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

Chloroplasts with clefts and holes: a reassessment of the chloroplast shape using 3D FE-SEM cellular reconstruction of two species of Chlamydomonas

Chloroplasts are usually considered spheroid organelles, but this is not the only shape of chloroplasts. The chloroplast of Chlamydomonas has been typically described as cup-shaped. However, in old studies, it was also modeled as a complex shape with "perforations" or windows. Here, we reconstructed the cellular architecture of Chlamydomonas reinhardtii and C. applanata using an array tomography system installed on a field emission scanning electron microscope. C. reinhardtii chloroplasts resembled a baseball glove or a cup without a side, featuring numerous large and small holes that may facilitate the transport of metabolites and proteins produced in the Golgi apparatus fitted in the holes. In a lipid-accumulating, high-light condition, the chloroplast volume increased by filling the side cleft with an entire wall. Many accumulated large lipid droplets were accommodated within the chloroplast holes, which could have been considered as "chloroplast lipid droplets." Mitochondrial meshworks surrounded the chloroplast. C. applanata chloroplasts appeared like a folded starfish or a cup with many side clefts and a few holes. There was a single mitochondrion or two that branched in a complex form. Tight contacts of various organelles were also found in C. applanata. These reconstructions illustrate the complexity of chloroplast shape, which necessitates a revised understanding of the localization of lipid droplets and the evolution of chloroplasts: The prevailing image of the spheroid chloroplasts that reminds us of the similarity between chloroplasts and cyanobacteria is no longer tenable.

Principles of amyloplast replication in the ovule integuments of Arabidopsis thaliana

Plastids in vascular plants have various differentiated forms, among which amyloplasts are crucial for starch storage and plant productivity. Despite the vast knowledge of the binary-fission mode of chloroplast division, our understanding of the replication of non-photosynthetic plastids, including amyloplasts, remains limited. Recent studies have suggested the involvement of stromules (stroma-filled tubules) in plastid replication when the division apparatus is faulty. However, details of the underlying mechanism(s) and their relevance to normal processes have yet to be elucidated. Here, we developed a live analysis system for studying amyloplast replication using Arabidopsis (Arabidopsis thaliana) ovule integuments. We showed the full sequence of amyloplast development and demonstrated that wild-type amyloplasts adopt three modes of replication, binary fission, multiple fission, and stromule-mediated fission, via multi-way placement of the FtsZ ring. The minE mutant, with severely inhibited chloroplast division, showed marked heterogeneity in amyloplast size, caused by size-dependent but wild-type modes of plastid fission. The dynamic properties of stromules distinguish the wild-type and minE phenotypes. In minE cells, extended stromules from giant amyloplasts acquired stability, allowing FtsZ ring assembly and constriction, as well as the growth of starch grains therein. Despite hyper-stromule formation, amyloplasts did not proliferate in the ftsZ null mutant. These data clarify the differences between amyloplast and chloroplast replication and demonstrate that the structural plasticity of amyloplasts underlies the multiplicity of their replication processes. Furthermore, this study shows that stromules can generate daughter plastids via the assembly of the FtsZ ring.

Adaptive evolution of chloroplast division mechanisms during plant terrestrialization

Despite extensive research, the origin and evolution of the chloroplast division machinery remain unclear. Here, we employ recently sequenced genomes and transcriptomes of Archaeplastida clades to identify the core components of chloroplast division and reconstruct their evolutionary histories, respectively. Our findings show that complete division ring structures emerged in Charophytes. We find that Glaucophytes experienced strong selection pressure, generating diverse variants adapted to the changing terrestrial environments. By integrating the functions of chloroplast division genes (CDGs) annotated in a workflow developed using large-scale multi-omics data, we further show that dispersed duplications acquire more species-specific functions under stronger selection pressures. Notably, PARC6, a dispersed duplicate CDG, regulates leaf color and plant growth in Solanum lycopersicum, demonstrating neofunctionalization. Our findings provide an integrated perspective on the functional evolution of chloroplast division machinery and highlight the potential of dispersed duplicate genes as the primary source of adaptive evolution of chloroplast division.

Plant environmental sensing relies on specialized plastids

In plants, plastids are thought to interconvert to various forms that are specialized for photosynthesis, starch and oil storage, and diverse pigment accumulation. Post-endosymbiotic evolution has led to adaptations and specializations within plastid populations that align organellar functions with different cellular properties in primary and secondary metabolism, plant growth, organ development, and environmental sensing. Here, we review the plastid biology literature in light of recent reports supporting a class of 'sensory plastids' that are specialized for stress sensing and signaling. Abundant literature indicates that epidermal and vascular parenchyma plastids display shared features of dynamic morphology, proteome composition, and plastid-nuclear interaction that facilitate environmental sensing and signaling. These findings have the potential to reshape our understanding of plastid functional diversification.

The Passage of H2O2 from Chloroplasts to Their Associated Nucleus during Retrograde Signalling: Reflections on the Role of the Nuclear Envelope

The response of chloroplasts to adverse environmental cues, principally increases in light intensity, stimulates chloroplast-to-nucleus retrograde signalling, which leads to the induction of immediate protective responses and longer-term acclimation. Hydrogen peroxide (H2O2), generated during photosynthesis, is proposed to both initiate and transduce a retrograde signal in response to photoinhibitory light intensities. Signalling specificity achieved by chloroplast-sourced H2O2 for signal transduction may be dependent upon the oft-observed close association of a proportion of these organelles with the nucleus. In this review, we consider more precisely the nature of the close association between a chloroplast appressed to the nucleus and the requirement for H2O2 to cross both the double membranes of the chloroplast and nuclear envelopes. Of particular relevance is that the endoplasmic reticulum (ER) has close physical contact with chloroplasts and is contiguous with the nuclear envelope. Therefore, the perinuclear space, which transducing H2O2 molecules would have to cross, may have an oxidising environment the same as the ER lumen. Based on studies in animal cells, the ER lumen may be a significant source of H2O2 in plant cells arising from the oxidative folding of proteins. If this is the case, then there is potential for the ER lumen/perinuclear space to be an important location to modify chloroplast-to-nucleus H2O2 signal transduction and thereby introduce modulation of it by additional different environmental cues. These would include for example, heat stress and pathogen infection, which induce the unfolded protein response characterised by an increased H2O2 level in the ER lumen.

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.

Arabidopsis EGY1 Is Critical for Chloroplast Development in Leaf Epidermal Guard Cells

In Arabidopsis thaliana, the Ethylene-dependent Gravitropism-deficient and Yellow-green 1 (EGY1) gene encodes a thylakoid membrane-localized protease involved in chloroplast development in leaf mesophyll cells. Recently, EGY1 was also found to be crucial for the maintenance of grana in mesophyll chloroplasts. To further explore the function of EGY1 in leaf tissues, we examined the phenotype of chloroplasts in the leaf epidermal guard cells and pavement cells of two 40Ar17+ irradiation-derived mutants, Ar50-33-pg1 and egy1-4. Fluorescence microscopy revealed that fully expanded leaves of both egy1 mutants showed severe chlorophyll deficiency in both epidermal cell types. Guard cells in the egy1 mutant exhibited permanent defects in chloroplast formation during leaf expansion. Labeling of plastids with CaMV35S or Protodermal Factor1 (PDF1) promoter-driven stroma-targeted fluorescent proteins revealed that egy1 guard cells contained the normal number of plastids, but with moderately reduced size, compared with wild-type guard cells. Transmission electron microscopy further revealed that the development of thylakoids was impaired in the plastids of egy1 mutant guard mother cells, guard cells, and pavement cells. Collectively, these observations demonstrate that EGY1 is involved in chloroplast formation in the leaf epidermis and is particularly critical for chloroplast differentiation in guard cells.

Stromules, functional extensions of plastids within the plant cell

Stromules are thin tubular extensions of the plastid compartment surrounded by the envelope membrane. A myriad of functions have been proposed for them, and they likely have multiple roles. Recent work has illuminated aspects of their formation, especially the important of microtubules in their movement and microfilaments in anchoring. A variety of biotic and abiotic stresses result in induction of stromule formation, and in recent years, stromule formation has been strongly implicated as part of the innate immune response. Both stromules and chloroplasts relocate to surround the nucleus when pathogens are sensed, possibly to supply signaling molecules such as reactive oxygen species. In addition to the nucleus, stromules have been observed in close proximity to other compartments such as mitochondria, endoplasmic reticulum, and the plasma membrane, potentially facilitating exchange of substrates and products to carry out important biosynthetic pathways. Much remains to be learned about the identity of proteins and other molecules released from chloroplasts and stromules and how they function in plant development and defense.

Spatial chloroplast-to-nucleus signalling involving plastid-nuclear complexes and stromules

Communication between chloroplasts and the nucleus in response to various environmental cues may be mediated by various small molecules. Signalling specificity could be enhanced if the physical contact between these organelles facilitates direct transfer and prevents interference from other subcellular sources of the same molecules. Plant cells have plastid-nuclear complexes, which provide close physical contact between these organelles. Plastid-nuclear complexes have been proposed to facilitate transfer of photosynthesis-derived H₂O₂ to the nucleus in high light. Stromules (stroma filled tubular plastid extensions) may provide an additional conduit for transfer of a wider range of signalling molecules, including proteins. However, plastid-nuclear complexes and stromules have been hitherto treated as distinct phenomena. We suggest that plastid-nuclear complexes and stromules work in a coordinated manner so that, according to environmental conditions or developmental state, the two modes of connection contribute to varying extents. We hypothesize that this association is dynamic and that there may be a link between plastid-nuclear complexes and the development of stromules. Furthermore, the changes in contact could alter signalling specificity by allowing an extended or different range of signalling molecules to be delivered to the nucleus. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Arabidopsis thaliana Leaf Epidermal Guard Cells: A Model for Studying Chloroplast Proliferation and Partitioning in Plants

The existence of numerous chloroplasts in photosynthetic cells is a general feature of plants. Chloroplast biogenesis and inheritance involve two distinct mechanisms: proliferation of chloroplasts by binary fission and partitioning of chloroplasts into daughter cells during cell division. The mechanism of chloroplast number coordination in a given cell type is a fundamental question. Stomatal guard cells (GCs) in the plant shoot epidermis generally contain several to tens of chloroplasts per cell. Thus far, chloroplast number at the stomatal (GC pair) level has generally been used as a convenient marker for identifying hybrid species or estimating the ploidy level of a given plant tissue. Here, we report that Arabidopsis thaliana leaf GCs represent a useful system for investigating the unexploited aspects of chloroplast number control in plant cells. In contrast to a general notion based on analyses of leaf mesophyll chloroplasts, a small difference was detected in the GC chloroplast number among three Arabidopsis ecotypes (Columbia, Landsberg erecta, and Wassilewskija). Fluorescence microscopy often detected dividing GC chloroplasts with the FtsZ1 ring not only at the early stage of leaf expansion but also at the late stage. Compensatory chloroplast expansion, a phenomenon well documented in leaf mesophyll cells of chloroplast division mutants and transgenic plants, could take place between paired GCs in wild-type leaves. Furthermore, modest chloroplast number per GC as well as symmetric division of guard mother cells for GC formation suggests that Arabidopsis GCs would facilitate the analysis of chloroplast partitioning, based on chloroplast counting at the individual cell level.

Arabidopsis PARC6 Is Critical for Plastid Morphogenesis in Pavement, Trichome, and Guard Cells in Leaf Epidermis

Recently, a recessive Arabidopsis thaliana mutant with abundant stromules in leaf epidermal pavement cells was visually screened and isolated. The gene responsible for this mutant phenotype was identified as PARC6, a chloroplast division site regulator gene. The mutant allele parc6-5 carried two point mutations (G62R and W700stop) at the N- and C-terminal ends of the coding sequence, respectively. Here, we further characterized parc6-5 and other parc6 mutant alleles, and showed that PARC6 plays a critical role in plastid morphogenesis in all cell types of the leaf epidermis: pavement cells, trichome cells, and guard cells. Transient expression of PARC6 transit peptide (TP) fused to the green fluorescent protein (GFP) in plant cells showed that the G62R mutation has no or little effect on the TP activity of the PARC6 N-terminal region. Then, plastid morphology was microscopically analyzed in the leaf epidermis of wild-type (WT) and parc6 mutants (parc6-1, parc6-3, parc6-4 and parc6-5) with the aid of stroma-targeted fluorescent proteins. In parc6 pavement cells, plastids often assumed aberrant grape-like morphology, similar to those in severe plastid division mutants, atminE1, and arc6. In parc6 trichome cells, plastids exhibited extreme grape-like aggregations, without the production of giant plastids (>6 µm diameter), as a general phenotype. In parc6 guard cells, plastids exhibited a variety of abnormal phenotypes, including reduced number, enlarged size, and activated stromules, similar to those in atminE1 and arc6 guard cells. Nevertheless, unlike atminE1 and arc6, parc6 exhibited a low number of mini-chloroplasts (< 2 µm diameter) and rarely produced chloroplast-deficient guard cells. Importantly, unlike parc6, the chloroplast division site mutant arc11 exhibited WT-like plastid phenotypes in trichome and guard cells. Finally, observation of parc6 complementation lines expressing a functional PARC6-GFP protein indicated that PARC6-GFP formed a ring-like structure in both constricting and non-constricting chloroplasts, and that PARC6 dynamically changes its configuration during the process of chloroplast division.

The Arabidopsis arc5 and arc6 mutations differentially affect plastid morphology in pavement and guard cells in the leaf epidermis

Chloroplasts, or photosynthetic plastids, multiply by binary fission, forming a homogeneous population in plant cells. In Arabidopsis thaliana, the division apparatus (or division ring) of mesophyll chloroplasts includes an inner envelope transmembrane protein ARC6, a cytoplasmic dynamin-related protein ARC5 (DRP5B), and members of the FtsZ1 and FtsZ2 families of proteins, which co-assemble in the stromal mid-plastid division ring (FtsZ ring). FtsZ ring placement is controlled by several proteins, including a stromal factor MinE (AtMinE1). During leaf mesophyll development, ARC6 and AtMinE1 are necessary for FtsZ ring formation and thus plastid division initiation, while ARC5 is essential for a later stage of plastid division. Here, we examined plastid morphology in leaf epidermal pavement cells (PCs) and stomatal guard cells (GCs) in the arc5 and arc6 mutants using stroma-targeted fluorescent proteins. The arc5 PC plastids were generally a bit larger than those of the wild type, but most had normal shapes and were division-competent, unlike mutant mesophyll chloroplasts. The arc6 PC plastids were heterogeneous in size and shape, including the formation of giant and mini-plastids, plastids with highly developed stromules, and grape-like plastid clusters, which varied on a cell-by-cell basis. Moreover, unique plastid phenotypes for stomatal GCs were observed in both mutants. The arc5 GCs rarely lacked chlorophyll-bearing plastids (chloroplasts), while they accumulated minute chlorophyll-less plastids, whereas most GCs developed wild type-like chloroplasts. The arc6 GCs produced large chloroplasts and/or chlorophyll-less plastids, as previously observed, but unexpectedly, their chloroplasts/plastids exhibited marked morphological variations. We quantitatively analyzed plastid morphology and partitioning in paired GCs from wild-type, arc5, arc6, and atminE1 plants. Collectively, our results support the notion that ARC5 is dispensable in the process of equal division of epidermal plastids, and indicate that dysfunctions in ARC5 and ARC6 differentially affect plastid replication among mesophyll cells, PCs, and GCs within a single leaf.