Reversal of chromoplasts to chloroplasts inBuxus leaves

The ultrastructural changes in plastids ofBuxus sempervirens L. leaves were observed during their seasonal yellowing and regreening. The disintegration of chloroplasts into globular type chromoplasts in yellowing leaves and their direct restoration to functional chloroplasts again in regreening leaves were followed. The results presented an example of recent information indicating the essential sense of the reversible reciprocation of plastid transformation.     

Ultrastructural characterization of the reversible differentiation of chloroplasts in cucumber fruit

The changes in plastid ultrastructure in the pericarp of cucumber (Cucumis sativus L) fruit were studied during fruit yellowing (which accompanied maturation) and regreening. In the course of fruit maturation, the thylakoid system was progressively reduced, and only a small number of membranes remained in the plastids of mature fruit. At the same time, the plastoglobules increased in size, often remaining in close proximity to the degrading thylakoids. In pericarp tissue which turned green again, the thylakoid network in the plastids was gradually reconstituted. Morphological similarities between the plastids in mature and regreening fruit indicated that the chloroplasts in regreened tissue were redifferentiated from the plastids of mature fruit. Reconstitution of the thylakoid system appeared to start from two morphologically distinct types of membranes: from double membranes which resembled thylakoids and from membrane-bound bodies (MBBs). The latter appeared to form thylakoids by two mechanisms: by detachment of extensions from their surfaces and by fragmentation. The plastoglobules remained in the plastids during thylakoid system reconstitution and were often observed in close proximity to developing thylakoids. In the course of chloroplast redifferentiation, several types of membraneous structures were found to be associated with the plastid envelope: (i) vesicles which appeared to separate from the envelope and to fuse subsequently with the developing thylakoids, (ii) tubules, and (iii) double-membrane sheets which appeared asde novo forming thylakoids.     

Regreening of senescent Nicotiana leaves. II. Redifferentiation of plastids

Single senescent leaves attached to decapitated shoots of Nicotiana rustica L. regreened, especially when treated with cytokinin. Regreening caused an increase in leaf thickness, due to cell expansion. Senescent leaf plastids (gerontoplasts) were smaller than green chloroplasts, with degenerated membrane systems and stroma, and larger plastoglobuli. At advanced senescence, micrographs showed disintegrating gerontoplasts, reduced numbers of plastids were counted, and regreening became variable. The redevelopment of grana and stroma in regreening plastids was accelerated by cytokinin. All plastids in regreening leaves were identifiable as redifferentiating gerontoplasts because of their content of plastoglobuli and starch. Immunogold labelling showed significant association of POR with etioplasts in cotyledons, but with mature plastids in regreening leaves. No proplastids or dividing chloroplasts were observed in regreening leaves. Plastids numbers declined during senescence and did not increase again during regreening. It is concluded that the chloroplasts of regreening leaves arose by redifferentiation of gerontoplasts.Keywords: Chloroplasts, cytokinin, Nicotiana, senescence, regreening.      

Studies on the Conversion of Chloroplast to Chromoplast During Fruit Ripening in Solanum pseudo-capsicum var. diflorum

Determination of chlorophyll and carotenoid contents in the ectocarp during fruit ripening in Solanum pseudo-capsicum var. diflorurn (Veil.) Bitter revealed that the changes of fruit colour coincided with the decline of chlorophyll and the increase of carotenoid contents. The conversion of chloroplasts to chromoplasts in the fruit was studied by electron microscopy. The early green fruit was characterized by chloroplasts with a typical grana-intergranal thylakoid structure. At yellow-green fruit stage the thylakoid system was disintegrated and replaced by few non-chlorophyllous single thylakoids, with accumulation of large osmiophilic plastoglobules. The plastids developed as the so-called proplastids. These indicated dedifferentiation of chloroplasts in a ripening fruit. When the fruit reached its yellow stage, numerous large plastoglobules contained in the young chromoplasts frequently showed transitional changes to plastid tubule structure. At first, the center of plastoglobules became semi-translucent. It was believed that the young chromoplast were in an initial state of carotenoid deposition, followed by plastoglobules elongation and tubule protrution from the globules. These tubules were surrounded with an electron dense membranous sheath leaving the core semi-translucent. Concurrently a series of vesicles in different developmental stages appeared from the stroma of the plastid, likely representing a process of formation of numerous small new plastoglobules. In the chromoplasts of a ripe orange-or orange red-colored fruit only numerous tubules and small plastoglobules were present. The plastid tubules increased in number and elongated in length filling the mature chromoplast. Numerous small plastoglobules also increased and distributed in the spaces between tubules. These results indicated that the reconstruction of a mature chromoplast from a dedifferentiated plastid was really a form of redifferentiation, and it might be concluded that the conversion of chloroplast to chromoplast in the fruit of S. pseudo-capsicum var. diflorum, in fact, was a processes of dedifferentiation and redifferentiation.     

珊瑚豆果实成熟过程中叶绿体转化为杂色体的研究

珊瑚豆 (Solanum pseudo- capsicum var.diflorum (Vell.) Bitter)果实成熟过程中 ,果实颜色的变化和叶绿素含量降低及类胡萝卜素含量增长相符合。对果实中叶绿体转化为杂色体进行了电镜观察。早期绿色果实的特点是叶绿体具典型的基粒 -基粒间类囊体结构。在黄绿色果实时期叶绿体类囊体系统解体 ,代之以少数非叶绿素的单个类囊体和积累大的嗜锇的质体小球。质体转变为所谓的原质体。这表明叶绿体在果实成熟中的脱分化过程。当果实达到黄色阶段 ,这些质体所含的质体小球开始从中央形成质体小管的结构。最初质体小球中央变为半透明 ,认为是质体累积胡萝卜素的开始。随着质体小球的延长 ,小管从小球中伸出。这些小管围以电子致密的膜 ,中央是半透明的轴心。与此同时 ,在质体基质中出现一系列发育不同阶段的小泡 ,似乎是形成新的质体小球的过程。在成熟的橙色和橙红色果实中的杂色体中只包含无数小管和小的质体小球。质体小管在数量和长度上增长 ,充满成熟的杂色体。无数质体小球分布在小管之间的空间中。成熟杂色体从脱分化的原质体的重建是真正的再分化过程。可以作出结论 ,珊瑚豆果实叶绿体转化为杂色体实质上是一个脱分化和再分化过程     

Ultrastructure of chromoplasts and other plastids in Crocus sativus L. (Iridaceae)

Saffron (Crocus sativus L. Iridaceae) chromoplasts and other plastids were studied by electron microscope to determine their structure, origin and pigment localization. Plastids from pistils of floral buds and flowers at anthesis, dried and decoloured stigmas, and green and senescent leaves were examined. Results indicated that mature saffron chromoplasts occur in the red parts of stigmas and have a reticulo-tubular structure. They contain a reticulum of tubules and plastoglobules. Tubules formed dilated vesicles mainly while plastoglobules appeared numerous and scattered on the whole chromoplast. Chromoplasts appeared in red stigma of very young floral buds. They originated from amyloplasts, the only plastids occurring in the colourless basal portion of style, as well as in the parenchyma of ovary and corm. Transition forms of plastid as amylo-chromoplast, occur in the yellow parts of stigma and style. Senescent leaves did not show plastids with structure similar to the chromoplast of red stigma. Red pigmented and scented stigmas might cooperate in saffron reproduction by attracting pollinator.     

Structural organization of chloroplast of tomato plants <Emphasis Type="Italic">Solanum lycopersicum</Emphasis> transformed by Fe-containing superoxide dismutase

In this work we analyzed the results of the transformation of tomato plants Solanum lycopersicum by gene FeSOD1 from Arabidopsis thaliana, equipped with the signal sequence for targeting into chloroplasts. PCR analysis showed that the gene was integrated into the genome of several tomato plants that underwent transformation followed by selection in the kanamycin-containing medium. Two lines, provisionally denoted as nos. 6 and 8, were selected from the independent transformants. Line 6 was characterized by a reduced growth rate and altered leaves and line 8, by normal growth and leaves typical for control plants. Both lines showed a significant increase in SOD activity. In line 8 the increase in SOD activity was accompanied by an increase of ascorbate peroxidase activity, and in line 6 this effect was not present. Electron microscopic analysis of parenchymal and guard cells of both lines was performed, with an emphasis on the ultrastructural organization of chloroplasts. It is shown that the chloroplasts of the two transgenic lines differ in the number and size of starch grains and deposited plastoglobules as well as in the organization of lamellae and grana. Taken together, the results indicate that the expression of the introduced gene FeSOD1 has a significant effect on metabolic processes in the plastids. The findings are discussed in relation to the hypothesis about the importance of low concentrations of ROS for the integration of structure and function of chloroplasts.     

Ultrastructural changes associated with June Yellows in strawberry and with leaf yellowing symptoms of viral and genetic origin in Fragaria, Rubus and Ribes

Light and electron microscopy was used to study the ultrastructural effects of June Yellows (JY) in leaves of strawberry. Four cultivars of strawberry, affected to different extents by JY, were compared with healthy (JY-free) cv. Cambridge Favourite and with strawberry infected with strawberry crinkle rhabdovirus, Fragaria vesca infected with strawberry mottle virus (SMotV), raspberry and black currant showing virus-induced yellowing and with strawberry and raspberry showing chaemeric yellow sectors in the leaves. Except for isometric virus-like particles detected in SMotV-infected F. vesca, no virus-like particles or structures of other pathogenic agents were found in any of the tissues examined. Leaf cells of JY-affected strawberry showed severe disruption of chloroplasts and plasmalemma, whorls of membranous vesicles and decreased vacuole size. The extent and severity of these abnormalities increased with increased severity of JY symptoms but, even in leaves with mild JY symptoms, chloroplast abnormalities were obvious. In the most severely affected leaves, the cells lacked discrete vacuoles and extensive hypertrophy was seen in other organelles such as nuclei and mitochondria. Few, if any, ultrastructural abnormalities were observed in virus-infected strawberry or F. vesca, or in chaemeric leaves of strawberry and raspberry. By contrast, in raspberry and black currant with yellowed leaves caused by virus infection, the cells showed enlarged chloroplasts, decreased vacuole size and vesicle formation. However, chloroplast enlargement and disruption in this material seemed due to increased size of starch grains which were largely absent from swollen chloroplasts of JY-affected strawberry. The ultrastructural abnormalities observed in JY-affected strawberry are, therefore, not inconsistent with the possibility that a pathogenic agent may be involved in the condition.     

Over-expression of a pepper plastid lipid-associated protein in tobacco leads to changes in plastid ultrastructure and plant development upon stress

Proteins homologous to fibrillin, a pepper plastid lipid-associated protein involved in carotenoid storage in fruit chromoplasts, have been recently identified in leaf chloroplasts from several species and shown to be induced upon environmental stress. To further investigate the role of the protein, transgenic Nicotiana tabacum plants over-expressing fibrillin using a constitutive promoter were generated. Transgenics grown under standard light intensities (300 micromol photons m-2 sec-1) were found to contain substantial amounts of fibrillin in flowers and leaves. In leaves, the protein was immunolocalized within chloroplasts in both stromal and thylakoid subfractions. No change was noticed in thylakoid structures from transgenics, but chloroplasts contained an increased number of plastoglobules organized in clusters. In petals, leucoplasts were also found to contain more agglutinated plastoglobules. The effects of environmental factors on fibrillin gene expression and protein localization were studied in tobacco leaves. Less fibrillin was present in plants grown under low light intensities, which can be explained by the involvement of a light-dependent splicing step in the control of fibrillin gene expression in leaves. Analysis of protein subfractions from plants subjected to drought or high light showed that both stresses resulted in fibrillin association with thylakoids. Whereas no growth difference between wild-type (WT) and transgenic plants was noticed under low light conditions, transgenics exhibit a longer main stem, enhanced development of lateral stems and accelerated floral development under higher light intensities. These data suggest that fibrillin-related proteins fulfil an important function in plant development in relation to environmental constraints.     

Hyperhydricity in pepper plants regenerated in vitro: involvement of BiP (Binding Protein) and ultrastructural aspects

 Hyperhydricity in regenerated pepper plants was monitored by the induction of the ER-luminal resident protein, as observed by immunoblotting. Immunoblotting of total protein using an anti-soybean BiP serum indicated that the induction and accumulation of an 80-kDa protein was related to BiP (Binding protein), a 78-kDa ER-resident molecular chaperone. The anti-BiP serum cross-reacted with an 80-kDa protein which was significantly induced by hyperhydricity. Based on similar molecular weight and immunological reactivity we concluded that the 80-kDa protein induced in hyperhydric plants is a BiP homologue. The ultrastructural organisation of leaves in non-hyperhydric and hyperhydric pepper (Capsicum annuum L.) plants was investigated with the aim of identifying the subcellular changes associated with this phenomenon. In non-hyperhydric leaves the chloroplasts of the palisade cells had normally developed thylakoids and grana and a low accumulation or absence of starch grains and plastoglobules. In the hyperhydric plants, however, the chloroplasts exhibited thylakoid disorganisation, low grana number, an accumulation of large starch grains and a low accumulation or absence of plastoglobules. Although the structure of mitochondria and peroxisomes did not change in hyperhydric plants, the number of peroxisomes did increase. Received: 23 July 1998 / Revision received: 26 February 1999 / Accepted: 17 March 1999     

The Chloroplast Min System Functions Differentially in Two Specific Nongreen Plastids in Arabidopsis thaliana

The nongreen plastids, such as etioplasts, chromoplasts, etc., as well as chloroplasts, are all derived from proplastids in the meristem. To date, the Min system members in plants have been identified as regulators of FtsZ-ring placement, which are essential for the symmetrical division of chloroplasts. However, the regulation of FtsZ-ring placement in nongreen plastids is poorly understood. In this study, we investigated the division site placement of nongreen plastids by examining the etioplasts as representative in Arabidopsis Min system mutants. Surprisingly, the shape and number of etioplasts in cotyledons of arc3, arc11 and mcd1 mutants were similar to that observed in wild-type plants, whereas arc12 and parc6 mutants exhibited enlarged etioplasts that were reduced in number. In order to examine nongreen plastids in true leaves, we silenced the ALB3 gene in these Min system mutant backgrounds to produce immature chloroplasts without the thylakoidal network using virus induced gene silencing (VIGS). Interestingly, consistent with our observations in etioplasts, enlarged and fewer nongreen plastids were only detected in leaves of parc6 (VIGS-ALB3) and arc12 (VIGS-ALB3) plants. Further, the FtsZ-ring assembled properly at the midpoint in nongreen plastids of arc3, arc11 and mcd1 (VIGS-ALB3) plants, but organized into multiple rings in parc6 (VIGS-ALB3) and presented fragmented filaments in arc12 (VIGS-ALB3) plants, suggesting that division site placement in nongreen plastids requires fewer components of the plant Min system. Taken together, these results suggest that division site placement in nongreen plastids is different from that in chloroplasts.     

Fine Structure and Carotenoid Composition of the Fibrillar Chromoplasts of Asparagus officinalis L.

The spindle-shaped chromoplasts of the ripe fruit of Asparagusofficinalis developed from chloroplasts that lost grana andstroma lamellae and accumulated large-sized plastoglobules,most of which transformed into fibrils of exceptionally largediameter. The carotenoid concentration was high in ripe fruits(2520 µg g^–1 fresh wt) and consisted mainly of capsanthin,ß-carotene and zeazanthin. A. officinalis is onlythe third species reported to contain both capsanthin and capsorubin. The large diameter of both fibrils and their associated plastoglobulesenabled verification that flbrils pass through plastoglobules.Unlike most fibrillar chromoplasts, the plastoglobules remainedassociated with flbrils even in fully-ripe fruit, and a straightline relationship can be established between plastoglobule andfibril diameter. The presence of more than one fibril developingfrom a plastoglobule, and the occasional existence of fibrilstwisted helically around one another, is interpreted to meanthat fibril growth by lateral addition of microfibrils is unlikely. The microfibrillar ultrastructure of Asparagus flbrils was differentto that found for Capsicum fibrils, and these differences areillustrated in two models. These differences, however, do notnecessarily mean that the fibrils of each species are constructedof different protein sub-units, although the bonding betweensub-units is different.     

Osservazioni Citologiche Su Peschi Clorotici E Rinverditi in Seguito A Trattamenti Con Soluzioni Minerali

Abstract

A Cytological study of peach leaves, chlorotic or regreening after treatment with salt solutions. — Whitening and regreening (for treatment with solutions containing Fe) leaves of chlorotic (Fe deficient) peaches were examined both at the optical and the electron microscope. The nuclei, as seen at the optical microscope, and the plastids infrastructures of chlorotic leaves sharply differ from the same structure of the leaves of virused plants. The differentiation of plastids of peach chlorotic leaves is arrested at a very early stage comparable to that of proplastids in etiolated plants before a vescicolar body is formed. In peach plastids a prolamellar body is never formed, not even during the greening of plastids. This is a further confirmation that such a structure, although normal in etiolated and genetically variegated plants, does not represent a fixed stage during differentiation of the lamellar system.

The present observations put into evidence that, even when the formation of lamellae is not immediate, the formation of the prolamellar body is not a necessary condition for the further development of the lamellar apparatus.     

Origin and developmental changes of envelope proteins and translocator activities from plastids of Secale cereale L.

To determine the sites of synthesis of chloroplast-envelope proteins, we have analysed several enzyme and translocator functions ascribed to the envelope membranes, and investigated the envelope polypeptide composition of plastids isolated from 70S ribosome-deficient leaves of rye (Secale cereale L.) generated by growing the plants at a temperature of 32°C. Since the ribosomedeficient plastids are also achlorophyllous in light-grown leaves, not only were chloroplasts from mature, green leaves used for comparison, but also those from yellowing, aged leaves as well as etioplasts from dark-grown leaves raised at a temperature of 22° C. A majority of the plastidenvelope polypeptides appeared to be of cytoplasmic origin. The envelopes of ribosome-deficient plastids possessed ATPase (EC 3.6.1.3) activity; this was not, however, dependent on divalent cations, in contrast to the Mn²⁺- or Mg²⁺-dependent ATPase which is associated with chloroplast envelopes. Adenylate kinase (EC 2.7.4.3) was present in the stromal fraction of ribosome-deficient plastids and the stromal form of this enzyme is, therefore, of cytoplasmic origin. In contrast to previous findings, adenylate kinase was not, however, specifically associated with the chloroplast-envelope membranes, either in rye or in spinach. Measurements of the uptake of l-[¹⁴C]-malate into ribosome-deficient plastids indicated the presence and cytoplasmic origin of the dicarboxylate translocator. Malate uptake into rye etioplasts was, however, low. The phosphate translocator was assayed by the uptake of 3-phospho-[¹⁴C]glycerate. While rapid 3-phosphoglycerate uptake was observed for rye chloroplasts and etioplasts, it was hardly detectable for ribosome-deficient, plastids and rather low for chloroplasts from aged leaves. A polypeptide of M _r approx. 30000 ascribed to the phosphate translocator was greatly reduced in the envelope patterns of ribosome-deficient plastids and of chloroplasts from aged leaves.     

The regreening of nitrogen-deficient Chlorella fusca

Chlorella fusca, strain 211-15, cells degreened in a nitrogen-deficient mineral growth medium in the light for 4–6 weeks were regreened for up to 24 hrs in a nitrogen rich medium that leads to synchronous cell division at 24–26 hrs. Structural changes in the plastid membranes during the regreening period were observed by thin section and freeze-fracture electron microscopy. Nitrogen-deficient plastids were found to have non-appressed lamellae, prolamellar body-like membrane aggregations, and only 2 types of freeze-fracture face. At this time no photosynthetic oxygen evolution could be demonstrated. After 6 hrs regreening the plastid lamellae had fused to form bands of appressed lamellae and the four types of freeze-fracture face, described previously, were visible. At this time photosynthetic oxygen evolution could be demonstrated. After 24 hrs regreening the plastids had an appearance typical of normally grown Chlorella and had commenced to divide. Supporting evidence for these developmental stages is presented from isolated chloroplast particle fractions. An unusual type of cell wall proliferation was observed in the nitrogen-deficient Chlorella cells that resulted in the laying down of several walls, each with a trilaminar component.     

Fine Chloroplast Structure in Cucumber and Pea Leaves Developed under Red Light

Photosynthetic activity, the content of various photosynthetic pigments, and the chloroplast ultrastructure were examined in the leaves of cucumber (Cucumis sativus L.) and pea (Pisum sativum L.) plants of different ages grown under red light (600–700 nm, 100 W/m²). In pea leaves tolerant to red-light irradiation, chloroplast ultrastructure did not essentially change. In the first true leaves of cucumber plants susceptible to red-light irradiation, we observed a considerable increase in the number and size of plastoglobules, the appearance of chloroplasts lacking grana or containing only infrequent grana, and stromal thylakoids. In the upper leaves of 22-day-old cucumber plants, the chloroplast structure was essentially similar to that of the control chloroplasts in white light, and we therefore suppose that these plants have acclimated to red light.     

Photosynthetic and Heterotrophic Ferredoxin Isoproteins Are Colocalized in Fruit Plastids of Tomato

Fruit tissues of tomato (Lycopersicon esculentum Mill.) contain both photosynthetic and heterotrophic ferredoxin (FdA and FdE, respectively) isoproteins, irrespective of their photosynthetic competence, but we did not previously determine whether these proteins were colocalized in the same plastids. In isolated fruit chloroplasts and chromoplasts, both FdA and FdE were detected by immunoblotting. Colocalization of FdA and FdE in the same plastids was demonstrated using double-staining immunofluorescence microscopy. We also found that FdA and FdE were colocalized in fruit chloroplasts and chloroamyloplasts irrespective of sink status of the plastid. Immunoelectron microscopy demonstrated that FdA and FdE were randomly distributed within the plastid stroma. To investigate the significance of the heterotrophic Fd in fruit plastids, Glucose 6-phosphate dehydrogenase (G6PDH) activity was measured in isolated fruit and leaf plastids. Fruit chloroplasts and chromoplasts showed much higher G6PDH activity than did leaf chloroplasts, suggesting that high G6PDH activity is linked with FdE to maintain nonphotosynthetic production of reducing power. This result suggested that, despite their morphological resemblance, fruit chloroplasts are functionally different from their leaf counterparts.     

Light-induced postharvest regreening of pummelo fruit*

Regreening was observed and measured in harvested pummelo fruit stored in the light. At temperatures of 22 - 28°C, regular daylight was sufficient for regreening to occur. The addition of continuous fluorescent light intensified the process. Pre-stored fruit held in darkness at 11°C and non-stored fruit responded to both light conditions in a similar manner. Electron microscopy has shown that globular chromoplasts revert to chloroplasts during regreening. The similarities between regreening processes in preharvest and postharvest fruits are discussed.     

Accumulation of Chlorophyll, Chloroplastic Proteins, and Thylakoid Membranes during Reversion of Chromoplasts to Chloroplasts in Citrus sinensis Epicarp

In vitro culture of pericarp segments from fruit of Citrus sinensis (L.) Osbeck cv Valencia was used to determine the temporal sequence in development of chloroplasts from chromoplasts during regreening of the epicarp. Regreening of chromoplasts closely resembled greening of etioplasts, except that regreening proceeded much more slowly. Chlorophyll, the light-harvesting chlorophyll a/b binding protein of photosystem II, the chlorophyll a binding protein of reaction center P-700 of photosystem I, thylakoid membranes, and adenosine triphosphate synthetase were all detected at very low levels in degreened epicarp. All of these increased in parallel during regreening of the epicarp. Ribulose 1,5-bisphosphate carboxylase (RuBPCase) levels were high in degreened epicarp and declined for the first 10 days of culture before reaccumulating in the regreening segments. Light was necessary for the accumulation of all of the chloroplastic components. A lack of exogenous nitrogen did not prevent the accumulation of any chloroplastic component except Ru-BPCase, although accumulation of the other components was reduced. Sucrose at 150 millimolar in media lacking nitrogen markedly inhibited the accumulation of chlorophyll and light-harvesting chlorophyll a/b-protein.     

ELECTRON MICROSCOPE STUDIES ON THE MORPHOGENESIS OF PLASTIDS. X. ULTRASTRUCTURAL CHANGES OF CHLOROPLASTS IN MORNING GLORY LEAVES EXPOSED TO ETHYLENE

Electron microscopic studies were made on chloroplasts of morning glory leaves exposed continuously to ethylene (6.5 ppm) for 5 days. The leaves gradually became tinged with yellow and finally were shed. The chloroplasts suffered severe injury when plants were exposed to ethylene, i.e., normal thylakoidal membranes collapsed resulting in the formation of macrograna, and then the plastids became filled with many plastoglobules. With such a metamorphosis of chloroplasts, a great deal of phytoferritin was produced in the stroma. Plastid degeneration in this manner was similar, to a certain extent, to that in morning glory leaves exposed to ozone and in leaves undergoing natural senescence. Ethylene exposure rapidly induces senescence of plant organelles, especially chloroplasts.