Hormonal factors controlling the initiation and development of lateral roots

As the first part of a comprehensive study of the hormonal control of lateral root initiation and development, the effect of surgical treatments such as removal of the root tip, one or more cotyledons, the young epicotyl, or combination of these treatments, on the induction and emergence of lateral roots on the primary root of pea seedlings has been examined. Results show that removal of the root tip leads to a rapid but transitory increase in the number of lateral primordia, the largest number arising in the most apical segment of decapitated roots suggesting the accumulation of acropetally moving promoter substances in this region. The cotyledons appear to be the main source of promoter substances for both the induction and emergence of lateral roots, although one or more promoters also appear to be produced in the epicotyl. The data further indicate that the root tip is possibly the source of a substance which moves basipetally and interacts with acropetally moving promoters to regulate the zone for lateral primordia initiation; the root tip also appears to be the source of a powerful inhibitor of lateral root emergence.     

A molecular mechanism that confines the activity pattern of miR165 in Arabidopsis leaf primordia

In Arabidopsis leaf primordia, the expression of HD‐Zip III, which promotes tissue differentiation on the adaxial side of the leaf primordia, is repressed by miRNA165/166 (miR165/166). Small RNAs, including miRNAs, can move from cell to cell. In this study, HD‐Zip III expression was strikingly repressed by miR165/166 in the epidermis and parenchyma cells on the abaxial side of the leaf primordia compared with those on the adaxial side. We also found that the MIR165A locus, which was expressed in the abaxial epidermis, was sufficient to establish the rigid repression pattern of HD‐Zip III expression in the leaf primordia. Ectopic expression analyses of MIR165A showed that the abaxial‐biased miR165 activity in the leaf primordia was formed neither by a polarized distribution of factors affecting miR165 activity nor by a physical boundary inhibiting the cell‐to‐cell movement of miRNA between the adaxial and abaxial sides. We revealed that cis‐acting factors, including the promoter, backbone, and mature miRNA sequence of MIR165A, are necessary for the abaxial‐biased activity of miR165 in the leaf primordia. We also found that the abaxial‐determining genes YABBYs are trans‐acting factors that are necessary for the miR165 activity pattern, resulting in the rigid determination of the adaxial–abaxial boundary in leaf primordia. Thus, we proposed a molecular mechanism in which the abaxial‐biased patterning of miR165 activity is confined.     

Benzyladenine-induced inhibition of flowering in Chenopodium rubrum in vitro is not related to the levels of isoprenoid cytokinins

The effect of 6-benzylaminopurine (BAP) on floweringand on endogenous levels of isoprenoid cytokinins wasinvestigated in explanted terminal shoots of Chenopodium rubrum cultivated in vitro. Themother plants were grown under continuous light andexplants were cut off when the 6th leaf primordiumoriginated at the shoot apex. The explants wereexposed to one dark period of 13 hours inductive forflowering or to continuous light on medium with orwithout BAP (0.05;0.2;0.4 mg.l^-1). Undernon-inductive conditions no flowering was observedeither in the control or after BAP treatment. Afterreceiving one inductive dark period, the controlexplants flowered. However, BAP application either atthe beginning of the inductive dark period and/orduring the following light cultivation inhibitedflowering and stimulated initiation and growth of leafprimordia. In the case of the most efficient BAPconcentration (0.05 mg.l^-1) flowering wasinhibited by 80% and the number of leaf primordia wasincreased by 3. Explantation caused a significantincrease in the total amount of endogenous cytokininsin the explants within first 13 h, provided they werekept in light. When explants were kept in darkness,only a slight increase in cytokinin levels wasobserved. BAP treatment had no influence on the levelsof endogenous cytokinins either in light or indarkness. We may thus conclude, that BAP applicationinhibited flowering of photoperiodically inducedterminal shoot explants and stimulated leaf primordiaformation with no significant effect on changes inlevels of endogenous isoprenoid cytokinins. This maysuggest the direct ability of BAP to regulate morphogenesis.     

Rapid in vitro propagation of Matteuccia struthiopteris (L.) Todaro – an edible fern

A rapid and efficient in vitro plant regeneration method was developed for Matteuccia struthiopteris (L.) Todaro (Ostrich fern). Side shoots, originating in meristems of sectioned rhizomes, were used as explant material. A very high rate of meristem multiplication was achieved by culturing the explants in half-strength MS liquid medium supplemented with 2.0 mg/l N-(4-Pyridyl)-N′-phenylurea (4-PU) and 0.5 mg/l thidiazuron (TDZ). Multiplication of the shoot primordia was faster in suspension culture than on solid medium. Rhizogenesis and growth of regenerants were best achieved on hormone-free one-quarter-strength MS solid medium amended with 0.4% agar and 1.0% activated charcoal. Regenerated plantlets continued to grow after transfer to soil in a phytotron. Received: 19 March 1998 / Revision received: 17 July 1998 / Accepted: 3 August 1998     

Following the initiation and development of individual leaf primordia at the level of the shoot apical meristem: the case of distichous phyllotaxis in Begonia

BACKGROUND AND AIMS: By using the technique of replicas of a developing apex it is possible to obtain a direct measure of phyllotactic parameters (plastochrone and platochronic ratio) involved in the initiation of two successive primordia at the level of the SAM. The goal of this study is to compare, in a real time setting, the value of phyllotactic parameters in distichous systems using Begonia as a case study, with the value of the same parameters in spiral phyllotactic systems. METHODS: To determine the real-time sequence of events at the level of the SAM, replicas were made of the developing apex at different intervals using previously described techniques. Impression moulds were made at 24-h intervals. The following phyllotactic parameters were measured: plastochrone, angle of divergence, plastochrone ratio and ratio between the diameter of the leaf and the apex. RESULTS: The time between the appearance of two successive leaves is 15-20 d. The average value of the plastochrone ratio (R) is 1.3, and the ratio of the leaf to the diameter of the apex (Gamma) is 2.5. The angle of divergence varies from 165 masculine to 180 masculine. The speed of advection of the primordium from the apex, varies from 0.28 to 0.37 microm d(-1). CONCLUSIONS: The speed of advection of primordia in Begonia is lower than that of Anagalis. This is not in accordance with theoretical simulations that predict the opposite. In Begonia, the plastochrone ratio does not reflect the real time of appearance of two successive primordia. The time separating the appearance of two primordia is not directly related to the distance of these two primordia from the centre of the apex but is related instead to the enlargement of leaves.     

Daily temperature gradients and processes of organogenesis in apical meristem of <Emphasis Type="Italic">Cucumis sativus</Emphasis> L.

We studied the influence of daily temperature gradients on organogenesis in apical and axil shoot meristems at different developmental stages in Cucumis sativus L. The level of organogenic activity of meristems was determined according to the number of leaf primordia on the main and lateral shoots, number of 2nd order shoots, and rudiments of flowers of different levels of development. At the studied ontogenetic stages (mesotrophic seedling or juvenile state), plants were grown under the controlled conditions: photoperiod 12 h, light intensity 100 Wt/m², range of mean daily temperatures 20…30°C, and daily temperature gradients ?20…+20°C. After the temperature treatment, some plants were returned to the optimal, for growth and development, conditions for two weeks (aftereffect). Three types of organogenic activity of meristems in response to the influence of variable daily temperatures were described: stimulation, inhibition, or absence of effect. The phenomenon of stimulation includes two subtypes: optimization, when a maximum effect, observed at other constant temperatures, was attained under the influence of variable temperatures and maximization, when maximum values markedly exceeded those at constant temperatures. The patterns described are preserved on the whole in the aftereffect of daily temperatures.     

Ultrastructure of Amoebophrya sp. and its changes during the course of infection

Amoebophrya is a syndinian parasite that kills harmful bloom forming algae. Previously uncharacterized ultrastructural aspects of infection and development were elucidated. The biflagellate dinospore has two mitochondria, electron-dense bodies, striated strips, trichocysts, and a nucleus with peripherally condensed chromatin. After finding an Akashiwo sanguinea host and adhering to its surface, the parasite penetrates the host surface, apparently using a microfilament based motility and electron-dense bodies within a microtubular basket in the process of parasitophorous vacuole membrane formation. After entering the host nucleus, possibly by a similar mechanism used to enter the host cell, the parasite cytosol expanded substantially prior to mitosis. From 12-36 hours mitochondria were inconspicuous but present. Chromatin condensation was variable. By 36 hours post-infection, parasites had multiple nuclei, a microtubule-supported cytopharynx, and were beginning to form a fully internal mastigocoel. By 48 hours, the characteristic "beehive" appearance was apparent with flagella projecting into a fully developed mastigocoel. The cytoplasm contained trichocysts, elongated mitochondria, and nuclei with peripherally condensed chromatin. Although Amoebophrya lacks an apical complex, its electron-dense bodies show functional similarities to apicomplexan rhoptries. Its lack of permanently condensed chromosomes, but compact dinospore chromatin, supports the idea that dinoflagellate permanently condensed chromosomes may be a remnant of a parasitic ancestor with a compact dispersal stage.     

Inflorescence and floral ontogeny in Crocus sativus L. (Iridaceae)

Inflorescence and floral ontogeny of the perennial, herbaceous crop Crocus sativus L. were studied using epi-illumination light microscopy. After production of leaves with helical arrangement a determinate inflorescence forms which becomes completely transformed into a single terminal flower. In some cases, bifurcation of the inflorescence meristem yields two or three floral meristems. The order of floral organs initiation is outer tepals – stamens – inner tepals – carpels. Stamens and outer tepals are produced from the lateral bifurcation of three common stamen-tepal primordia. Within each whorl, organs start developing unidirectionally from the adaxial side, except for the stamens which begin to grow from the abaxial side. Specialized features during organ development include interprimordial growth between tepals forming a perianth tube, fusion at the base of stamen filaments, and formation of an inferior ovary with unfused styles.     

荔枝草花的形态发生

在体式显微镜系统观察的基础上,对唇形科鼠尾草属植物荔枝草(Salvia plebeiaR.Br.)的花发育过程进行了扫描电镜观察.发现荔枝草的轮伞花序由多数交互对生的聚伞花序单位组成,花器官发育形式为向心式",各部分花器官从外向内依次形成;共形成4个雄蕊原基,其中2个雄蕊原基在形成后不再发育,另2个雄蕊原基每个均发育出1可育药室和1不育药室,不育药室膨大连接,并在花成熟之前参与组成特殊杠杆结构;子房四深裂"的形成实际是由4个原基分别发育,而后相互靠拢而成.     

Rhythmic plant morphogenesis: Recurrent patterns of idioblast cell production

Most plants are constructed from repeating modular units such as phytomers, merophytes, and cell packets. Even an organism as simple as the filamentous cyanobacterium Anabaena shows recurrent patterns of differentiated cellular structures, notably with respect to its heterocysts. These examples reflect the inherent rhythms established within developmental processes of living organisms. In the present article, attention is paid to repetitious production of idioblasts—isolated cells, or clusters of cells, with an identity different to that of neighbouring cells from which they are derived. In higher plant root tissues, idioblasts are contained within cell packets that grow up from mother cells during the course of a number of cycles of cell production. The heterocysts of Anabaena are also discussed; they, too, are a type of idioblast. The idioblasts of root tissues originate as small cells which result from unequal cell divisions. Such divisions are usually the final ones within a cell packet which has already undergone a number of division cycles and are characteristically located at one or both ends of a packet. The packet end walls are suggested to have a role in regulating division asymmetry. Idioblastic systems discussed are root cortical trichosclereids and diaphragm cells; in their earliest stage, the cells from which lateral root primordia arise are also considered as clusters of idioblasts because they, too, are the products of asymmetric divisions of pericyclic mother cells. The division patterns of all these idioblastic systems were modelled in a consistent way using L-systems, with the assumption that the age of a cell-packet end wall plays a special role in cell determination. This article is dedicated to Vsevelod Ya. Brodsky, doyen of Russian studies of rhythms in cell division and development, who celebrates his 80th birthday on August 4, 2008 This article was presented in original.