Tova Arzee

Bursts of incorporation into RNA and ribonuclease activities associated with induction of morphogenesis inPharbitis

Summary Uridine-5- 3 H incorporation into the macromolecules of the apices of Pharbitis nil , Chois, cv. Violet, a species requiring an induction of a single dark period of about 14 hours in order to flower, was measured. Uridine incorporation was greater in plants induced to flower: The two peaks with the maximum rate of incorporation occurred at 14 and 17 hours after the beginning of the dark period. If the dark period was broken by a short illumination, flowering did not occur and uridine incorporation was reduced almost to the control level. RNase activity was higher in the apices of induced plants than in the controls, with a peak activity between the peaks of uridine incorporation, followed by a second increase after the second peak of uridine activity. Methods were devised to extract RNA in the presence of these increased levels of RNase. Fractionation on polyacrylamide gels showed no great differences in the pattern of incorporation during the first period of maximum incorporation. There was greatly increased incorporation into RNA in plants induced to flower. This increase seemed to be the same for rRNA, sRNA, and possibly the relatively non-UV-absorbing (high specific activity) RNA.

PHOTOINDUCTION OF PHARBITIS FLOWERING: RELATIONSHIP TO RNA SYNTHESIS AND OTHER METABOLIC EVENTS

We have previously shown that a wave of enhanced uridine incorporation into RNA occurs in the more vegetative parts of the plumule at the end of the single dark period that evokes flowering in Pharbitis nil. We demonstrate here that a light break that suppresses flowering suppresses this wave as well. It does not shift the kinetics of the wave of uridine incorporation to a different time. The enhanced incorporation is into all RNA fractions.

Abscission in the Tumbleweed Kochia indica: Ethylene, Cellulase, and Anatomical Structure

The transition region between the root and the stem is the site of abscission in the tumbleweed Kochia indica. A comparison of the stem, transition region, and root with regard to their rate of ethylene emanation, cellulase activity, and anatomical features showed that, prior to abscission, high rates of ethylene evolution and cellulase activity exist in the root and the transition region but not in the stem. An extended lag period of 35 days was found between the maxima of ethylene emanation and cellulase activity. Both in the root and the transition region, parenchyma is more abundant than in the stem, but in the transition region the vascular rays are interlocked and form a network of parenchymatous tissue across the organ. We suggest that ethylene induces cellulase activity, probably through intermediate steps, which in turn dissolves the walls of the parenchyma cells. In the transition region this causes abscission, while in the root this causes only softening.

Morphological patterns and heterogeneity in populations of duckweeds

Morphological variation among several populations from the ‘Lemna gibba—Lemna minor complex’, were studied quantitatively in steady-state, light-grown axenic cultures. Although some of the features were modified, consistent differences among clones were maintained. Morphological patterns based on frond length and width, localization of daughter fronds, root length and colony size, can be used to characterize such clones. Morphological patterns seen in Spirodela clones in culture indicate that the idea of a ‘complex’ based on frond size and root number may apply equally well to this genus.

Thymidine incorporation into Pharbitis DNA: I. Adenosine effects on degradation by the thymidine phosphorylase system

Abstract Much of the 3H-methyl thymidine supplied to Pharbitis is degraded by thymidine phosphorylase (E.C.2.4.2.4.) to thymine and by other enzymes to β-amino isobutyric acid and presumably further to CO2 + water. At a certain time, adenosine application partially prevents this degradation at the level of thymidine to thymine, and also thymine to β-amino isobutyric acid.

Thymidine incorporation into Pharbitis DNA: II. Adenosine enhancement in vivo; total incorporation versus autoradiography

The rate of 3H-methyl thymidine incorporation increased in two waves following floral induction in Pharbitis. Adenosine only enhanced thymidine incorporation at non-peak times. This enhancement was studied by autoradiography and found to be both nuclear and cytoplasmic. Our interpretation of the results is that there was increased thymidine labeling of nuclei already undergoing DNA synthesis and not an increase of cells in S phase. Various vagaries and possible artifacts in the use of thymidine to measure DNA synthesis quantitatively are pointed out.

INHIBITION OF POLYAMINE BIOSYNTHESIS BY L-CANAVANINE AND ITS EFFECT ON MERISTEMATIC ACTIVITY, GROWTH, AND DEVELOPMENT OF ZEA MAYS ROOTS

ABSTRACT The effects of L-canavanine, a structural analogue of L-arginine and a competitive inhibitor of arginine decarboxylase, were studied in corn (Zea mays L.) on primary root elongation and lateral root differentiation, meristematic activity, and on polyamine content and biosynthesis. Incubation of seedlings in 10−3 M L-canavanine for 3 h inhibited both elongation of the primary root and differentiation of lateral root primordia. A marked decrease in cell division and DNA synthesis, along with an inhibition of the activation of the primary root quiescent center, was clearly evident in L-canavanine-treated roots. Putrescine content in subapical segments of the primary root decreased significantly during the first 24 h, while spermidine content increased at 48–72 h, coinciding with lateral root emergence and elongation. A 3 h incubation with 103 M L-canavanine completely inhibited the increase in spermidine content, thus modifying the ratio of putrescine to spermidine tissue concentrations. The activit...

Twofold Pathways of Apical Determination in the Thorn System of Carissa grandiflora

Morphological and anatomical analyses of Carissa grandiflora show that each pair of thorny branches is homologous to an inflorescence. We suggest that, in this species, floral evocation involves more than a single step and that incomplete induction can lead to apical arrest or the formation of a thorn. These alternative pathways of morphogenesis may account for the variability in cyme development.

A negative image of the quiescent centre in regenerating root apices of Zea mays

Usually the presence of the quiescent centre in roots is demonstrated by the absence of labelled nuclei following treatment of the root with appropriate radioactive markers. By modification of the pulselabelling technique, a “negative” image of the quiescent center, showing more intense labelling from [3H]thymidine than the surrounding area, was obtained in regenerating root apices of Zea mays L.

Ontogeny of Periderm and Phellogen Activity in Ceratonia siliqua L.

In the shoots of Ceratonia siliqua, except for the hypocotyl and the first internode, a continuous superficial phellogen and complete cork cover are established only after a prolonged period of growth has elapsed. The initiation of sequent periderms is restricted to tree parts estimated to be more than 40 yr old. Microautoradiographs indicate year-round phellogen activity, which supports the idea that the carob tree is most likely tropical in origin.