MORPHOGENETIC ASPECTS OF A LEAFLESS MUTANT IN TOMATO I. GENERAL PATTERNS IN DEVELOPMENT

The embryo of the reduced form of the lanceolate mutant in tomato fails to undergo the heart-shaped stage of development. Cells in the shoot apical region of this leafless mutant lose their meristematic character and develop into mature parenchyma during embryogenesis. This early loss of meristem tissue leads to the determinate growth which is evident in the seedling. In contrast to normal, starch grains are visible with the light microscope in cells of the shoot tip of the mutant hypocotyl from early embryogeny up to and including the seedling stage, and protein bodies are abundant in the same tissue of fully developed mutant embryos. The shoot apical region in homozygous mutant embryos with cotyledons or cotyledon-like structures exhibits some cytochemical and morphological similarity with the normal shoot apex. Morphological variation in these forms appears to be in a continuous pattern. The extent of their development and consequent longevity is related to possible differences in rates of cell expansion and variation in environmental factors during the early stages of embryogenesis.     

ONTOGENY OF THE PRIMARY THICKENING MERISTEM IN SEEDLINGS OF BOUGAINVILLEA SPECTABILIS

Anomalous secondary thickening occurs in the main axis of Bougainvillea spectabilis as a result of a primary thickening meristem which differentiates in pericycle. The primary thickening meristem first appears in the base of the primary root about 6 days after germination and differentiates acropetally as the root elongates. It begins differentiating from the base of the hypocotyl toward the shoot apex about 33 days after germination. The primary thickening meristem is first observable at the base of the first internode about 60 days after germination. It then becomes a cylinder in the main axis of the seedling. No stelar cambial cylinder forms in the primary root, hypocotyl, or stem because vascular cambium differentiation occurs neither in the pericycle opposite xylem points in the primary root nor in interfascicular parenchyma in the hypocotyl or stem. The primary vascular system of the stem appears anomalous because an inner and an outer ring of vascular bundles differentiate in the stele. Bundles of the inner ring anastomose in internodes, whereas those of the outer ring do not. Desmogen strands each of which is composed of phloem, xylem with both tracheids and vessels, and a desmogic cambium, differentiate from prodesmogen strands in conjunctive tissue. The parenchymatous cells surrounding desmogen strands then differentiate into elongated simple-pitted fibers and thick-walled fusiform cells that are about the same length as the primary thickening meristem initials.     

FRD3 controls iron localization in Arabidopsis

The frd3 mutant of Arabidopsis exhibits constitutive expression of its iron uptake responses and is chlorotic. These phenotypes are consistent with defects either in iron deficiency signaling or in iron translocation and localization. Here we present several experiments demonstrating that a functional FRD3 gene is necessary for correct iron localization in both the root and shoot of Arabidopsis plants. Reciprocal grafting experiments with frd3 and wild-type Arabidopsis plants reveal that the phenotype of a grafted plant is determined by the genotype of the root, not by the genotype of the shoot. This indicates that FRD3 function is root-specific and points to a role for FRD3 in delivering iron to the shoot in a usable form. When grown under certain conditions, frd3 mutant plants overaccumulate iron in their shoot tissues. However, we demonstrate by direct measurement of iron levels in shoot protoplasts that intracellular iron levels in frd3 are only about one-half the levels in wild type. Histochemical staining for iron reveals that frd3 mutants accumulate high levels of ferric iron in their root vascular cylinder, the same tissues in which the FRD3 gene is expressed. Taken together, these results clearly indicate a role for FRD3 in iron localization in Arabidopsis. Specifically, FRD3 is likely to function in root xylem loading of an iron chelator or other factor necessary for efficient iron uptake out of the xylem or apoplastic space and into leaf cells.     

Evidence for a revertant at the La locus in regenerating hypocotyl segments in tomato

Summary Leafy and leafless phenotypes were regenerated in vitro from hypocotyl segments of leafless forms (reduced and modified) of the homozygous lanceolate (La) mutant in tomato. Segregation of progeny of leafy regenerates into homozygous. mutant (La La), heterozygote (La La ⁺) and normal (La ⁺ La ⁺) indicates that cells forming the shoot apical meristems undergo a genetic reversion, and that the nutrient medium might be selecting for the heterozygote. Among the progeny of the regenerates is a true breeding, unlobed variant. Leaves of the variant are pinnately compound and the margins are entire. Opposite cotyledons followed in development by two simple leaves before the appearance of a pinnately compound leaf with an occasional lanceolate-shaped leaflet suggests that the unlobed variant is morphologically intermediate between La La ⁺ and La ⁺ La ⁺.     

APPLICATION OF THE 22.5 MEV DEUTERON MICROBEAM TO THE STUDY OF MORPHOGENETIC PROBLEMS WITHIN THE SHOOT APEX OF OSMUNDA CLAYTONIANA

Excised shoot apices of Osmunda claytoniana were grown under controlled sterile conditions. Histological examination of the normal shoot apex shows that it is comprised of: (1) a promeristem, which possesses 1 or more apical initiating cells at its center; (2) a prestelar tissue consisting of an incipient vascular tissue which flanks the pith-mother-cell zone; the pith-mother-cell zone gives rise to the pith rib meristem and subsequently to the fundamental parenchyma of the pith; (3) the fundamental parenchyma of the cortex and the fundamental parenchyma of the dermal system both arising from flank cells of the promeristem. Apical initial cells of meristems irradiated with a 127,000 rad acute exposure of a deuteron beam having a diameter of 25μ, histologically examined at 7-day intervals for a 12-week period, as early as 3 weeks’ postirradiation, showed the apical initiating cell(s) together with certain of the cells of the pith-mother-cell zone to be destroyed. A wound response develops peripherally to the destroyed initials. In addition, an isolated, organized growth center is observed to develop from normal promeristem cells. Incipient vascular tissue and a new pith-mother-cell zone are also observed to develop in association with the new center of growth. Implications of the role of the interrelationships between apical initiating cell(s) and other cells of the meristem and the role they may play in maintenance of meristematic integrity within the shoot meristem are discussed.     

植物离体茎段嫁接

植物体茎段嫁接系统是在无菌条件下将茎切段嫁接后放入培养基中、使接穗和砧木分别与含不同成分的培养基接触,再署光下培养的一个模拟植物正常生理过程、环境条件可控的实验系统,离体茎段嫁接体的发育与整体类拟,包括接穗与砧木粘连、愈伤组织产生、次生甩间连丝形成和维管束分化等几个步骤,发育进程受植物激素如生长素和细胞分裂素调节。该系统的建立为阐明嫁接体发机理及嫁接亲和性机制提供了重要的依据。     

Morphology and Anatomy of Stems and Pedicels of Spring Flush Shoots Associated with Citrus Fruit Set

Flowering and vegetative shoots of ‘Shamouti’ orange[Citrus sinensis (L.) Osbeck] and ‘Marsh’ seedlessgrapefruit (Citrus paradisi Macf.) were examined for correlationof their morphology and anatomy with fruit set. Fruit set isfavoured on leafy inflorescences whereas abortion is nearlycomplete on leafless inflorescences. Leafless inflorescencesof ‘Shamouti’ with one flower were found to havea very thin stem which contained few vascular bundles, whereasthose with three flowers had better-developed vascular systems.The vascular system of leafy inflorescences is significantlydifferent from that of leafless ones and contains a distinctcentral xylem cylinder. The vascular area of leafless inflorescencesis only about one-quarter of that of the leafy ones. The vascularsystem of grapefruit resembles that of the ‘Shamouti’orange. This study emphasizes the importance of the dimensionof the vascular system for fruit set and provides a possibleexplanation for the better fruit set on both leafy and leaflessinflorescences with several flowers compared with single-floweredinflorescences. Anatomy; citrus; fruit set; leafless inflorescence; leafy inflorescence; pedicel; vascular system; vegetative shoot     

Bud formation in leaves,leaf fragments and midrib pieces of Heloniopsis orientalis (Liliaceae)

Summary Isolated leaves, leaf fragments and pieces of the midrib portion devoid of lamina, of Heloniopsis orientalis were grown on an inorganic nutrient medium without organic nutrients and growth regulators in order to investigate their regenerative ability. Bud formation in intact, attached leaves occurs only at the tip, in isolated leaves at the tip and the base, whereas leaf fragments cut transversely at a distance from the tip and isolated midrib pieces form numerous shoot buds in a random distribution. Lamina fragments lacking midrib frequently fail to regenerate even after a long time of culture. It is suggested that endogeneous growth regulators in the leaf, especially the vascular tissues, play an important role in bud initiation. Very young leaves of Heloniopsis are capable forming buds and roots when isolated from the mother plants.     

蔗糖诱导拟南芥幼苗胚轴维管束细胞数量增多

为揭示蔗糖能否引起植物胚轴维管束细胞数量增多，将拟南芥播种于添加88mmol·L-1蔗糖和不添加糖的MS培养基上，对生长在不同培养基上的幼苗胚轴横切，显微镜下统计切片上维管束细胞数量。结果显示，与不加糖相比，加糖条件下萌发4d后幼苗维管束细胞总数增加约70％，维管薄壁细胞和导管分子都增加100％以上，筛管分子增加约90％，中柱鞘细胞数量不变。显然，蔗糖不仅使维管束薄壁细胞数量增多，也使筛管分子和导管分子数量增多。因此认为，添加蔗糖对拟南芥幼苗胚轴维管束具有双重效应，既引起维管薄壁细胞增殖，又促进维管薄壁细胞分化，从而使导管分子和筛管分子数量增多。     

喜树原形成层到形成层转化的研究

观察了枣树茎中原形成层到形成层的转化过程。距茎端0．5mm处,节间开始伸长之前,横切面上4—5个原形成束及束间的分生组织组成原形成层环。径向切面观,原形成层环呈现出较均一的结构。随着节间开始伸长,由于原形成层细胞发生假横向分裂,出现了长短两类细胞,长细胞多数端壁倾斜,短细胞多数端壁平截。以后,长细胞发育为纺缍状原始细胞,短细胞发育为射线原始细胞,部分射线原始细胞可以伸长井侵入生长而转化为纺缍状原始细胞。在节间伸长将停止时,此种转化基本完成。喜树为非叠生形成层,纺缍状原始细胞和射线原始细胞都有多核现象发生。     

CYP78A5 encodes a cytochrome P450 that marks the shoot apical meristem boundary in Arabidopsis

The normal development of shoot structures depends on controlling the growth, proliferation and differentiation of cells derived from the shoot apical meristem. We have identified the CYP78A5 gene encoding a putative cytochrome P450 monooxygenase that is the first member of the CYP78 family from Arabidopsis. This gene is strongly expressed in the peripheral regions of the vegetative and reproductive shoot apical meristems, defining a boundary between the central meristematic zone and the developing organ primordia. In addition, CYP78A5 shows a dynamic pattern of expression during floral development. Overexpression of CYP78A5 affects multiple cell types, causing twisting and kinking of the stem and defects in floral development. To define the relationship of CYP78A5 to genes controlling meristem function, we examined CYP78A5 expression in plants mutant for SHOOT MERISTEMLESS, ZWILLE and ARGONAUTE, and have found that CYP78A5 expression is altered in these mutant backgrounds. We propose that CYP78A5 has a role in regulating directional growth in the peripheral region of the shoot apical meristem in response to cues established by genes regulating meristem function.     

Molecular analysis of SCARECROW function reveals a radial patterning mechanism common to root and shoot

Mutation of the SCARECROW (SCR) gene results in a radial pattern defect, loss of a ground tissue layer, in the root. Analysis of the shoot phenotype of scr mutants revealed that both hypocotyl and shoot inflorescence also have a radial pattern defect, loss of a normal starch sheath layer, and consequently are unable to sense gravity in the shoot. Analogous to its expression in the endodermis of the root, SCR is expressed in the starch sheath of the hypocotyl and inflorescence stem. The SCR expression pattern in leaf bundle sheath cells and root quiescent center cells led to the identification of additional phenotypic defects in these tissues. SCR expression in a pin-formed mutant background suggested the possible origins of the starch sheath in the shoot inflorescence. Analysis of SCR expression and the mutant phenotype from the earliest stages of embryogenesis revealed a tight correlation between defective cell divisions and SCR expression in cells that contribute to ground tissue radial patterning in both embryonic root and shoot. Our data provides evidence that the same molecular mechanism regulates the radial patterning of ground tissue in both root and shoot during embryogenesis as well as postembryonically.     

SHOOT TIP ABORTION IN ULMUS AMERICANA

Millington , W. F. (Marquette U., Milwaukee, Wis.) Shoot tip abortion in Ulmus americana. Amor. Jour. Bot. 50(4): 371–378. Illus. 1963.—Phenological observations of American elm have shown that the phenomenon of shoot tip abortion in which the distal several plastochrons of each shoot turn yellow and abort, inducing sympodial growth, occurs over a period of several weeks starting at the time fruits are shed from older trees. Time of abortion varies among plants of different age, among individuals of the same age, and among shoots on the same individual. In the latter case, time of abortion is inversely correlated with the vigor of the shoot, the most vigorous shoots on a branch being the last to abort. Abortion is first evident in the yellowing of the entire shoot tip. Cytohistological studies show that necrosis commences at the sixth to eighth node back of the apex, where it is apparent in autolysis of internal cells of the stipules. Necrosis progresses acropetally in the stipules and young leaves and ultimately involves the leaf primordia at the shoot apex. Mitosis ceases in the shoot apex and its meristematic appearance is lost. These changes follow in basipetal sequence in the axillary buds down to the bud below the abscission site. This bud remains active and will resume growth the following season. The abscission site is evident externally as a green-yellow boundary in the basal part of the internode. No protective layer is present at the time of abscission, but it develops after the shoot tip abscises. There is no indication of blocking of vascular tissues before shoot tip abortion and limitation of water supply probably is not a causal factor. Photoperiod studies show that shoot tip abortion is accelerated in short days and delayed but not prevented in long days. Greenhouse experiments show that abortion is delayed also in seedlings, in plants supplied with organic fertilizer, or grown with the roots unconfined. Plants grown in a nutrient solution deficient in nitrogen aborted ahead of controls and plants deficient in calcium. Although shoot tip abortion occurs coincident with fruit drop, there is no indication of a causal relationship. The literature relating to shoot tip abortion is discussed in relation to the above observations.     

phot1 inhibition of ABCB19 primes lateral auxin fluxes in the shoot apex required for phototropism

It is well accepted that lateral redistribution of the phytohormone auxin underlies the bending of plant organs towards light. In monocots, photoreception occurs at the shoot tip above the region of differential growth. Despite more than a century of research, it is still unresolved how light regulates auxin distribution and where this occurs in dicots. Here, we establish a system in Arabidopsis thaliana to study hypocotyl phototropism in the absence of developmental events associated with seedling photomorphogenesis. We show that auxin redistribution to the epidermal sites of action occurs at and above the hypocotyl apex, not at the elongation zone. Within this region, we identify the auxin efflux transporter ATP-BINDING CASSETTE B19 (ABCB19) as a substrate target for the photoreceptor kinase PHOTOTROPIN 1 (phot1). Heterologous expression and physiological analyses indicate that phosphorylation of ABCB19 by phot1 inhibits its efflux activity, thereby increasing auxin levels in and above the hypocotyl apex to halt vertical growth and prime lateral fluxes that are subsequently channeled to the elongation zone by PIN-FORMED 3 (PIN3). Together, these results provide new insights into the roles of ABCB19 and PIN3 in establishing phototropic curvatures and demonstrate that the proximity of light perception and differential phototropic growth is conserved in angiosperms.     

TEMPERATURE-INDUCED CAVITIES AND SPECIALIZED PARENCHYMA CELLS IN THE VASCULAR CYLINDER OF PEA ROOTS

Pea seeds (Pisum sativum L.) of six cultivars were planted in the field, in the greenhouse, or in growth chambers, in five different media, in light or dark, and at various temperatures (10–32 C). Under all conditions above 15 C the central portion of the vascular cylinder, in all cultivars except “Ageotropum,” tended to form cavities in almost every primary root examined. These cavities then became filled by the ingrowth of specialized parenchyma cells (SP cells). The formation of cavities and SP cells was temperature dependent since the roots grown below 15 C always formed central metaxylem tracheary elements (MTEs), without cavities and SP cells. Cavities and SP cells did not form over the entire root length. When the roots were longer than 3 cm, they started to form cavities and SP cells and continued for an additional 10–30 cm. After that, late MTEs formed in the central vascular cylinder, and no cavities and SP cells occurred regardless of temperature. Within an individual root grown above 15 C, cavities and SP cells tended to form during periods of fast growth, while during periods of slow growth large central MTEs formed instead.     

A mutation in the ProteosomalRegulatory Particle AAA-ATPase-3 in Arabidopsis impairs the light-specific hypocotyl elongation response elicited by a glutamate receptor agonist, BMAA

BMAA is a cycad-derived glutamate receptor agonist that causes a two- to three-fold increase in hypocotyl elongation on Arabidopsis seedlings grown in the light. To probe the role of plant glutamate receptors and their downstream mediators, we utilized a previously described genetic screen to identify a novel, BMAA insensitive morphology (bim) mutant, bim409. The normal BMAA-induced hypocotyl elongation response observed on wild-type seedlings grown in the light is impaired in the bim409 mutant. This BMAA-induced phenotype is light-specific, as the bim409 mutant exhibits normal hypocotyl elongation in etiolated (dark grown) plants (+ or − BMAA). The mutation in bim409 was identified to be in a gene encoding the Proteosomal Regulatory Particle AAA-ATPase-3 (RPT3). Possible roles of the proteosome in Glu-mediated signaling in plants is discussed. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The SCHIZOID gene regulates differentiation and cell division in Arabidopsis thaliana shoots

 Cell division and cell differentiation are key processes in shoot development. The Arabidopsis thaliana (L.) Heynh. SCHIZOID (SHZ) gene appears to influence cell differentiation and cell division in the shoot. The shz-2 mutant is notable in that distinct phenotypes develop, depending on the environment in which the plants are grown. When shz-2 mutants are grown in petri dishes, callus develops from the petiole and hypocotyl. In contrast, when the mutants are grown on soil, shoots appear externally stunted with malformed leaves. However, detailed examination of soil-grown mutants shows that the two phenotypes are related. Soil-grown mutants form adventitious meristems, produce a large amount of vascular tissues and have aberrant cell divisions in the meristem. Cells with abnormal cell-division patterns were found in the apical and vascular meristems, suggesting SHZ influences cell division. Development of callus in petri dishes, development of adventitious meristems and aberrations in leaves on soil suggest that SHZ influences cell differentiation. The distinct, but related phenotypes on soil and in petri dishes suggests that SHZ normally functions to regulate differentiation and/or cell division in a manner that is responsive to environmental conditions. Received: 30 July 1999 / Accepted: 22 September 1999     

THE RELATIONSHIP BETWEEN THE PRIMARY THICKENING MERISTEM AND THE SECONDARY THICKENING MERISTEM IN YUCCA WHIPPLEI TORR. I. HISTOLOGY OF THE MATURE VEGETATIVE STEM

Anatomical observations were made on 1-, 2-, and 3-yr-old plants of Yucca whipplei Torr, ssp. percursa Haines grown from seed collected from a single parent in Refugio Canyon, Santa Barbara, California. The primary body of the vegetative stem consists of cortex and central cylinder with a central pith. Parenchyma cells in the ground tissue are arranged in anticlinal cell files continuous from beneath the leaf bases, through the cortex and central cylinder to the pith. Individual vascular bundles in the primary body have a collateral arrangement of xylem and phloem. The parenchyma cells of the ground tissue of the secondary body are also arranged in files continuous with those of the primary parenchyma. Secondary vascular bundles have an amphivasal arrangement and an undulating path with frequent anastomoses. Primary and secondary vascular bundles are longitudinally continuous. The primary thickening meristem (PTM) is longitudinally continuous with the secondary thickening meristem (STM). Axillary buds initiated during primary growth were observed in the leaf axils. The STM becomes more active prior to and during root initiation. Layers of secondary vascular bundles are associated with root formation.     

ONTOGENY AND CORRELATIVE RELATIONSHIPS OF THE PRIMARY THICKENING MERISTEM IN FOUR-O'CLOCK PLANTS (NYCTAGINACEAE) MAINTAINED UNDER LONG AND SHORT PHOTOPERIODS

The developmental anatomy of Mirabilis jalapa was investigated during the first 90 days of growth. The primary thickening meristem (PTM) initially differentiates in the pericycle at the top of the cotyledonary node 18 days after germination, then basipetally in the pericycle through the hypocotyl. The PTM differentiates acropetally into the stem and in the pericycle of the primaiy root, commencing 22 days after germination. Endodermis is easily identifiable in hypocotyls as well as in primary roots because of Casparian thickenings in its cells. It has not been definitely identified in stems. There are three rings of primary vascular bundles in the stem. The PTM differentiates as segments of cambium in a layer of cells (probably in the pericycle) on an arc between vascular bundles of the outer bundle ring. Later, arcs of PTM differentiate externally to the phloem of each bundle. Each arc forms a connection between original segments of PTM lying on either side of each vascular bundle. Thus, the PTM becomes a continuous cylinder. The PTM differentiates in the pericycle outside vascular tissue in the hypocotyl and root. Differentiation of the PTM and the mode of secondary thickening is similar in plants exposed to short (8-hr) and to long (18-hr) photoperiods, but some differences were observed. The PTM differentiates closer to the stem apex in all plants over 18 clays of age growing vegetatively under long photoperiods. That is, the diffuse lateral meristem, in whose cells the PTM differentiates in young intemodes, is shorter in nearly all investigated plants growing in long photoperiods. The hypocotyl and base of the primary root of 40-day-old plants in short photoperiods were more enlarged than those of the same age plants in long photoperiods; but, at the end of 64 days, the hypocotyl and primaiy root base were larger in plants growing under short photoperiods. Thirty-four days after seed germination, flower initiation occurs in plants exposed to short photoperiods. One hundred fifty days after seed germination, flowers differentiate on plants exposed to long photoperiods.