TGD5 is required for normal morphogenesis of non-mesophyll plastids, but not mesophyll chloroplasts, in Arabidopsis.

Stromules are dynamic membrane-bound tubular structures that emanate from plastids. Stromule formation is triggered in response to various stresses and during plant development, suggesting that stromules may have physiological and developmental roles in these processes. Despite the possible biological importance of stromules and their prevalence in green plants, their exact roles and formation mechanisms remain unclear. To explore these issues, we obtained Arabidopsis thaliana mutants with excess stromule formation in the leaf epidermis by microscopy-based screening. Here, we characterized one of these mutants, stromule biogenesis altered 1 (suba1). suba1 forms plastids with severely altered morphology in a variety of non-mesophyll tissues, such as leaf epidermis, hypocotyl epidermis, floral tissues, and pollen grains, but apparently normal leaf mesophyll chloroplasts. The suba1 mutation causes impaired chloroplast pigmentation and altered chloroplast ultrastructure in stomatal guard cells, as well as the aberrant accumulation of lipid droplets and their autophagic engulfment by the vacuole. The causal defective gene in suba1 is TRIGALACTOSYLDIACYLGLYCEROL5 (TGD5), which encodes a protein putatively involved in the endoplasmic reticulum (ER)-to-plastid lipid trafficking required for the ER pathway of thylakoid lipid assembly. These findings suggest that a non-mesophyll-specific mechanism maintains plastid morphology. The distinct mechanisms maintaining plastid morphology in mesophyll versus non-mesophyll plastids might be attributable, at least in part, to the differential contributions of the plastidial and ER pathways of lipid metabolism between mesophyll and non-mesophyll plastids.

Isolation and analysis of a stromule-overproducing Arabidopsis mutant suggest the role of PARC6 in plastid morphology maintenance in the leaf epidermis.

Stromules, or stroma-filled tubules, are thin extensions of the plastid envelope membrane that are most frequently observed in undifferentiated or non-mesophyll cells. The formation of stromules is developmentally regulated and responsive to biotic and abiotic stress; however, the physiological roles and molecular mechanisms of the stromule formation remain enigmatic. Accordingly, we attempted to obtain Arabidopsis thaliana mutants with aberrant stromule biogenesis in the leaf epidermis. Here, we characterize one of the obtained mutants. Plastids in the leaf epidermis of this mutant were giant and pleomorphic, typically having one or more constrictions that indicated arrested plastid division, and usually possessed one or more extremely long stromules, which indicated the deregulation of stromule formation. Genetic mapping, whole-genome resequencing-aided exome analysis, and gene complementation identified PARC6/CDP1/ARC6H, which encodes a vascular plant-specific, chloroplast division site-positioning factor, as the causal gene for the stromule phenotype. Yeast two-hybrid assay and double mutant analysis also identified a possible interaction between PARC6 and MinD1, another known chloroplast division site-positioning factor, during the morphogenesis of leaf epidermal plastids. To the best of our knowledge, PARC6 is the only known A. thaliana chloroplast division factor whose mutations more extensively affect the morphology of plastids in non-mesophyll tissue than in mesophyll tissue. Therefore, the present study demonstrates that PARC6 plays a pivotal role in the morphology maintenance and stromule regulation of non-mesophyll plastids.

A novel function for the sperm adhesion protein IZUMO1 in cell-cell fusion.

Mammalian sperm-egg adhesion depends on the trans-interaction between the sperm-specific type I glycoprotein IZUMO1 and its oocyte-specific GPI-anchored receptor JUNO. However, the mechanisms and proteins (fusogens) that mediate the following step of gamete fusion remain unknown. Using live imaging and content mixing assays in a heterologous system and structure-guided mutagenesis, we unveil an unexpected function for IZUMO1 in cell-to-cell fusion. We show that IZUMO1 alone is sufficient to induce fusion, and that this ability is retained in a mutant unable to bind JUNO. On the other hand, a triple mutation in exposed aromatic residues prevents this fusogenic activity without impairing JUNO interaction. Our findings suggest a second function for IZUMO1 as a unilateral mouse gamete fusogen.

Live imaging-based assay for visualising species-specific interactions in gamete adhesion molecules.

Successful gamete fusion requires species-specific membrane adhesion. However, the interaction of adhesion molecules in gametes is difficult to study in real time through low-throughput microscopic observation. Therefore, we developed a live imaging-based adhesion molecule (LIAM) assay to study gamete adhesion molecule interactions in cultured cells. First, we modified a fusion assay previously established for fusogens introduced into cultured cells, and confirmed that our live imaging technique could visualise cell-cell fusion in the modified fusion assay. Next, instead of fusogen, we introduced adhesion molecules including a mammalian gamete adhesion molecule pair, IZUMO1 and JUNO, and detected their temporal accumulation at the contact interfaces of adjacent cells. Accumulated IZUMO1 or JUNO was partly translocated to the opposite cells as discrete spots; the mutation in amino acids required for their interaction impaired accumulation and translocation. By using the LIAM assay, we investigated the species specificity of IZUMO1 and JUNO of mouse, human, hamster, and pig in all combinations. IZUMO1 and JUNO accumulation and translocation were observed in conspecific, and some interspecific, combinations, suggesting potentially interchangeable combinations of IZUMO1 and JUNO from different species.