Activation of nucleoli in tuber slices and the function of nucleolar vacuoles

rRNA intermediates coordinate the formation of nucleolar vacuoles in C. elegans

The nucleolus is the most prominent membraneless organelle within the nucleus. How the nucleolar structure is regulated is poorly understood. Here, we identified two types of nucleoli in C. elegans. Type I nucleoli are spherical and do not have visible nucleolar vacuoles (NoVs), and rRNA transcription and processing factors are evenly distributed throughout the nucleolus. Type II nucleoli contain vacuoles, and rRNA transcription and processing factors exclusively accumulate in the periphery rim. The NoV contains nucleoplasmic proteins and is capable of exchanging contents with the nucleoplasm. The high-order structure of the nucleolus is dynamically regulated in C. elegans. Faithful rRNA processing is important to prohibit NoVs. The depletion of 27SA2 rRNA processing factors resulted in NoV formation. The inhibition of RNA polymerase I (RNAPI) transcription and depletion of two conserved nucleolar factors, nucleolin and fibrillarin, prohibits the formation of NoVs. This finding provides a mechanism to coordinate structure maintenance and gene expression.

Ethylene is a selective ribosomal cistron regulator in Allium cepa epidermal cells

In Allium cepa L. (onion) the number, size, and morphology of visible nucleoli per nucleus varies during cell division, growth, differentiation, storage, activation of quiescent tissue, senescence, wounding, and disease (host-pathogen interactions). Since there are two types (differentiated visually by size) of ribosomal cistrons in onion (major and minor nucleoli differ in the external spacer nucleotide sequences), we inferred that they function separately. In controls, major nucleolar organizer regions (NORs) were visible in epidermal cell nuclei as nucleoli. After exposure of the tissue to ambient conditions, these nucleoli enlarged and changed morphology. Minor NORs (when in the genome) required more than 6 h to become developed as visible nucleoli. In the ethylene treatments, the major NORs (visible nucleoli in quiescent epidermal cells) increased in size a (activation of a greater number of tandem rRNA genes) as in the controls. The minor, quiescent NORs became active and visible (activation of quiescent, inactive NORs) within 3 h. Actinomycin D, and cycloheximide, with or without ethylene, inhibited the increase in size of the major NORs, and prevented activation of the minor NORs (i.e. they did not become visible). Silver nitrate and cobalt chloride had no effect on major NORs but inhibited the appearance of minor nucleoli when combined with ethylene treatments. We infer: ethylene acts to regulate the expression of the minor NORs (selective ribosomal cistron regulation); and, other hormone(s) are involved in the regulation of the major NORs. Yellow and white sweet Spanish onions we studied had two major NORs and two minor NORs. Red sweet Spanish onions we studied had only the two major NORs.

Age-dependent variations in transcriptional response to wounding and gibberellic acid in a higher plant

The reaction of potato tuber tissue upon wounding and gibberellic acid (GA3) treatment is strictly dependent on the tuber age. Young, rapidly growing tubers decline both chromatin-bound, DNA-dependent RNA polymerase (EC 2.7.7.6) activity and template availability as a consequence of wounding and are not responsive toward GA3. At the onset of dormancy, ripened tubers do not at all respond to wounding or hormones, but later on develop the ability to increase their transcriptional rate and template accessibility, both after injury and treatment with 10(-7) mol l(-1) GA3. The size of the nucleolus and the rRNA content of the ribosomal population follows the same pattern.

Effect of hormones on nucleolar growth and vacuolation in elongating cotton fibers

A rather precise combination of the three phytohormones, gibberellic acid, auxin, and abscisic acid, is necessary for the considerable growth of fiber nucleoli in Gossypium hirsutum L. for about 8 days after anthesis and for nucleolar "vacuolation" in the second half of that period. Nucleolar growth and vacuolation must occur in a precise sequence for the fiber to reach a good final length. Nucleolar vacuolation indicates simultaneous output and neosynthesis of nucleolar material.

Activation of ribosomal and messenger RNA synthesis in excised Jerusalem artichoke tuber slices

RNA synthesis was studied in Jerusalem artichoke (Helianthus tuberosus L.) tuber slices immediately following excision and during the early period of aging in water. Incorporation of [(3)H]adenosine into RNA was detected as early as 20 min after excision. Measurement of the specific activities of RNA (cpm/μg) and of ATP showed that RNA synthesis proceeded at a constant rate for the first several hours of aging and then increased moderately. [(3)H]adenosine was incorporated into polysomes throughout the aging period examined. Sucrose gradient fractionation of EDTA-dissociated polysomes showed that during the first 2 h of aging most of this incorporation was not into ribosome subunits but into presumed mRNA. Autoradiographic analysis of [(3)H]adenosine labelled nuclei showed that this was caused, at least in part, by a delay in the onset of rRNA synthesis synthesized during this time chromatographed as poly(A)-RNA on oligo(dT)-cellulose, indicating that a large part of the mRNA was not polyadenylated.

Ribosome metabolism in hormone-treated Jerusalem artichoke tuber slices in the absence and presence of 5-fluorouracil

In order to examine the relation of protein synthesis to the onset of growth, changes in ribosome content and activity were compared in aged, metabolically active Jerusalem artichoke (Helianthus tuberosus L.) slices incubated in water or 2,4-dichlorophenoxyacetic acid+kinetin. In water, cells do not grow or divide and rRNA and protein levels remain constant. The percentage membrane-bound (mb) ribosomes drops from 25% to 16% during 24h. At the same time the proportion of ribosomes active in protein synthesis in both free and mb populations declines from about 69% to 54%. In auxin+kinetin, cell expansion occurs and is accompanied by a 3-fold increase in rRNA and a 50% increase in total protein content. The percentage mb ribosomes remains at 25% throughout 48 h of growth. During the first 24h of growth 70% of ribosomes in both free and mb populations are active; this value declines to near water levels at 48 h. Considering the large increase in total ribosomes the number of synthetically active ribosomes is substantially increased during growth. 5-Fluorouracil (5-FU) does not inhibit hormone induced growth but does depress total rRNA content by about one-third. It also reduces [(3)H]uridine incorporation into ribosomes by 70% and the newly made ribosomes are mostly inactive in protein synthesis. On the other hand, the inhibitor does not significantly affect the proportion of total ribosomes active in protein synthesis and only partially reduces protein accumulation during the second 24 h of growth. It is suggested that while ribosome production is reduced in 5-FU, ribosome turnover is also retarded resulting in retention of near normal capacity for protein synthesis and growth.

Autoradiographic and ultrastructural study of Cucurbita pepo root cells during their growth and differentiation

Cortex cells of the root meristem of Cucurbita pepo (0.0-0.5 mm from the cap junction), in the 3-4, 5-6 and 7-8 mm segments above the root tip, and the cells of the first three layers of lateral part of root cap were the object of the present study. The volume of cortex cells increases more than 20 times in the 7-8 mm segment as compared with meristematic cells, and the volume of cytoplasm about sevenfold. The largest increment of the cytoplasmic volume occurs between 0.5-6.0 mm. In consecutive root segments the sustained increase of the volume of nuclei takes place. By applying autoradiography the following processess have been investigated: DNA synthesis (3H thymidine uptake), template activity of DNA (3H actinomycin D(3H AMD)-binding), RNA synthesis (3H uridine incorporation), and protein synthesis (3H leucine). In the root cap cells and in segments where meristematic activity is over, DNA is replicated by endomitosis. On the basis of nuclear labelling it appears that nuclei in the 3-4 mm segment reach 4C ploidy state, but in the 7-8 mm segment half of the nuclei reach the 8C ploidy state. Most of the root cap cells are 4C, the remaining cells are 8C. Considering the uptake of 3H thymidine into nucleoli one may suppose that in the root cap cells nucleolar DNA is underreplicated, and to a lesser degree in 5-6 and 7-8 mm segments, while in 3-4 mm segment DNA is overreplicated as compared to meristem cells. Measurements of nucleolar volume, 3H uridine uptake, 3H AMD binding and quantity of granular component, indicate that the most noticeable nucleolar activity takes place in meristematic zone and in root parts showing the highest increase of cytoplasmic volume (3-4 and 5-6 mm segments). 3H leucine is still incorporated intensely into 7-8 mm segment, in which the concentration of ribosomes is low, however they are present in the form of polysomes. Comparison of 3H thymidine uptake into nuclear DNA with 3H AMD binding and 3H uridine incorporation into nuclei indicates that endomitotic DNA replication results in an increase of DNA template activity in root cap cells as well as in 3-4 and 5-6 mm segments; in the 7-8 mm segment binding of 3H AMD slightly decreases, while 3H uridine incorporation is considerably reduced. Divergence between the ploidy state, 3H AMD binding and 3H uridine incorporation can be due to the increment of the condensed chromatin area in differentiated cells. Plastids and mitochondria reach full maturity in 3-4 mm segment. The increasing volume density of ER and diminishing volume density of Golgi structures is accompanied by differentiation of cortex cells.

Metabolism of free and membrane-bound ribosomes during aging of Jerusalem artichoke tuber slices

Changes in ribosomes of artichoke (Helianthus tuberosus L.) tuber cells following excision and aging of tissue slices in water were studied using biochemical techniques. During the first 2 h of aging total rRNA dropped 28% and then remained constant for a subsequent 46 h. Since ribosome synthesis occurs through at least the first 24 h of aging, turnover of ribosomes must take place in this period. Cells of the dormant tuber gave essentially no membrane-bound (mb) ribosomes. On aging, the mb ribosome fraction rose and reached a maximum of 25% of total ribosomes at 24 h. Density gradient analysis showed that the ribosomes of dormant cells were present largely as monosomes. After 4 h aging a significant number of ribosomes in both free and mb populations sedimented as polysomes and the number of polysomes in both populations increased to a maximum at 24 h. The direct polysome analysis was confirmed by estimates of synthetically "active" ribosomes obtained using 0.8 M KCl to isolate monosomes carrying nascent polypeptides. This approach showed that while unaged cells had only 13% of total ribosomes active, on aging the active fraction rose to about 68% at 24 h. Both free and mb populations showed the same percentage of ribosomes active at all times studied. [(3)H]uridine showed significant incorporation into ribosomes during three periods studied; 2-4h, 12-14h, 22-24h. At the two latter periods the specific activity of the free ribosomes was greater than that of the mb ribosomes. Uridine was incorporated into both active and inactive ribosomes of both populations, judged by KCl fractionation, with the inactive fraction having greater specific activity in both cases. These differences in labelling possibly result from relatively slow mixing of different ribosome populations. Uptake of soluble [(3)H]uridine into the tissue increased 4-fold between 4 h and 14 h accounting at least in part for greater overall specific activity of ribosomes at later aging times.