particularly in the CG and CHG contexts. And the total number of T32 (Topas 32 °C for 6 h) vs. T0 (Topas 0 h) differentially methylated region-related genes (DRGs) was approximately two-fold higher than that of T18 (Topas 18 °C for 6 h) vs. T0 DRGs, which suggested that 32 °C might be a more intense external stimulus than 18 °C resulting in more changes in the DNA methylation status of cultured microspores. Additionally, 32 °C treatment for 6 h led to increased CHG differential methylations of transposons (DMTs), which were mainly constituted by overlaps between the hypomethylated differentially methylated regions (hypo-DMRs) and transposon elements (TEs). Further analysis demonstrated that the DRGs and their paralogs exhibited differential methylated/demethylated patterns. To summarize, the present study is the first methylome analysis of cultured microspores in response to STHS and may provide valuable information on the roles of DNA methylation in heat response.

TURAN and EVAN mediate pollen tube reception in Arabidopsis Synergids through protein glycosylation

Pollen tube (PT) reception in flowering plants describes the crosstalk between the male and female gametophytes upon PT arrival at the synergid cells of the ovule. It leads to PT growth arrest, rupture, and sperm cell release, and is thus essential to ensure double fertilization. Here, we describe TURAN (TUN) and EVAN (EVN), two novel members of the PT reception pathway that is mediated by the FERONIA (FER) receptor-like kinase (RLK). Like fer, mutations in these two genes lead to PT overgrowth inside the female gametophyte (FG) without PT rupture. Mapping by next-generation sequencing, cytological analysis of reporter genes, and biochemical assays of glycoproteins in RNAi knockdown mutants revealed both genes to be involved in protein N-glycosylation in the endoplasmic reticulum (ER). TUN encodes a uridine diphosphate (UDP)-glycosyltransferase superfamily protein and EVN a dolichol kinase. In addition to their common role during PT reception in the synergids, both genes have distinct functions in the pollen: whereas EVN is essential for pollen development, TUN is required for PT growth and integrity by affecting the stability of the pollen-specific FER homologs ANXUR1 (ANX1) and ANX2. ANX1- and ANX2-YFP reporters are not expressed in tun pollen grains, but ANX1-YFP is degraded via the ER-associated degradation (ERAD) pathway, likely underlying the anx1/2-like premature PT rupture phenotype of tun mutants. Thus, as in animal sperm–egg interactions, protein glycosylation is essential for the interaction between the female and male gametophytes during PT reception to ensure fertilization and successful reproduction.

Protein glycosylation is essential for gametophyte interactions between the male pollen tube and the female ovule in plants, reminiscent of gamete interactions during fertilization in mammals.

In flowering plants, gametes are produced by the haploid, multicellular male (pollen), and female (embryo sac) gametophytes, which develop within the reproductive organs of the flower. Successful fertilization depends on delivery of the sperm cells to the embryo sac, which is embedded in the ovule, by the pollen tube. Upon arrival of the pollen tube at the opening of the ovule, crosstalk between male and female gametophytes, known as pollen tube reception, ensues; the pollen tube slows or stops its growth, then resumes rapid growth, and finally bursts to release the sperm cells and effect double fertilization. Although several members of the pollen tube reception pathway, including the receptor-like kinase FERONIA, have been identified, the molecular mechanisms underlying this communication process remain unclear. Here, we show that protein N-glycosylation is required for normal pollen tube reception. A mutant screen identified two genes, TURAN and EVAN, which are involved in protein N-glycosylation in the endoplasmic reticulum. Both genes act in the FERONIA-mediated pollen tube reception pathway, which is impaired in these mutants. Thus, in plants, a “dual recognition system,” involving interactions between both protein and glycosyl residues on the surface of male and female gametophytes, appears to be required for successful pollen tube reception, conceptually similar to sperm–egg interactions in mammals, for which N-glycosylation of cell surface proteins also plays an important role.