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PubMed2006

Analysis of Euglena gracilis plastid-targeted proteins reveals different classes of transit sequences.

The plastid of Euglena gracilis was acquired secondarily through an endosymbiotic event with a eukaryotic green alga, and as a result, it is surrounded by a third membrane. This membrane complexity raises the question of how the plastid proteins are targeted to and imported into the organelle. To further explore plastid protein targeting in Euglena, we screened a total of 9,461 expressed sequence tag (EST) clusters (derived from 19,013 individual ESTs) for full-length proteins that are plastid localized to characterize their targeting sequences and to infer potential modes of translocation. Of the 117 proteins identified as being potentially plastid localized whose N-terminal targeting sequences could be inferred, 83 were unique and could be classified into two major groups. Class I proteins have tripartite targeting sequences, comprising (in order) an N-terminal signal sequence, a plastid transit peptide domain, and a predicted stop-transfer sequence. Within this class of proteins are the lumen-targeted proteins (class IB), which have an additional hydrophobic domain similar to a signal sequence and required for further targeting across the thylakoid membrane. Class II proteins lack the putative stop-transfer sequence and possess only a signal sequence at the N terminus, followed by what, in amino acid composition, resembles a plastid transit peptide. Unexpectedly, a few unrelated plastid-targeted proteins exhibit highly similar transit sequences, implying either a recent swapping of these domains or a conserved function. This work represents the most comprehensive description to date of transit peptides in Euglena and hints at the complex routes of plastid targeting that must exist in this organism.

Algal Proteins↗
PubMed2005

Multiple metabolic roles for the nonphotosynthetic plastid of the green alga Prototheca wickerhamii.

The presence of plastids in diverse eukaryotic lineages that have lost the capacity for photosynthesis is well documented. The metabolic functions of such organelles, however, are poorly understood except in the case of the apicoplast in the Apicomplexa, a group of intracellular parasites including Plasmodium falciparum, and the plastid of the green alga Helicosporidium sp., a parasite for which the only host-free stage identified in nature so far is represented by cysts. As a first step in the reconstruction of plastid functions in a nonphotosynthetic, predominantly free-living organism, we searched for expressed sequence tags (ESTs) that correspond to nucleus-encoded plastid-targeted polypeptides in the green alga Prototheca wickerhamii. From 3,856 ESTs, we found that 71 unique sequences (235 ESTs) correspond to different nucleus-encoded putatively plastid-targeted polypeptides. The identified proteins predict that carbohydrate, amino acid, lipid, tetrapyrrole, and isoprenoid metabolism as well as de novo purine biosynthesis and oxidoreductive processes take place in the plastid of P. wickerhamii. Mg-protoporphyrin accumulation and, therefore, plastid-to-nucleus signaling might also occur in this nonphotosynthetic organism, as we identified a transcript which encodes subunit I of Mg-chelatase, the enzyme which catalyzes the first committed step in chlorophyll synthesis. Our data indicate a far more complex metabolism in P. wickerhamii's plastid compared with the metabolic pathways predicted to be located in the apicoplast of P. falciparum and the plastid of Helicosporidium sp.

Algal Proteins↗
PubMed2004

In vivo analysis of interactions between GFP-labeled microfilaments and plastid stromules.

BACKGROUND: Plastid stromules are stroma-filled tubules that extend from the surface of plastids in higher plants and allow the exchange of protein molecules between plastids. These structures are highly dynamic; stromules change both their shape and position in the cytoplasm very rapidly. Previous studies with microfilament inhibitors indicated that stromule shape and movement are dependent on the actin cytoskeleton. To learn more about the nature of the interactions of stromules and the cytoskeleton, we imaged fluorescently-labeled microfilaments and plastids. RESULTS: We have used Arabidopsis thaliana plants expressing green fluorescent protein fused to the human actin-binding protein talin to observe microfilaments and their relationship to stromules in vivo. Microfilaments were observed in close contact with stromules and plastid bodies of hypocotyl epidermis. Time-lapse confocal microscopy revealed that microfilament rearrangements were associated with changes in plastid and stromule morphology and position. We also observed close interactions between mitochondria and stromules in double-labeled cells. CONCLUSION: Our results indicate a correlation between the rearrangement of microfilaments and changes in the shape and position of plastids and stromules. Stromules interact with microfilaments that may also be utilized by mitochondria and other organelles. The interaction of microfilaments and plastids is likely to be mediated by actin-binding proteins on the plastid envelope membrane.

Actin Cytoskeleton↗
PubMed2006

The complete plastid genome sequence of the parasitic green alga Helicosporidium sp. is highly reduced and structured.

BACKGROUND: Loss of photosynthesis has occurred independently in several plant and algal lineages, and represents a major metabolic shift with potential consequences for the content and structure of plastid genomes. To investigate such changes, we sequenced the complete plastid genome of the parasitic, non-photosynthetic green alga, Helicosporidium. RESULTS: The Helicosporidium plastid genome is among the smallest known (37.5 kb), and like other plastids from non-photosynthetic organisms it lacks all genes for proteins that function in photosynthesis. Its reduced size results from more than just loss of genes, however; it has little non-coding DNA, with only one intron and tiny intergenic spaces, and no inverted repeat (no duplicated genes at all). It encodes precisely the minimal complement of tRNAs needed to translate the universal genetic code, and has eliminated all redundant isoacceptors. The Helicosporidium plastid genome is also highly structured, with each half of the circular genome containing nearly all genes on one strand. Helicosporidium is known to be related to trebouxiophyte green algae, but the genome is structured and compacted in a manner more reminiscent of the non-photosynthetic plastids of apicomplexan parasites. CONCLUSION: Helicosporidium contributes significantly to our understanding of the evolution of plastid DNA because it illustrates the highly ordered reduction that occurred following the loss of a major metabolic function. The convergence of plastid genome structure in Helicosporidium and the Apicomplexa raises the interesting possibility that there are common forces that shape plastid genomes, subsequent to the loss of photosynthesis in an organism.

Chlorophyta↗
PubMed2003

Plastid ultrastructure defines the protein import pathway in dinoflagellates.

Eukaryotic cells contain a variety of different compartments that are distinguished by their own particular function and characteristic set of proteins. Protein targeting mechanisms to organelles have an additional layer of complexity in algae, where plastids may be surrounded by three or four membranes instead of two as in higher plants. The mechanism of protein import into dinoflagellates plastids, however, has not been previously described despite the importance of plastid targeting in a group of algae responsible for roughly half the ocean's net primary production. Here, we show how nuclear-encoded proteins enter the triple membrane-bound plastids of the dinoflagellate Gonyaulax. These proteins all contain an N-terminal leader sequence with two distinct hydrophobic regions flanking a region rich in hydroxylated amino acids (S/T). We demonstrate that plastid proteins transit through the Golgi in vivo, that the first hydrophobic region in the leader acts as a typical signal peptide in vitro, and that the S/T-rich region acts as a typical plastid transit sequence in transgenic plants. We also show that the second hydrophobic region acts as a stop transfer sequence so that plastid proteins in Golgi-derived vesicles are integral membrane proteins with a predominant cytoplasmic component. The dinoflagellate mechanism is thus different from that used by the phylogenetically related apicomplexans, and instead, is similar to that of the phylogenetically distant Euglena, whose plastids are also bound by three membranes. We conclude that the protein import mechanism is dictated by plastid ultrastructure rather than by the evolutionary history of the cell.

5' Untranslated Regions↗
PubMed2005

Plastids unleashed: their development and their integration in plant development.

Derived by endosymbiosis from ancestral cyanobacteria, chloroplasts integrated seamlessly into the biology of their host cell. That integration involved a massive transfer of genes to the cell's nucleus, with the modification of pre-existing processes, like plastid division and the operation of the plastid genetic machinery and the emergence of new ones, like the import of proteins translated in the cytoplasm. The uncovering in molecular detail of several of these processes reveals a merger of mechanisms of symbiont and host origin. Chloroplasts acquired roles as part of the biology of land plants by differentiating into a variety of interconvertible plastid forms according to the cell type. How these conversions take place, or how new problems, like the regulation of the plastid population size in cells, have been solved, is barely starting to be understood. Like the whole plant and as a result of the requirements and dangers associated with photosynthetic activity, chloroplasts in particular are under the control of environmental cues. Far from being passive targets of cellular processes, plastids are sources of signals of plastid-nuclear communication, which regulate activities for their own biogenesis. Plastids are also sources of developmental signals, in whose absence tissue architecture or cell differentiation are aberrant, in a cell-autonomous fashion. Over evolutionary time, plastids also contributed many genes for activities that are no longer directly associated with them (like light perception or hormone function). The overall picture is one in which plastids are at both the receiving and the acting ends in plant development, in both ontogenic and evolutionary terms.

Biological Evolution↗
PubMed2006

A truncated hepatitis E virus ORF2 protein expressed in tobacco plastids is immunogenic in mice.

AIM: To cost-effectively express the 23-ku pE2, the most promising subunit vaccine encoded by the E2 fragment comprising of the 3'-portion of hepatitis E virus (HEV) open reading frame 2 (ORF2) in plastids of tobacco (Nicotiana tabacum cv. SR1), to investigate the transgene expression and pE2 accumulation in plastids, and to evaluate the antigenic effect of the plastid-derived pE2 in mice. METHODS: Plastid-targeting vector pRB94-E2 containing the E2 fragment driven by rice psbA promoter was constructed. Upon delivery into tobacco plastids, this construct could initiate homologous recombination in psaB-trnfM and trnG-psbC fragments in plastid genome, and result in transgene inserted between the two fragments. The pRB94-E2 was delivered with a biolistic particle bombardment method, and the plastid-transformed plants were obtained following the regeneration of the bombarded leaf tissues on a spectinomycin-supplemented medium. Transplastomic status of the regenerated plants was confirmed by PCR and Southern blot analysis, transgene expression was investigated by Northern blot analysis, and accumulation of pE2 was measured by ELISA. Furthermore, protein extracts were used to immunize mice, and the presence of the pE2-reactive antibodies in serum samples of the immunized mice was studied by ELISA. RESULTS: Transplastomic lines confirmed by PCR and Southern blot analysis could actively transcribe the E2 mRNA. The pE2 polypeptide was accumulated to a level as high as 13.27 microg/g fresh leaves. The pE2 could stimulate the immunized mice to generate pE2-specific antibodies. CONCLUSION: HEV-E2 fragment can be inserted into the plastid genome and the recombinant pE2 antigen derived is antigenic in mice. Hence, plastids may be a novel source for cost-effective production of HEV vaccines.

Animals↗
PubMed2005

Microarray profiling of plastid gene expression in a unicellular red alga, Cyanidioschyzon merolae.

Plastid genomes of red algae contain more genes than those of green plant lineages, and it is of special interest that four transcription factors derived from ancestral cyanobacteria are encoded therein. However, little is known about transcriptional regulation of the red algal plastid genome. In this study, we constructed a red algal plastid DNA microarray of Cyanidioschyzon merolae covering almost all protein coding genes, and found that plastid genes are differentially activated by illumination. Run-on transcription assays using isolated plastids confirmed that activation takes place at the transcriptional level. In bacteria and plants, sigma factors determine the genes that are to be transcribed, and four plastid sigma factors (Cm_SIG1-4) encoded in the nuclear genome of C. merolae may be responsible for differential gene expression of the plastid genome. We found that transcripts for all Cm_SIG genes accumulated transiently after a shift from dark to light, whereas only the Cm_SIG2 transcript was increased after a shift from low to high light, suggesting that Cm_SIG2 is a sigma factor that responds to high light. Phylogenetic analysis of plastid sigma factors suggested that sigma factors of red and green algal plastids and the group 1 sigma factors of cyanobacteria form a monophyletic group.

Algal Proteins↗
PubMed1992

Gene phylogenies and the endosymbiotic origin of plastids.

The endosymbiotic origin of chloroplasts from cyanobacteria has long been suspected and has been confirmed in recent years by many lines of evidence. Debate now is centered on whether plastids are derived from a single endosymbiotic event or from multiple events involving several photosynthetic prokaryotes and/or eukaryotes. Phylogenetic analysis was undertaken using the inferred amino acid sequences from the genes psbA, rbcL, rbcS, tufA and atpB and a published analysis (Douglas and Turner, 1991) of nucleotide sequences of small subunit (SSU) rRNA to examine the relationships among purple bacteria, cyanobacteria and the plastids of non-green algae (including rhodophytes, chromophytes, a cryptophyte and a glaucophyte), green algae, euglenoids and land plants. Relationships within and among groups are generally consistent among all the trees; for example, prochlorophytes cluster with cyanobacteria (and not with green plastids) in each of the trees and rhodophytes are ancestral to or the sister group of the chromophyte algae. One notable exception is that Euglenophytes are associated with the green plastid lineage in psbA, rbcL, rbcS and tufA trees and with the non-green plastid lineage in SSU rRNA trees. Analysis of psbA, tufA, atpB and SSU rRNA sequences suggests that only a single bacterial endosympbiotic event occurred leading to plastids in the various algal and plant lineages. In contrast, analysis of rbcL and rbcS sequences strongly suggests that plastids are polyphyletic in origin, with plastids being derived independently from both purple bacteria and cyanobacteria. A hypothesis consistent with these discordant trees is that a single bacterial endosymbiotic event occurred leading to all plastids, followed by the lateral transfer of the rbcLS operon from a purple bacterium to a rhodophyte.

Amino Acid Sequence↗
PubMed1999

Tracing the Thread of Plastid Diversity through the Tapestry of Life.

Plastids (chloroplasts) are endosymbiotic organelles derived from previously free-living cyanobacteria. They are dependent on their host cell to the degree that the majority of the proteins expressed in the plastid are encoded in the nuclear genome of the host cell, and it is this genetic dependency that distinguishes organelles from obligate endosymbionts. Reduction in the size of the plastid genome has occurred via gene loss, substitution of nuclear genes, and gene transfer. The plastids of Chlorophyta and plants, Rhodophyta, and Glaucocystophyta are primary plastids (i.e., derived directly from a cyanobacterium). These three lineages may or may not be descended from a single endosymbiotic event. All other lineages of plastids have acquired their plastids by secondary (or tertiary) endosymbiosis, in which a eukaryote already equipped with plastids is preyed upon by a second eukaryote. Considerable gene transfer has occurred among genomes and, at times, between organisms. The eukaryotic crown group Alveolata has a particularly complex history of plastid acquisition.

Journal Article↗
PubMed1992

Plastid DNA in developing maize endosperm : genome structure, methylation, and transcript accumulation patterns.

Amyloplasts in storage organs such as maize (Zea mays L.) endosperm are plastid-derived, nonphotosynthetic, starch-accumulating organelles. This study was initiated to characterize the plastid genome in maize endosperm cells containing differentiated amyloplasts and to determine whether plastid genes are transcribed during the period of amyloplast biogenesis in endosperm development. Four cosmid clones representing the total sequence diversity of the maize plastid genome were hybridized to restriction digests of total cellular DNA from isolated 16-day-old endosperms. The hybridization patterns indicated that the plastid DNA present in endosperm tissue was indistinguishable from that in leaf total DNA. Methylation of maize endosperm amyloplast DNA or leaf chloroplast DNA was not detected with the methylation-sensitive enzymes HpaII and EcoRII. Transcripts homologous to the 17 specific plastid DNA BamHI fragments tested were detectable in total RNA prepared from 16-day-old endosperm tissue. Compared with leaf transcripts, the abundance of endosperm transcripts was substantially lower for transcripts detected by 12 different BamHI fragments and was similar or relatively higher for some transcripts homologous to five BamHi fragments. Transcripts homologous to genes for plastid ribosomal small subunit proteins 7 and 12 on fragments 10 and 23 and to an open reading frame on fragment 14 accumulated primarily as unprocessed or partially processed species in endosperm RNA. The demonstration that maize endosperm cells contain an intact, transcriptionally active plastid genome indicates that plastid genes could contribute to amyloplast biogenesis, although no transcripts unique to endosperm were identified.

Journal Article↗
PubMed1992

Regulation of Plastid Gene Expression during Photooxidative Stress.

We have used the carotenoid biosynthesis inhibitor norflurazon to study the relationship between chloroplast and nuclear gene expression and the mechanisms by which plastid mRNA accumulation is regulated in response to photooxidative stress. By treating 4-week-old hydroponic spinach plants (Spinacea oleracea), we were able to determine the response at two distinct stages of chloroplast development. For all parameters studied, differences were found between the norflurazon-treated young and mature leaves. Young leaves lost essentially all pigment content in the presence of norflurazon, whereas mature leaves retained more than 60% of their chlorophyll and carotenoids. The accumulation of plastid mRNA was determined for several genes, and we found a decrease in mRNA levels for all genes except psbA in herbicide-treated young leaves. For genes such as atpB, psbB, and psaA, there was a corresponding change in the relative level of transcription, but for psbA and rbcL, transcription and mRNA accumulation were uncoupled. In norflurazon-treated mature leaves, all plastid mRNAs except psaA accumulated to normal levels, and transcription levels were either normal or higher than corresponding controls. This led to the conclusion that plastid mRNA accumulation is regulated both transcriptionally and posttranscriptionally in response to photooxidative stress. Although direct photooxidative damage is confined to the plastid and peroxisome, there is a feedback of information controlling the transcription of nuclear-encoded plastid proteins. Considerable evidence has accumulated implicating a "plastid factor" in this control. Therefore, the expression of several nuclear-encoded plastid proteins and the corresponding mRNAs were determined. Although the levels of both the small subunit of ribulose-1,5-bisphosphate carboxylase and the light harvesting chlorophyll a/b-binding protein and corresponding mRNAs were reduced, a 28-kilodalton chloroplast RNA-binding protein and corresponding mRNA were at normal levels in norflurazon-treated plants. Changes in mRNA and protein levels were not the result of a general loss due to photooxidation but rather the result of selective stabilization of certain components. The response of both genomes to photooxidative stress is discussed in terms of the postulated plastid factor.

Journal Article↗
PubMed1997

Exchange of protein molecules through connections between higher plant plastids.

Individual plastids of vascular plants have generally been considered to be discrete autonomous entities that do not directly communicate with each other. However, in transgenic plants in which the plastid stroma was labeled with green fluorescent protein (GFP), thin tubular projections emanated from individual plastids and sometimes connected to other plastids. Flow of GFP between interconnected plastids could be observed when a single plastid or an interconnecting plastid tubule was photobleached and the loss of green fluorescence by both plastids was seen. These tubules allow the exchange of molecules within an interplastid communication system, which may facilitate the coordination of plastid activities.

Amino Acid Sequence↗
PubMed1983

Plastid translation in organello and in vitro during light-induced development in Euglena.

Plastids were isolated from dark-grown Euglena gracilis, from cells exposed to light for 3 h, and from normal, light-grown cells. Plastid protein synthesis was then measured either in organello, by incorporation of [35S]Met into isolated plastids, or in vitro, by programming wheat germ, reticulocyte, or Escherichia coli translation systems with RNA isolated from the plastids. Specific and total rates of protein synthesis in organello increased during light-induced development about 100-fold. In contrast, specific mRNA activity of plastid RNA increased no more than 3-fold, and the estimated total mRNA activity of plastids increased less than 10-fold. Protein synthesis in plastids appears therefore to be subject to strong regulation at a level other than that of transcription superimposed on weak transcriptional regulation. The synthesis of the large subunit of ribulose-bisphosphate carboxylase in particular appears to be under translational control. There is also qualitative evidence for regulation. Most plastid mRNAs, as indicated by translation in vitro, electrophoresis, and fluorography of their translation products, increase with light-induced development as a coordinate set. A second or proplastid set can be detected in RNAs from immature plastids, but not from mature chloroplasts. A few mRNAs appear only later in development, corresponding to a delayed set. Most translation products measured in organello also appear to belong to the coordinate set, a few to the proplastid and delayed sets, and a further few to a transient set, which appears only early in development.

Euglena gracilis↗
PubMed1995

Analyses of ribosomal RNA sequences from glaucocystophyte cyanelles provide new insights into the evolutionary relationships of plastids.

Glaucocystophyte algae (sensu Kies, Berl. Deutsch. Bot. Ges. 92, 1979) contain plastids (cyanelles) that retain the peptidoglycan wall of the putative cyano-bacterial endosymbiont; this and other ultrastructural characters (e.g., unstacked thylakoids, phycobilisomes) have suggested that cyanelles are "primitive" plastids that may represent undeveloped associations between heterotrophic "host" cells (i.e., glaucocystophytes) and cyanobacteria. To test the monophyly of glaucocystophyte cyanelles and to determine their evolutionary relationship to other plastids, complete 16S ribosomal RNA sequences were determined for Cyanophora paradoxa, Glaucocystis nostochinearum, Glaucosphaera vacuolata, and Gloeochaete wittrockiana. Plastid rRNAs were analyzed with the maximum-likelihood, maximum-parsimony, and neighbor-joining methods. The phylogenetic analyses show that the cyanelles of C. paradoxa, G. nostochinearum, and G. wittrockiana form a distinct evolutionary lineage; these cyanelles presumably share a monophyletic origin. The rDNA sequence of G. vacuolata was positioned within the nongreen plastid lineage. This result is consistent with analyses of nuclear-encoded rRNAs that identify G. vacuolata as a rhodophyte and support is removal from the Glaucocystophyta. Results of a global search with the maximum-likelihood method suggest that cyanelles are the first divergence among all plastids; this result is consistent with a single loss of the peptidoglycan wall in plastids after the divergence of the cyanelles. User-defined tree analyses with the maximum-likelihood method indicate, however, that the position of the cyanelles is not stable within the rRNA phylogenies. Both maximum-parsimony and neighbor-joining analyses showed a close evolutionary relationship between cyanelles and non-green plastids; these phylogenetic methods were sensitive to inclusion/exclusion of the G. wittrockiana cyanelle sequence.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Sequence↗
PubMed2000

Engineering of the rpl23 gene cluster to replace the plastid RNA polymerase alpha subunit with the Escherichia coli homologue.

The Escherichia coli RNA polymerase (RNAP) alpha, beta, and beta' core subunits are evolutionarily conserved among bacteria and plastids, and the plastid specificity factors form a functional holoenzyme with the E. coli core. To investigate whether the E. coli core subunits may form a functional hybrid enzyme with the plastid core subunits, we replaced the tobacco plastid RNAP alpha subunit gene (rpoA) with the E. coli alpha subunit gene by targeted gene insertion. The transplastomic tobacco plants look similar to tobacco rpoA deletion mutants in that they are chlorophyll-deficient and nonphotoautotrophic. In addition, they lack transcripts from promoters recognized by the E. coli-like plastid RNA polymerase. These results indicate that evolutionary conservation between the E. coli and plastid RNA polymerase alpha subunits is insufficient to allow substitution of the tobacco alpha subunit with its bacterial counterpart. Interestingly, the cyanobacterial alpha subunits are as different as the E. coli alpha subunits; and therefore it is unlikely that replacement of the tobacco alpha subunit with cyanobacterial alpha subunits would yield a functional enzyme. Replacement of plastid rpoA with the E. coli RNA polymerase alpha subunit gene represents the first engineering of a plastid operon in higher plants.

Amino Acid Sequence↗
PubMed1998

Transcript analysis of the tobacco plastid operon rps2/atpI/H/F/A reveals the existence of a non-consensus type II (NCII) promoter upstream of the atpI coding sequence.

The plastid ATP synthase complex is composed of nine subunits, of which six are encoded in the plastome. The plastid-encoded genes are arranged in two transcriptional units: atpB/E and atpI/H/F/A. We have recently reported that besides containing four -10 and -35 consensus-type (CT) promoters, the atpB/E operon also contains a non-consensus type (NCII) promoter that alone is responsible for its expression in non-photosynthetic plastids. As the functionality of ATP synthase requires expression of all nine subunits, NCII promoter-driven transcription of the atpI/H/F/A operon is to be expected in non-photosynthetic plastids. Therefore, a detailed transcriptional analysis of this operon was carried out using RNA samples from tobacco leaf, cultured cells (BY-2) and seedlings grown on streptomycin and spectinomycin; which contain chloroplasts, translationally active non-photosynthetic plastids and translationally inactive plastids, respectively. We identified a total of three transcription initiation sites (TIS) and four transcript processing sites in the non-coding regions of this operon. Our results also demonstrate that rps2 is co-transcribed with the atpI/H/F/A genes. One of the TIS (-208 atpI) is characterized by an NCII type promoter, while other two primary transcripts (-131 atpI and -384 atpH) initiate from CT promoters. In non-photosynthetic plastids the atpI/H/F/A-specific transcript pool seems to be solely contributed by initiation at the -208 atpI (NCII type) promoter, because transcripts from CT promoters do not accumulate in these plastid types.

Base Sequence↗
PubMed2006

Cytoplasmic channels and their association with plastids in male meiocytes of tobacco, onion and lily.

The ultrastructures of male meiocytes in tobacco, onion and lily were studied to elucidate the interaction between cytoplasmic channels (CCs) and plastids. Before meiosis, the male sporogenous cells had identically thickened cell walls (CWs) traversed by typical plasmodesmata (PDs). After entering meiosis, their CWs became uneven in thickness and 80-500nm aperture CCs were formed. Simultaneously, plastids or plastid-like bodies (PLBs) differing in size and morphology assembled at one or both ends of the CCs. These plastids and PLBs commonly orientated their sharper ends to face the CCs and were co-orientated on the axial line crossing the CC. Such pairs of plastids were often interconnected through the CC by thin (50-100nm) threads emanating from their membranes. Sometimes, plastids or PLBs extended directly from one side of a CW to the other, forming a bridge via the CC. In some cases, several plastids formed bridges between cells via one common CC. This is the first report that clearly demonstrates an intercellular continuum of, or communication between, plastids in male plant meiocytes.

Intercellular Junctions↗