Young Plastids Research Articles

Plastid alternative oxidase functions as an alternative electron acceptor in Arabidopsis and may protect young plastids from photodamage

Plastid alternative oxidase functions as an alternative electron acceptor in Arabidopsis and may protect young plastids from photodamage

Going green with every developmental stage

Plastid biogenesis involves the import of most of the organelle proteins from the cytoplasm. While plastid homeostasis is ongoing during the lifetime of the organelle, it makes sense that the highest protein-import activity would be found in young, developing plastids during their biosynthesis. This notion was previously demonstrated experimentally1. However, the reasons underlying the high protein trafficking activity of young plastids were not known. The paper by Jarvis et al. provides perhaps the first glimpse into the mechanism of this developmental change in the efficiency of protein import into plastids.

Electron Microscopy of the Reserves Mobilization in Germinating Common Bean (Phaseolus vulgaris L.) Seeds.

The ultrastructure of the reserves mobilization in common bean cotyledon parenchyma cells during germination was investigated using scanning and transmission electron microscopes. At the first germination stage, the ultrastructural chages in cotyledon parenchyma cells were observed with the formation of small vacuoles from the protein bodies (about 5μm in diameter). These vacuoles developed to large vacuoles by fusing with each other. Large vacuoles were surrounded by developed rough endoplasmic reticulum (RER) and high electron-dense vesicles. During the middle germination stage, most of the parenchyma cells were filled with large vacuoles, and these cells had numerous high electron-dense vesicles and small vesicles. By obsevation with a transmission electron microscope, starch grain breakdown rapidly proceeded in amyloplasts from the middle germination stage. Additionally, the young plastids had small starch grains, and few inner membranes were observed during the middle germination stage. By the last germination stage, starch grains became smaller and eventually disappeared, and the young plastids ptogressed into mature chloroplasts. From the observations by scanning and transmission electron microscopes, the developed RER system, developed large vacuoles and numerous small vesicles seem to closely related to the protein breakdown during the germination of common bean seed. In addition, starch breakdown proceeded to the last from the middle germination stage. The newly formed plastids with developed inner membranes were observed in the cotyledon with starch breakdown.

Dynamics of localization and protein composition of plastid nucleoids in light-grown pea seedlings

Localization and protein composition of plastid nucleoids was analyzed in light-grown pea seedlings at various stages of leaf development. In young plastids of unopened leaf buds, nucleoids were abundant and localized in the periphery of plastids, whereas, in mature leaves, chloroplasts contained nucleoids within narrow spaces restricted by thylakoids or grana. The migration of nucleoids into the interior of plastids preceded the formation of grana, and hence, the maturation of the photosynthetic apparatus. The protein composition of nucleoids was considerably different in young plastids and mature chloroplasts. Polypeptides with a molecular mass of 70–100 kDa predominated in the nucleoids of young plastids, whereas polypeptides with molecular mass of 20–30 kDa were abundant in the nucleoids of mature chloroplasts. Immuno-blot analysis with antibodies against the nucleoids of young plastids identified various polypeptides that were significantly more abundant in the nucleoids of young plastids than in the nucleoids of mature chloroplasts. These results demonstrate that plastid nucleoids are subject to dynamic changes in both localization and composition during the normal development of chloroplasts in the light.

Occurrence of unidentified bodies that resemble ?plastid initials? in poplar cells after breaking of dormancy in midwinter

Electron-microscopic studies of plastids in cortical cells of poplar (Populus euramericana cv. gelrica) were carried out to examine whether any structural changes were initiated after breaking of dormancy in midwinter under non-growing conditions. After the breaking of dormancy, ultrastructural changes became evident and the profiles of plastids became heterogeneous. Organelles resembling the “plastid initials” proposed by Muhlenthaler and Frey-Wyssling in 1965 were frequently observed concomitant with changes in the plastid envelope. The formation of “plastid initials” appeared to be initiated by the formation of septa in pre-existing plastids. After this stage, narrow connections appeared between the “initials” and the parent plastids. Approximately 50 days after the breaking of dormancy in late March, further heterogeneity in the profiles of plastids was observed. At this stage, young plastids (plastids without starch granules) were frequently observed and the formation of “plastid initials” was hardly ever observed. These observations suggest that the “plastid initials” may be present for only a limited period in the cortical cells of the poplar and may be the precursors of the proplastids. Similar ultrastructural profiles were found in cortical cells of mulberry and in leaf buds of apple trees, suggesting that such changes in the ultrastructure of plastids are a general feature of perennials.

Developmental Pattern and Structural Organisation of Leaf Chloroplasts in Lepidozamia peroffskyana

The ontogenesis of chloroplasts during leaf development in Lepidozamia peroffskyana Regel is described; this cycad lives in the ground layer of the evergreen subtropical closed forest of south-eastern Australia. Characteristic features in young plastids are long bands of multilayered thylakoids, resembling photosynthetic structures of some green algae and ferns. 'Optional' structures, namely prothylakoid bodies and crystalline inclusions, are present in the pre-grana stages, in relation to the slow rate of leaf development. Mature chloroplasts are characterised by a great number of randomly oriented high grana stacks, a lack of starch, a high ratio of appressed to exposed thylakoids, and a low ratio of Chl a to Chl b; these shade features appear coherent with the natural habitat of Lepidozamia. However, since similar structures have been reported in mature chloroplasts of other cycads living in exposed sites, the existence of a genetically superimposed structural pattern is hypothesised, reflecting a primitive adaptation to weak light conditions of this group of ancient seed plants.

Thylakoid protein kinase activity and associated control of excitation energy distribution during chloroplast biogenesis in wheat

The activity of thylakoid protein kinase and the regulation of excitation energy distribution between photosystems I and II was examined during chloroplast biogenesis in light-grown Triticum aestivum (wheat) leaves. The specific activity of the thylakoid protein kinase decreased some six-fold during development from the young plastids at the base of the 7-d-old leaf to the mature chloroplasts at the leaf tip. Appreciable activity was also detected in plastids isolated from etiolated leaves. In mature chloroplasts the majority of phosphate was incorporated into the Mr=26,000 apo-proteins of the light-harvesting chlorophyll a/b-protein complex (LHCP). However, at early stages of chloroplast development and in the etioplast, the phosphate was predominantly incorporated into a polypeptide of Mr=9,000 dalton. Immature thylakoids, isolated from the base of the leaf, had relatively low concentrations of LHCP and could perform a State 1-State 2 transition, as demonstrated by ATP-induced quenching of photosystem II fluorescence. Analyses of photosystem I and photosystem II fluorescence-induction curves from intact leaf tissue demonstrated that this transition occurs in vivo at early stages of leaf development and, therefore, may play an important role in regulating energy transduction during chloroplast biogenesis.

Chloroplast ontogenesis inHelianthus annuus L.

Research carried out into successively older sunflower leaves has allowed us to follow the different ontogenetic stages leading to the building up of a well-differentiated chloroplast. In all the intrathylakoid compartments of the young plastids there is very electron-dense material whose accumulation causes the dilatation of some thylakoid profiles and, consequently, the forming of roundish bodies. The thylakoid membranes appear as “Lithtly Stained Membranes” which, according to the most recent interpretation, are considered as incompletely organized ones. At a subsequent stage there is an increase of the thylakoid membrane staining along with a decrease of the electron-density of the intrathylakoid compartments that, at the end of the ontogenetic process, appear electrontransparent and flattened. Therefore, the intrathylakoid material appears to have been used up in the forming of well-organized membranes. This particular situation, present in the early ontogenetic stages of the plastids, may be attributed to an initial disequilibrium between the synthesis speed of membranal material and the capability to build up well-organized membranes.

Young plastids in shoot apical cells of a genetic albino strain of Nicotiana (Solanaceae)

The ultrastructure of young plastids in shoot apices of a genetic albino strain of Nicotiana was examined by transmission electron microscopy. Typical young plastids were variable in shape and contained prolamellar bodies in various stages of organization. Thylakoids were few in number, some originating from the prolamellar body while others appeared free in the densely ribosomal stroma. Peripheral areas of the young plastids contained electron-transparent vesicles and tubules which were either in proximity or connected to the inner plastid envelope. Osmiophilic globules occasionally occurred in association with prolamellar bodies. Relatively large starch grains were common; cytoplasmic invaginations into the plastids were less common. Young plastids in the genetic green shoot apex closely resembled those of the genetic albino. These results indicate that organization of large-type grana in plastids of mature phenotypically green, genetic albino leaves occurs beyond the apex stage of germinated seedlings of this strain of Nicotiana.

CHLOROPLAST DEVELOPMENT IN YOUNG SESAME PLANTS

SUMMARYElectron microscopic examination of developing chloroplasts of sesame indicates that plastids in the shoot apex and leaf buds contain a membrane‐bound inclusion with electron‐dense, homogenous contents. The incipient grana of these young plastids have enlarged loculi filled with an electron‐dense material, and the membranes exhibit a ‘negative’ image. The bounding membrane of the inclusion is frequently observed in intimate contact with, and possibly contiguous with, the grana‐fretwork system of the developing chloroplast. As development continues, many of the inclusions are observed to contain partly or wholly crystalline contents, the locular content of electron‐dense material decreases, and the ‘negative’ membrane image disappears. With further maturation, the inclusions become smaller, and eventually disappear.It is suggested that, in accordance with other studies, the membrane‐bound inclusion of developing sesame leaf chloroplasts is a storage centre for thylakoid precursor materials. The ‘negative’ image observed in the incipient grana may also be a reflection of incomplete assembly of the membranes associated with the accumulation of material within the loculus.

Plastid development in a virescens barley mutant and chloroplast microtubules

Development of plastids in a virescens mutant of ‘Gateway' barley was studied during greening of dark-grown seedlings and in seedlings grown in continuous light. Etioplasts of the mutant did not have a crystalline prolamellar body: however, prolamellar bodies were present in chloroplasts of the 4-day-old mutant when grown in continuous light. Upon illumination, the vesicles within etioplasts and young plastids of the mutant joined together to form grana lamellae and these were formed before the stroma lamellae. Formation of the lamellar system in the mutant was slow and the granal stacks were oriented irregularly rather than being all in the same direction within the plastid.The microtubules seen in the stroma of chloroplasts resembled cytoplasmic microtubules, but they were smaller in diameter and they were unlike them in their fixation properties and cold sensitivity. In the normal, microtubules were present in etioplasts and during formation of the lamellar system. However, in the mutant, they were observed only after formation of the lamellar system. From these findings, it was thought that the microtubules might perform a function in orienting the lamellar system of chloroplasts during their development.

Seasonal changes in structure and function of spruce chloroplasts

Seasonal changes of ultrastructure were studied by electron microscopy and by determining the chlorophyll and starch content of the plastids. Young plastids of spruce (Picea abies (L.) Karst.) first function as amyloplasts which store reserve material for the growth of the young needles. Then they develop a normal thylakoid system and produce assimilation starch during the day. In autumn, starch synthesis ceases and the plastids group together. In winter they swell and their membrane system becomes disorganized and reduced. With increasing temperatures in spring the chloroplasts recover, but then they accumulate large amounts of starch, which is not broken down during the night or even during a dark period of several days. As in the previous year they now function as amyloplasts providing reserve material for the new shoot. In summer these plastids are again converted into typical chloroplasts. The same seasonal changes of structure and function could be observed in chloroplasts from 2- or 3-year old needles. Thus these changes represent cyclic processes, which repeat each year. Features of slow aging are superimposed on to these cycles.

Ultrastructural development of plastids in cherry peppers during fruit ripening

The fine structure of plastids in the fruit of cherry peppers was studied during the various stages of ripening. The color change of fruit during ripening is due to the quantitative change of such pigment components as chlorophyll, carotenoid and anthocyanin. Plastid metamorphosis takes place in relationship to the disappearance of chlorophyll and the new formation of carotenoids. The membrane system of plastids degenerates through ripening, although a little differentiation is observed in young plastids of creamy fruits. In parallel wity the color change of fruit from cream to orange, the osmiophilic globules increase in both number and size. As ripening proceeds further, the large osmiophilic globules seem to be gradually transformed into the needle shaped crystalloids of carotenoid pigments which are the remarkable feature of the chromoplasts in red-ripe fruit. The relationship between the development of chromoplasts and the increase and decrease of some pigments is also discussed.

Variable amounts of DNA related to the size of chloroplasts. IV. Three-dimensional arrangement of DNA in fully differentiated chloroplasts of Beta vulgaris L.

ABSTRACT Glutaraldehyde-fixed strips of fully differentiated leaves of Beta vulgaris L. were treated with proteases. With this method it is possible to display DNA-regions in chloroplasts selectively and to reconstruct them from serial sections. As in young plastids, the DNA in mature plastids is also distributed within several regions which are separated rather clearly by thylakoids. The number of such regions depends upon the size of the organelle and thus upon its developmental state. The shape of the regions is, in contrast to less-differentiated plastids, mostly elongated. The individual regions seem to contain unequal quantities of DNA-fibrils. A comparison of our ultrastructural results and biochemical data (DNA-amounts per plastid depending on the size of the organelle; kinetic complexity of chloroplast-DNA of Beta) as well as a comparison of the chloroplast with similar prokaryotic systems (as in bacteria, blue-green algae, and mitochondria) leads to the suggestion that each DNA-containing region can be regarded as a single nucleoid. In addition, each nucleoid already contains several (on average, 4–8) genetic units. Thus the chloroplast of Beta seems genetically polyvalent in at least 2 respects: (i) it is of polyenergidic organization, and (ii) the individual nucleoids can be polyploid to varying degrees.

Structure of plastids of a variegated Betula pubescens mutant

The patterns of variegation in the leaves of Betula pubescens only become evident in the mature leaves. However, the young plastids are also abnormal and pigment analysis does not suggest that the deformation of the plastids is caused by carotenoid deficiency. In mature leaves the mutant plastids are badly deformed. Among the most typical defects are an amoeboid form and the budding of peripheral reticular parts from the mutant plastids. These budding parts can easily be mistaken for mitochondria, but a closer inspection reveals differences in their tubular structures. These buds should therefore only be considered a special type of abnormal plastid.

Electron Microscope Observations of Plastids in the Root Cells of Some Hydrophytes

The root cell plastids and their development were studied in seven genera of hydrophytes: Lemna, Nymphoides, Hydrocharis, Trapa, Eichhornia, Najas, and Hydrilla under the electron microscope.Roots of these hydrophytes with exception of Eichhornia appear slightly greenish and their plastids are composed of grana-lamellar structures. In some of the young plastids, the development of arranged vesicles into lamellar structures has been observed.In the plastids of Eichhdrnia root cells, no grana-lamellar structure has been observed.

Development of Normal and Divergent Plastid Types in Zea Mays

Plastid primordia in the root tips of Zea mays are mitochondria. In the root cap their development into mature plastids can be traced within a single microscopic field. In the region of differentiation of the root proper two distinct types of mitochondria occur, the larger of which develops into plastids while the smaller apparently undergoes no further change. In the stem growing point the primordia are generally minute plastids, that is, hollow spheroids containing starch grains 0.5 μ in diameter in the central vacuoles, although occasionally they are mitochondria. Back from the growing point of the stem in the epicotyl, the primordia quickly develop into plastids. In the procambial cells plastid-like bodies can be observed to fragment, and there is some evidence that these fragments develop into mitochondria. In the vascular bundles many transitional stages can be observed in the course of development of mitochondria into plastids. In the leaves the primordia develop into mitochondria in the phloem, into large starch-containing plastids with prominent pores in the cells immediately surrounding the vascular bundles, and into smaller, apparently denser plastids in the chlorenchymous cells between the bundles. In all aberrant types investigated, the plastid primordia were normal so far as could be observed, the divergent types being due to (1) a delayed development, (2) a stoppage of the development, and (3) a stoppage of development followed by degeneration. In the first albino stock investigated the mode of development in the young plastids suggested an influence exerted by the genetic constitution of the mother plant. A disintegration of the partly developed plastids in the leaves of this stock occurred in all except the earliest-formed cells of the foliage tissue, that is, the tips of the first, second, and third leaves. The second albino stock was evidently an extreme case of virescence. All of the mesophyll tissue of the leaves contained minute plastids. Both albino stocks died while starch was still stored in the epicotyl, indicating that starvation was not the primary cause of death. The divergent plastids in the maternal inheritance stock developed as in normal tissue until they were about half-grown. The first signs of abnormality were not in the plastids themselves but in their number and arrangement within the cells. In tissue of half-grown leaves, a distinct size difference could be observed between the plastids in the light and the dark stripes. In old tissue the plastids in the lighter areas degenerated and disappeared. Mature leaves showed but two kinds of plastids, and contained no intermediate forms on the border-line of the two regions. In the green-yellow lethal stock the plastids degenerated after reaching one-fourth the normal diameter. In "aurea" plastid development was quite normal except for the pigment content. In both the "argentea" and the expallescent-virescent stock the abnormality was due to a checking of plastid development early in the life of the seedling, and to the later normal plastid growth. In general, the abnormal period was between the formation of the tip of the first leaf and the formation of the base of the fifth leaf. Both stocks were very variable.