Accumulation of non-utilizable starch in laticifers of Euphorbia heterophylla and E. myrsinites

Starch biosynthesis and degradation was studied in seedlings and mature plants of Euphorbia heterophylla L. and E. myrsinites L. Mature embryos, which lack starch grains in the non-articulated laticifers, develop into seedlings that accumulate starch rapidly when grown either in the light or the dark. Starch accumulation in laticifers of dark-grown seedlings was ca. 47 and 43% of total starch in light-grown controls in E. heterophylla and E. myrsinites, respectively. In light-grown seedlings, starch was present in laticifers as well as parenchyma of stems and leaves, whereas in dark-grown seedlings starch synthesis was almost exclusively limited to laticifers. In 7-month-old plants placed into total darkness, the starch in chyma was depleted within 6 d, whereas starch in laticifers was not mobilized. The starch content of latex in plants during development of floral primordia, flowering, and subsequent fruit formation remained rather constant. The results indicate that laticifers in seedlings divert embryonal storage reserves to synthesize starch even under stress conditions (darkness) in contrast to other cells, and that starch accumulated in laticifers does not serve as a metabolic reserve. The laticifer in Euphorbia functions in the accumulation and storage of secondary metabolites yet retains the capacity to produce, but not utilize starch, a primary metabolite.     

LATEX AND LATICIFER STARCH CONTENT OF DEVELOPING LEAVES OF EUPHORBIA PULCHERRIMA

Laticifer starch accumulation was compared to laticifer growth for developing leaves of Euphorbia pulcherrima Willd. (poinsettia). Measurements of the laticifer-specific triterpenol, cycloartenol, in latex and in whole leaf extracts were used to calculate the total latex volume in leaves of different developmental stages. Latex volume and starch concentration in the latex were used to determine total laticifer starch and to compare laticifer growth and starch synthesis. Young leaves contained the highest latex and laticifer starch contents on dry wt and leaf area bases. In older expanding leaves, laticifer growth produced an increase in total latex volume accompanied by an increase in total laticifer starch. Laticifer growth and starch accumulation stopped upon cessation of leaf expansion. Starch concentration was similar in latex from all leaves, but differed between plant organs. Thus, laticifer starch accumulation correlated with laticifer growth, but mobilization of the starch out of the laticifer was not observed in old or senescent leaves. This is evidence that laticifer starch grains function within the laticifer independently of degradation or export to other cell types.     

Ultrastructure and development of nonarticulated laticifers in seedlings ofEuphorbia maculata L.

The ultrastructure of nonarticulated laticifers in the seedlings ofEuphorbia maculata was studied at various developmental stages. The apical regions of the seedling laticifers growing intrusively contained large nuclei with mainly euchromatin and dense cytoplasm possessing various and many organelles such as rich ribosomes, several small vacuoles, giant mitochondria with dense matrices, rough endoplasmic reticulum, dictyosomes, and proplastids. This result suggested that the apical regions of laticifers were metabolically very active. Laticifers in seedlings at the first-leaf developmental stage did not contain latex particle. In seedlings at second-leaf growth stage, the laticifer cells contained numerous and elongated small vacuoles. These vacuoles appeared to arise by dilation of the endoplasmic reticulum and frequently possessed osmiophilic or electron-dense latex particles. The small vacuoles fused with the large vacuole occupying the central portion of the subapical region of laticifers, and then the latex particles were released into the large central vacuole. The latex particles varied in size and were lightly or darkly stained. Proplastids with a dense matrix and a few osmiophilic plastoglobuli were filled with an elongated starch grain and thus were transformed into amyloplasts. Latex particles were initially produced in the laticifers after seedlings had developed their second young leaves. In seedlings at forth-leaf stage, latex particles with an alveolated rim were found in the laticifers.     

Differentiation of Non-articulated Laticifers in Poinsettia (Euphorbia pulcherrima Willd.)

Differentiation of non-articulated laticifers in poinsettia(Euphorbia pulcherrima Willd.) was studied ultra-structurally.Growing laticifers show: (1) a multinucleate apical region containingabundant ribosomes but few other differentiated organelles and(2) a sub-apical zone where the cytoplasm is dominated by vacuolesof diverse morphology with latex particles. These particlesappear first within narrow tubular vacuoles developed especiallyin the peripheral cytoplasm. During vacuolation of the laticifer,portions of cytoplasm, including some of the nuclei, becomeisolated by the enlarging and fusing vacuoles; eventually thesebecome lysed, except the latex particles which remain in thecentral vacuole. During differentiation of a laticifer branch,the cytoplasm contains the usual organelles, including a fewmicrobodies and coated vesicles. The plastids that lie withinthe peripheral cytoplasm differentiate into amyloplasts witha single elongated starch grain. Towards the end of differentiationthe cytoplasm becomes restricted to a thin parietal layer, withthe remaining organelles reduced or degenerate, surroundinga central vacuole filled with latex particles. Euphorbia pulcherrima Willd, poinsettia, ultrastructure, differentiation, laticifers     

Localization of starch in shoot apices of vegetative and photoperiodically induced plants ofChenopodium rubrutn

Starch was determined by means of IKI reaction in shoot apices ofChenopodium rubrum plants induced to flowering by two short days and in non-induced plants. Small starch grains were already observed in the meristematic cells at an age of four days after sowing. Larger grains were found in the subapical region of the apex. Heterogeneity increases during further growth of the plants in induced, as well as in non-induced vegetative plants. Starch disappears from the cells potentially giving rise to axillary buds, while the number and size of starch grains increase in cells from which leaf primordia will be formed. This metabolic specifity of leaf and bud primordia is preserved during morphological differentiation and applies to vegetative, as well as to prefloral apices of photoperiodically induced plants. The amount of starch in the different regions of the apex is linked rather with organogenesis than with the quantitative growth in the apex.     

Laticifers in Camptotheca acuminata Decne: distribution and structure

Summary. In this paper, a system of laticifers in Camptotheca acuminata Decne (Nyssaceae) is described. Laticifers were already present in the leaf primordia of the shoot apex. In the mature leaves, laticifers were found in the midrib and in the larger veins, both in the parenchymatic region delimited by vascular bundles and in the cortex just external to the phloem. In the stem, laticifers were present in both the primary and secondary body, running parallel to the longitudinal axis. They were located in the pith and in the cortex proximal to the phloem. No laticifers were found in the roots. The histochemical analyses indicated that the main compounds accumulated in laticifers were phenols. Neutral lipids and fatty acids were also present. Ultrastructural observations showed osmiophilic globules both in the vacuoles and in the peripheral regions of the cytoplasm of the laticifer cells. Plastids were present, although altered, with some parallel membranes and lacking starch grains. The discovery in C. acuminata of a laticifer system, which had never been described for the order Cornales, could be of taxonomic value, also considering that this order has traditionally represented one of the most problematic groups of flowering plants. Correspondence and reprints: Dipartimento di Biologia Vegetale, Università “La Sapienza”, Piazzale Aldo Moro 5, 00185 Rome, Italy.     

Characteristics of Starch Grains Isolated from Mature Pepper Leaves Grown under Different Irradiances and Daylengths

The size, shape and number of starch grains have been determinedin mature pepper leaves taken from plants grown under definedconditions of daylength and irradiance. Starch grains were 0.2–7.0 µm diameter and 02–1.5µm in thickness. Grain diameter was positively relatedto daylength and the number of grains per unit leaf area inverselyrelated to daylength. Mean grain diameter was also positivelyrelated to leaf area. Analysis of starch grains from leaves having a wide range ofstarch contents showed that grain diameter was linearly relatedto leaf starch content. However, mean diameter only doubledwith a 10-fold increase in starch content. The number of grainsincreased from approximately 5 ? 10¹⁰ m^–2 of leaf to over200 ? 10¹⁰ m^–2 with increasing starch content. The totalsurface area of grains increased from less than 1.0 m² m^–2leaf to over 20 m² m^–2 leaf. Leaf starch grain shape and size are compatible with both efficientstorage as disc-shaped chloroplasts and the maintenance of hightotal grain surface area by increasing grain number more thandiameter. Possible mechanisms for the control of grain initiation,growth and degradation are suggested. Key words: Starch grains, size, shape, pepper leaves     

Starch storage in the stems of wheat plants: localization and temporal changes

Background and Aims

Carbohydrate temporarily accumulates in wheat stems during the early reproductive growth phase, predominantly as water soluble carbohydrate (WSC), and is subsequently remobilized during grain filling. Starch has also been reported as a minor storage carbohydrate component in wheat stems, but the details are lacking.

Methods

The accumulation and localization of starch in wheat stem and leaf sheath tissue over a developmental period from 6 d before anthesis to 35 d after anthesis was investigated.

Key Results

The region of the peduncle enclosed by the flag-leaf sheath, and the penultimate internode were the main tissues identified as containing starch, in which the starch grains localized to the storage parenchyma cells. In contrast, the exposed peduncle lacked starch grains. Starch grains were also found in the flag-leaf and second-leaf sheath. Plants grown in low-nitrogen conditions exhibited increased storage of both starch and WSC compared with plants grown in high-nitrogen supply.

Conclusions

The major accumulation and decrease of starch occurred temporally independently to that for WSC, suggesting a different functional role for starch in wheat stems. Starch reutilization concomitant with peduncle growth, and the early development of the reproductive structures, suggested a role in provision of energy and/or carbon scaffolds for these growth processes.Key words: Carbohydrate partitioning, peduncle, starch, wheat stem, storage parenchyma, Triticum aestivum     

Anther starch variations in Lilium during pollen development

Starch was cytologically localized and biochemically assayed in different anther cell layers of Lilium cv. Enchantment during pollen development and its presence was correlated with anther growth. Two phases could be distinguished: the first, the growth phase, extends from the beginning of meiosis to the vacuolated microspore stage and corresponds to maximum increase in anther size and weight. During this period, microspores lack amyloplasts and starch is degraded in the outer staminal wall layers. The tapetum does not contain starch reserves but accumulates a PAS-positive substance in its vacuole. The second phase, the maturation phase, begins with the late vacuolated microspore stage and lasts until pollen maturation. Anther growth is slowed during this phase. A wave of amylogenesis/ amylolysis occurs first in the late vacuolated-microspores and young pollen grains and, next, in the staminal envelopes. In the pollen grain, the cytoplasm of the vegetative cell is filled with starch, but amyloplasts are not detected in the generative cell. When pollen grains ripen, amylaceous reserves are replaced with lipids. In the staminal envelopes, the second amylogenesis is particularly evident in the endothecium and the middle layers; the peak of starch is reached at the young bicellular pollen grain stage; starch disappears from the anther wall early during the maturation phase. The wave of amylogenesis/amylolysis occurring in the staminal envelopes during the maturation phase is peculiar to Lilium. It is interpreted as a sudden increase in carbohydrate level caused by lower anther needs when the growth is completed. Staminal envelopes may act as a physiological buffer and regulate soluble sugar level in the anther. Stages of anther growth correlate with starch content variations and this suggests that during the growth phase, products of starch hydrolysis in the staminal envelopes may be consumed partly by anther cell layers and partly by microspores.     

A COMPARISON OF ALPHA-AMYLASES FROM THE LATEX OF THREE SELECTED SPECIES OF EUPHORBIA (EUPHORBIACEAE)

A technique for the partial purification of α-amylases from latex of Euphorbia heterophylla, E. marginata, and E. tirucalli is described. The enzymes were found to be similar to other higher plant amylases using the criteria of molecular weight, pH characteristics, kinetics, number of isozymes, and blue value-reducing value patterns. Carbohydrases other than α-amylases were not detectable in latex. The amylases were employed to examine their capacity to digest latex starch grains which are common components of the laticiferous cell in this genus. Laticifer starch grains are not susceptible to in vitro amylolysis. Removal of the starch grain membrane with Triton X-100, damaging the grain, or treating the grains with α-amylases from diverse biological sources had little effect upon hydrolysis. Grains incubated with pullulanase followed by α-amylase caused a slight but significant increase in hydrolysis of raw laticifer starch grains. These studies indicate that the nonarticulated laticifer in Euphorbia is a cul de sac for certain primary and secondary metabolic products and that the indigestible and morphologically complex starch grains in the latex have evolved to function in a secondary role within the laticifer.     

EVOLUTION OF THE LATICIFER IN EUPHORBIA AS INTERPRETED FROM STARCH GRAIN MORPHOLOGY

The morphology of plastid starch grains from several succulent and nonsucculent species of Euphorbia was examined in parenchyma and in non-articulated laticifiers. Several classes of grains were identified: small oblong-round, rod, somewhat osteoid, osteoid, lobed osteoid, discoid, and round grains. Parenchyma possessed only small oblong-round grains, whereas grains of different morphology were present in laticifers. Different species of this genus can be characterized by the morphology of the large, or mature, starch grains present in the laticifer. The rod-shaped grain, which was somewhat wider at the midregion than at the ends, was present in several nonsucculent forms, E. terracina, E. pulcherrima and E. heterophylla. The somewhat osteoid grain was represented by the succulent species E. viguieri, E. milii, and E. mauritanica, where the mature grains developed somewhat enlarged ends. Grains with much enlarged ends were represented in the succulent species of E. abyssinica, E. pseudocactus, and E. tirucalli. Alteration of the pattern of starch deposition in which several lobes were formed at the ends of the grain has given rise to a lobed osteoid class (E. inconstantia). Lobes also may be formed with greater frequency along the midregion of the grain in this than in other species. Euphorbia lactea had the most complex grain in which lobing was frequent at the ends as well as along the midregion, resulting in a large discoid grain. Subclasses in which grains differed significantly in length between species were evident in all classes containing several species. The average length of grains in any subclass was similar for subclasses between the classes. The study suggests that the elongated grain of the laticifer was derived from the round or oblong grain present in the more primitive parenchymatous cell. Progressive changes in the pattern of starch deposition have given rise to osteoid and discoid grains of increasing morphological complexity which is interpreted to represent trends in laticifer evolution between different species of Euphorbia as reflected by subtle changes for starch deposition within the plastids of this cell.     

基于植株碳流的水稻籽粒淀粉积累模拟模型

通过解析水稻(Oryza sativa)植株碳素积累和转运的动态规律及其与环境因子和基因型之间的定量关系, 构建基于植株碳流动态的水稻籽粒淀粉积累模拟模型。水稻籽粒中的淀粉积累速率取决于库限制下的淀粉积累速率和源限制下的可获取碳源。库限制下的淀粉积累速率是潜在淀粉积累速率及温度、水分、氮素、淀粉合成能力等因子综合影响的结果; 源限制下的可获取碳源取决于花后光合器官生产的即时光合产物和营养器官向籽粒转运的储存光合产物。花后植株即时光合产物随花后生长度日呈对数递减。花后营养器官向籽粒转运的储存光合产物又分为叶片和茎中积累碳素的转运。利用不同栽培条件下的独立田间试验资料对籽粒淀粉积累的动态模型进行了检验, 结果显示籽粒淀粉积累量和含量的模拟值和观测值之间的根均方差均值分别为3.61%和4.51%, 决定系数分别为0.994和0.959, 表明该模型对不同栽培条件下的水稻单籽粒淀粉积累量和含量具有较好的预测性, 为水稻生产中籽粒淀粉指标的动态预测和管理调控提供了量化工具。     

遮光对蚕豆花荚形成和脱落的影响

本文旨在研究蚕豆开花前后不同时期光照强度对花荚形成和脱落的影响。产量补偿能力以及花荚形成和脱落的生理生态原因。蚕豆花前遮光,开花总数和结荚数降低,但花荚脱落率下降,粒重增加。花期和花后遮光,对开花总数没有明显影响,但花荚脱落严重,减产最多。任何时期遮光均使比叶重、遮光后期叶绿素含量、光合生产量、生殖器官干物质分配率,可溶性糖和含N量下降,但成熟期可溶性糖和含N量,营养元素吸收量不受影响。遮光导致花荚形 成和产量减少的主要原因是C／N比值下降,而不是改变营养元素的丰度所致。     

Genetic controls on starch amylose content in wheat and rice grains

Starch accumulates in plants as granules in chloroplasts of source organs such as leaves (transitory starch) or in amyloplasts of sink organs such as seeds, tubers and roots (storage starch). Starch is composed of two types of glucose polymers: the essentially linear polymer amylose and highly branched amylopectin. The amylose content of wheat and rice seeds is an important quality trait, affecting the nutritional and sensory quality of two of the world’s most important crops. In this review, we focus on the relationship between amylose biosynthesis and the structure, physical behaviour and functionality of wheat and rice grains. We briefly describe the structure and composition of starch and then in more detail describe what is known about the mechanism of amylose synthesis and how the amount of amylose in starch might be controlled. This more specifically includes analysis of GBSS alleles, the relationship between waxy allelic forms and amylose, and related quantitative trait loci. Finally, different methods for increasing or lowering amylose content are evaluated.     

Knockdown of NtMed8, a Med8-like gene, causes abnormal development of vegetative and floral organs in tobacco (Nicotiana tabacum L.)

Med8, a subunit of mediator complex, has proved to possess crucial functions in many organisms from yeast to human. In plant, the med8 mutant of Arabidopsis thaliana displayed delayed anthesis and increased number of leaves during the vegetative period. However, the roles of Med8 in other flowering plants are still unknown. To investigate the function of Med8 ortholog in tobacco (Nicotiana tabacum L.; named as NtMed8), we created transgenic tobacco plants with repressed NtMed8 expression mediated by RNA interference (RNAi). Compared with the wild type, the NtMed8-RNAi plants exhibited: more leaves with smaller but thicker blades; larger cells and vascular bundles with lower stomata density in leaves; swelled chloroplasts with thicker and lumen-enlarged thylakoids; weaker root system with fewer lateral roots; larger flowers and floral organs; flowering earlier under long day, but later under short day conditions; and male sterile with larger but less germinable pollens. In addition, quantitative RT-PCR indicated that NtMed8 is expressed in both vegetative and floral tissues. Subcellular localization analysis by transient expression of fusion protein in Nicotiana benthamiana leaves showed that NtMed8 was located in both plasma membrane and nucleus. These results suggest that NtMed8 plays important roles in both vegetative and reproductive development, and the function of Med8 appears to be, at least partially, conserved in flowering plants.     

Light-induced degradation of starch granules in turions of Spirodela polyrhiza studied by electron microscopy

Spirodela polyrhiza forms turions, starch-storing perennial organs. The light-induced process of starch degradation starts with an erosion of the surface of starch grains. The grain size decreases over a period of red irradiation and the surface becomes rougher. The existence of funnel-shaped erosion structures demonstrates that starch degradation is also possible inside the grains. Neither etioplasts nor clues as to their transition into chloroplasts were found in the storage tissue by transmission electron microscopy. Juvenile chloroplasts always contained the starch grains which remained from amyloplasts. No chloroplasts were found which developed independently of starch grains. Amyloplasts are therefore the only source of chloroplasts in the cells of irradiated turions. The intactness of amyloplast envelope membranes could not be directly proved by electron microscopy. However, the light-induced transition of amyloplasts into chloroplasts provides indirect evidence for the integrity of the envelope membranes throughout the whole process. The starch grains are sequestered from the cytosolic enzymes, and only plastid-localized enzymes, which have access to the starch grains, can carry out starch degradation. In this respect the turion system resembles transitory starch degradation as known from Arabidopsis leaves. On the other hand, with α-amylase playing the dominant role, it resembles the mechanism operating in the endosperm of cereals. Thus, turions appear to possess a unique system of starch degradation in plants combining elements from both known starch-storing systems.     

FLOWERING OF DICHONDRA IN RESPONSE TO TEMPERATURE AND DAYLENGTH

Experiments using controlled environment facilities showed that flowering of Dichondra repens was promoted by chilling plants at 10 C or below. Optimum length of the chilling period was 5–6 weeks. Unchilled plants did not flower. The flowering stimulus resulting from chilling was destroyed by temperatures above 21 C. Rate of flowering was increased by short days during chilling, but short days could not substitute for the chilling requirement. Optimum daylength for flower initiation following chilling was approximately 14 hr and the optimum temperature was approximately 15 C. Flower buds developed in leaf axils of primary stems and laterals, but stem apices remained vegetative. When the chilling requirement was met flowering continued indefinitely as the plants grew.     

Manipulation of starch granule size distribution in potato tubers by modulation of plastid division

Starch granule size is an important parameter for starch applications in industry. Starch granules are formed in amyloplasts, which are, like chloroplasts, derived from proplastids. Division processes and associated machinery are likely to be similar for all plastids. Essential roles for FtsZ proteins in plastid division in land plants have been revealed. FtsZ forms the so-called Z ring which, together with inner and outer plastid division rings, brings about constriction of the plastid. It has been shown that modulation of the expression level of FtsZ may result in altered chloroplast size and number. To test whether FtsZ is also involved in amyloplast division and whether this, in turn, may affect the starch granule size in crop plants, FtsZ protein levels were either reduced or increased in potato. As shown previously in other plant species, decreased StFtsZ1 protein levels in leaves resulted in a decrease in the number of chloroplasts in guard cells. More interestingly, plants with increased StFtsZ1 protein levels in tubers resulted in less, but larger, starch granules. This suggests that the stoichiometry between StFtsZ1 and other components of the plastid division machinery is important for its function. Starch from these tubers also had altered pasting properties and phosphate content. The importance of our results for the starch industry is discussed.     

In Vitro Biosynthesis of Phosphorylated Starch in Intact Potato Amyloplasts

Intact amyloplasts from potato (Solanum tuberosum L.) were used to study starch biosynthesis and phosphorylation. Assessed by the degree of intactness and by the level of cytosolic and vacuolar contamination, the best preparations were selected by searching for amyloplasts containing small starch grains. The isolated, small amyloplasts were 80% intact and were free from cytosolic and vacuolar contamination. Biosynthetic studies of the amyloplasts showed that [1-¹⁴C]glucose-6-phosphate (Glc-6-P) was an efficient precursor for starch synthesis in a manner highly dependent on amyloplast integrity. Starch biosynthesis from [1-¹⁴C]Glc-1-P in small, intact amyloplasts was 5-fold lower and largely independent of amyloplast intactness. When [³³P]Glc-6-P was administered to the amyloplasts, radiophosphorylated starch was produced. Isoamylase treatment of the starch followed by high-performance anion-exchange chromatography with pulsed amperometric detection revealed the separated phosphorylated α-glucans. Acid hydrolysis of the phosphorylated α-glucans and high-performance anion-exchange chromatography analyses showed that the incorporated phosphate was preferentially positioned at C-6 of the Glc moiety. The incorporation of radiolabel from Glc-1-P into starch in preparations of amyloplasts containing large grains was independent of intactness and most likely catalyzed by starch phosphorylase bound to naked starch grains.     

Cytoplasmic DNA apportionment and plastid differentiation during male gametophyte development in Pelargonium zonale

In the male gametophyte of Pelargonium zonale, generative and sperm cells contain cytoplasmic DNA in high density compared to vegetative cells. Cytoplasmic DNA was examined using the DNA fluorochrome DAPI (4'6-diamidino-2-phenylindole) and observed with epifluorescence and electron microscopy. The microspore cell contains a prominent central vacuole before mitosis; mitochondria and plastids are randomly distributed throughout the cytoplasm. Following the first pollen grain mitosis, neither the vegetative cell nor the early generative cell display a distributional difference in cytoplasmic DNA, nor is there in organelle content at this stage. During the maturation of the male gametophyte, however, a significant discrepancy in plastid abundance develops. Plastids in the generative cell return to proplastids and do not contain large starch grains, while those in the vegetative cell develop starch grains and differentiate into large amyloplasts. Plastid nucleoids in generative and sperm cells in a mature male gametophyte are easily discriminated after DAPI staining due to their compactness, while those in vegetative cells stained only weakly. The utility of the hydrophilic, non-autofluorescent resin Technovit 7100 in observing DAPI fluorescence is also demonstrated.