Seed size plasticity in response to embryonic lethality conferred by ectopic CYCD activation is dependent on plant architecture.

The size of seeds is the result of cell proliferation and growth in the three seed compartments: the embryo, endosperm and integuments. Targeting expression of the D-type cyclin CYCD7;1 to the central cell and early endosperm (FWA:CYCD7;1) triggered nuclear divisions and partial ovule abortion, reducing seed number in each silique and leading to increased seed size. A similar effect on seed size was observed with other segregating embryo lethal mutations, suggesting caution is needed in interpreting apparent seed size phenotypes. Here, we show that the positive effect of FWA:CYCD7;1 on Arabidopsis seed size is modulated by the architecture of the mother plant. Larger seeds were produced in FWA:CYCD7;1 lines with unmodified inflorescences, and also upon removal of side branches and axillary stems. This phenotype was absent from inflorescences with increased axillary floral stems produced by pruning of the main stem. Given this apparent confounding influence of resource allocation on transgenes effect, we conclude that plant architecture is a further important factor to consider in appraising seed phenotypes.

Spatiotemporal regulation of cell-cycle genes by SHORTROOT links patterning and growth.

The development of multicellular organisms relies on the coordinated control of cell divisions leading to proper patterning and growth. The molecular mechanisms underlying pattern formation, particularly the regulation of formative cell divisions, remain poorly understood. In Arabidopsis, formative divisions generating the root ground tissue are controlled by SHORTROOT (SHR) and SCARECROW (SCR). Here we show, using cell-type-specific transcriptional effects of SHR and SCR combined with data from chromatin immunoprecipitation-based microarray experiments, that SHR regulates the spatiotemporal activation of specific genes involved in cell division. Coincident with the onset of a specific formative division, SHR and SCR directly activate a D-type cyclin; furthermore, altering the expression of this cyclin resulted in formative division defects. Our results indicate that proper pattern formation is achieved through transcriptional regulation of specific cell-cycle genes in a cell-type- and developmental-stage-specific context. Taken together, we provide evidence for a direct link between developmental regulators, specific components of the cell-cycle machinery and organ patterning.

Competency of Nicotiana tabacum L. stem tissues to dedifferentiate is associated with differential levels of cell cycle gene expression and endogenous cytokinins.

The expression of the mitotic cyclin Arath; CYCB1;1 and of the cyclin-dependent kinase Arath; CDC2a was located by beta-glucuronidase histochemical detection and in situ hybridization, and was quantified by 4-methylumbelliferyl beta- D-glucuronide assay in tobacco stem tissues during both in vivo differentiation and in vitro dedifferentiation. The changes in localization of endogenous cytokinins were also determined during both processes using quantitative analysis and in situ immunocytochemistry. The CDC2a promoter was expressed continuously during stem development, with particularly high expression in the shoot apical meristem and in the internal and external primary phloem. CYCB1 expression was not restricted to the dividing cells; its expression in the shoot apical meristem was particularly high in the leaf-forming peripheral cells but the gene was also expressed throughout development in the internal and external phloem in which the rate of cell division was reduced or zero. Following in vitro culture, the internal phloem cells appeared to be particularly competent to re-enter the cell cycle within a short lag phase while the pith tissue reactivated later. In culture, cells that resumed division were found to accumulate cytokinins. The high competency of primary phloem to dedifferentiate was associated with its capacity to express CDC2a and CYCB genes and the presence of high cytokinin levels, providing some insights into the determinants of competency for resuming cell division.

Overexpression of KNAT1 in lettuce shifts leaf determinate growth to a shoot-like indeterminate growth associated with an accumulation of isopentenyl-type cytokinins.

Leaves are specialized organs characterized by defined developmental destiny and determinate growth. The overexpression of Knotted1-like homeobox genes in different species has been shown to alter leaf shape and development, but a definite role for this class of genes remains to be established. Transgenics that overexpress Knotted1-like genes present some traits that are characteristic of altered cytokinin physiology. Here we show that lettuce (Lactuca sativa) leaves that overexpress KNAT1, an Arabidopsis kn1-like gene, acquire characteristics of indeterminate growth typical of the shoot and that this cell fate change is associated with the accumulation of specific types of cytokinins. The possibility that the phenotypic effects of KNAT1 overexpression may arise primarily from the modulation of local ratios of different cytokinins is discussed.

In situ localization of endogenous cytokinins during shooty tumor development on Eucalyptus globulus Labill.

Our previous results demonstrated that endogenous cytokinins are involved in the shooty potential of tumors initiated on Eucalyptus globulus plantlets inoculated with Agrobacterium tumefaciens strain 82.139 [A. Azmi et al. (1997a) Plant Sci 127: 81-90]. In order to investigate whether or not these hormones are distributed homogeneously in the tumors prior to the onset of bud regeneration, decapitated hypocotyls were inoculated with the strain C58pMP90/T139 GUS-INT harboring the wild transferred DNA (T-DNA) of strain 82.139 tagged with the beta-glucuronidase (gus)-reporter gene. In situ immunolocalization of zeatin, dihydrozeatin and isopentenyladenine was performed in the developing tumors and combined with the histo-enzymological beta-glucuronidase assay. It was found that the expression of the T-DNA was restricted to only some small areas located deeply in the tumors. These sites were also provided with a high cytokinin signal while the untransformed parts of the tumors displayed a weaker signal, except in the early differentiating tracheary elements. The regenerated buds were untransformed and originated from superficial parts of the tumors provided with a moderate signal for cytokinins. The method of colocalization of both cytokinins and gus expression developed here might be helpful for further studies concerning the role of these hormones in controlling gene expression at cell and tissue levels.

Immunolocalization of actin in root statocytes of Lens culinaris L.

Lentil root statocytes show a strict structural polarity of their organelles with respect to the g vector. These cells are involved in the perception of gravity and are responsible for the orientation of the root. Actin filaments take part in the positioning of their organelles and could also be involved in the transduction of the gravitropic signal. A pre-embedding immunogold silver technique was carried out with a monoclonal antibody in order to study the distribution of actin cytoskeleton in the statocytes at the electron microscopic level. Some areas were never labelled (cell wall, vacuole, nucleoplasm, mitochondria, starch grains of the amyloplasts) or very slightly labelled (stroma of the amyloplasts). The labelling was scattered in the cytoplasm always close to, or on the nuclear and amyloplast envelopes and the tonoplast. Associations of 2 to 6 dots in file were observed, but these short files were not oriented in one preferential direction. They corresponded to a maximum distance of 0.9 micron. This work demonstrated that each statocyte organelle was enmeshed in an actin web of short filaments arranged in different ways. The images obtained by rhodaminephalloidin staining were in accordance with those of immunogold labelling. The diffuse fluorescence of the cytoplasm could be explained by the fact that the meshes of the web should be narrow. The vicinity of actin and of the amyloplasts envelope could account for the movement of these organelles that was observed in spatial microgravity.

Dynamics of cytokinins in apical shoot meristems of a day-neutral tobacco during floral transition and flower formation

This study considered cytokinin distribution in tobacco (Nicotiana tabacum L.) shoot apices in distinct phases of development using immunocytochemistry and quantitative tandem mass spectrometry. In contrast to vegetative apices and flower buds, we detected no free cytokinin bases (zeatin, dihydrozeatin, or isopentenyladenine) in prefloral transition apices. We also observed a 3-fold decrease in the content of cytokinin ribosides (zeatin riboside, dihydrozeatin riboside, and isopentenyladenosine) during this transition phase. The group concluded that organ formation (e.g. leaves and flowers) is characterized by enhanced cytokinin content, in contrast to the very low endogenous cytokinin levels found in prefloral transition apices, which showed no organogenesis. The immunocytochemical analyses revealed a differing intracellular localization of the cytokinin bases. Dihydrozeatin and isopentenyladenine were mainly cytoplasmic and perinuclear, whereas zeatin showed a clear-cut nuclear labeling. To our knowledge, this is the first time that this phenomenon has been reported. Cytokinins do not seem to act as positive effectors in the prefloral transition phase in tobacco shoot apices. Furthermore, the differences in distribution at the cellular level may be indicative of a specific physiological role of zeatin in nuclear processes.

Zeatin is indispensable for the G2-M transition in tobacco BY-2 cells.

The importance of N6-isoprenoid cytokinins in the G2-M transition of Nicotiana tabacum BY-2 cells was investigated. Both cytokinin biosynthesis and entry in mitosis were partially blocked by application at early or late G2 of lovastatin (10 microM), an inhibitor of mevalonic acid synthesis. LC-MS/MS quantification of endogenous cytokinins proved that lovastatin affects cytokinin biosynthesis by inhibiting HMG-CoA reductase. Out of eight different aminopurines and a synthetic auxin tested for their ability to override lovastatin inhibition of mitosis, only zeatin was active. Our data point to a key role for a well-defined cytokinin (here, zeatin) in the G2-M transition of tobacco BY-2 cells.

Diurnal Fluctuations in Ethylene Formation in Chenopodium rubrum.

Ethylene formation was studied in 5- to 6-d-old Chenopodium rubrum seedlings under the following light regimes: continuous light (CL), continuous darkness (CD), and alternating light/darkness (12 h of each). No significant regular oscillations in ethylene formation were found in either the CL or CD groups. In the light/dark regime, pronounced diurnal fluctuations in ethylene formation were observed. Activity of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase was transiently increased on transfer from light to dark and vice versa. In CL, ACC oxidase activity did not change significantly, whereas in CD, it decreased continuously after the initial increase. The in vivo levels of ACC and N-malonyl-ACC (MACC) were constant for the first few hours of darkness, then decreased dramatically, but increased again in the light. In constant darkness, the level of ACC displayed endogenous rhythm, with minimum values at h 12 and 44, and a maximum value at h 32 to 36. The level of MACC in both shoots and roots decreased in the CD group until h 12, and then remained constant until h 30 before decreasing continuously. We conclude that the photoperiodic regime affects both ACC and MACC levels, as well as the conversion of ACC to ethylene. Correlation of the described changes in ethylene formation to photoperiodic flower induction is discussed.

Superroot, a recessive mutation in Arabidopsis, confers auxin overproduction.

We have isolated seven allelic recessive Arabidopsis mutants, designated superroot (sur1-1 to sur1-7), displaying several abnormalities reminiscent of auxin effects. These characteristics include small and epinastic cotyledons, an elongated hypocotyl in which the connection between the stele and cortical and epidermal cells disintegrates, the development of excess adventitious and lateral roots, a reduced number of leaves, and the absence of an inflorescence. When germinated in the dark, sur1 mutants did not develop the apical hook characteristic of etiolated seedlings. We were able to phenocopy the Sur1- phenotype by supplying auxin to wild-type seedlings, to propagate sur1 explants on phytohormone-deficient medium, and to regenerate shoots from these explants by the addition of cytokinins alone to the culture medium. Analysis by gas chromatography coupled to mass spectrometry indicated increased levels of both free and conjugated indole-3-acetic acid. sur1 was crossed to the mutant axr2 and the altered-auxin response mutant ctr1. The phenotype of both double mutants was additive. The sur1 gene was mapped on chromosome 2 at 0.5 centimorgans from the gene encoding phytochrome B.

The metabolism of benzyladenine in Spathiphyllum floribundum 'Schott Petite' in relation to acclimatisation problems.

In Spathiphyllum floribundum 'Petite', which was cultured on medium containing benzyladenine (BA), uptake of this cytokinin and its conversion to 9-ß-D-ribofuranosyl-benzyladenine (9R-BA) or 9-ß-glucopyranosyl-benzyladenine (9G-BA) was monitored. BA and extremely large quantities of 9G-BA were exclusively located in the basal part of the plant (callus and meristems). 9R-BA was found in the basal part, the petioles and the leaf blades. After an acclimatisation period of 9 weeks the plants still contained high levels of 9G-BA, but BA and 9R-BA could no longer be detected after one week. The possible role of BA and its derivatives on inhibition of root initiation or irreversible chloroplast deficiency is discussed.

High-pressure liquid chromatographic analysis of aniline and its metabolites.

A high-pressure liquid chromatographic method has been developed for the determination of nanomole quantities of aniline; its metabolites o- and p-aminophenol, phenylhydroxylamine, nitrosobenzene and nitrobenzene; and azobenzene and azoxybenzene which form non-enzymatically by condensation of reactive metabolites. These compounds were separated by reverse-phase chromatography (mu-Bondapak C18 column) and detected spectrophotometrically. The first four components were eluted using methanol-water (15:85) containing 0.26 M ammonium acetate and 0.015 M nickel acetate as mobile phase. The remaining compounds were eluted with methanol-water (50:50). The stabilities of the metabolites were studied electrochemically and results were used in the development of the chromatographic system.