Ezh2-dependent H3K27me3 modification dynamically regulates vitamin D3-dependent epigenetic control of CYP24A1 gene expression in osteoblastic cells

Epigenetic control is critical for the regulation of gene transcription in mammalian cells. Among the most important epigenetic mechanisms are those associated with posttranslational modifications of chromosomal histone proteins, which modulate chromatin structure and increased accessibility of promoter regulatory elements for competency to support transcription. A critical histone mark is trimethylation of histone H3 at lysine residue 27 (H3K27me3), which is mediated by Ezh2, the catalytic subunit of the polycomb group complex PRC2 to repress transcription. Treatment of cells with the active vitamin D metabolite 1,25(OH)₂ D₃ , results in transcriptional activation of the CYP24A1 gene, which encodes a 24-hydroxylase enzyme, that is, essential for physiological control of vitamin D3 levels. We report that the Ezh2-mediated deposition of H3K27me3 at the CYP24A1 gene promoter is a requisite regulatory component during transcriptional silencing of this gene in osteoblastic cells in the absence of 1,25(OH)₂ D₃ . 1,25(OH)₂ D₃ dependent transcriptional activation of the CYP24A1 gene is accompanied by a rapid release of Ezh2 from the promoter, together with the binding of the H3K27me3-specific demethylase Utx/Kdm6a and thereby subsequent erasing of the H3K27me3 mark. Importantly, we find that these changes in H3K27me3 enrichment at the CYP24A1 gene promoter are highly dynamic, as this modification is rapidly reacquired following the withdrawal of 1,25(OH)₂ D₃ .

Identification of a novel long noncoding RNA that promotes osteoblast differentiation

Long noncoding RNAs (lncRNAs) are a heterogeneous class of transcripts, longer than 200 nucleotides, 5'-capped, polyadenylated, and poorly conserved among mammalian species. Several studies have shown the contribution of lncRNAs to different cellular processes, including regulation of the chromatin structure, control of messenger RNA translation, regulation of gene transcription, regulation of embryonic pluripotency, and differentiation. Although limited numbers of functional lncRNAs have been identified so far, the immense regulatory potential of these RNAs is already evident, indicating that a functional characterization of lncRNAs is needed. In this study, mouse preosteoblastic cells were induced to differentiate into osteoblasts. At 3 sequential differentiation stages, total RNA was isolated and libraries were constructed for Illumina sequencing. The resulting sequences were aligned and transcript abundances were determined. New lncRNA candidates that displayed differential expression patterns during osteoblast differentiation were identified by combining bioinformatics and reverse transcription polymerase chain reaction analyses. Among these, lncRNA-1 that exhibited increased expression during osteogenesis and was downregulated during myogenesis. Importantly, knockdown of lncRNA-1 expression in primary mouse preosteoblasts was found to inhibit osteogenic differentiation, reflected by a reduced transcription of the Runx2/p57 and Sp7 bone master genes. Together, our results indicate that lncRNA-1 represents a new regulatory RNA that plays a relevant role during the early stages of osteogenesis.

CHACO outreach project: the development of a primary health care-based medical genetic service in an Argentinean province

Dissemination of knowledge in genetics to be applied in medicine has created a growing need for capacity building in health care workers. The CAPABILITY ARGENTINA outreach project protocol was designed as a model to introduce genetics in areas without genetic services. Our aim was for genetic health care to become part of primary care in an Argentine province lacking genetic services. The program was innovative as professionals from the referral center (Garrahan Hospital S.A.M.I.C.) traveled to remote areas to train professionals through problem-based education. A logical framework was designed for a local needs assessment. Teaching materials (Powerpoint presentations, printed syllabus, and CD) and a web page were developed. A demonstration project was carried out in the Province of Chaco, Argentina. A total of 485 health workers were trained. The number of consultations increased significantly in participating areas comparing before and after the training period. To support this increase, a complementary project was set up from a public hospital sponsored from within Argentina to build a cytogenetic laboratory in the capital of the Province of Chaco. The model was improved for reproduction in other areas in Argentina. CAPABILITY ARGENTINA is a capacity building model for training of primary care professionals in genetics that may be applied to other medical specialties. The outcomes of the programme have a direct impact on clinical practice.

Maize leaf elongation: continuous measurements and close dependence on plant water status

A simple method was developed for measuring extensive intact leaves of monocots on a minute-by-minute basis. Growth was markedly reduced by a slight reduction in leaf water potential. When plants mildly deficient in water were irrigated, growth resumed virtually instantly. The transitional rapid growth aftr watering suggests that water deficit increased cell extensibility.

[Vibrio cholerae sepsis in the neonate]

Vibrio cholerae sepsis is infrequent, especially in neonates although sporadic cases have been reported in older patients. We report the case of a neonate who was admitted to the intensive care unit for hypovolemic shock secondary to diarrhea caused by V. cholerae that developed into bacteremia. The predisposing factors were low socioeconomic status, home delivery, delayed presentation at the health center, and active maternal gastrointestinal infection with V. cholerae. The organism identified in blood and feces culture was identified as V. cholerae 0 -1, biotype Thor, serotype Ogawa, which correlated with the clinical presentation.

Tumour necrosis factor microsatellites and HLA-DRB1*, HLA-DQA1*, and HLA-DQB1* alleles in Peruvian patients with rheumatoid arthritis

OBJECTIVE

To study the association between rheumatoid arthritis (RA) and HLA and tumour necrosis factor (TNF) polymorphism in Peruvian mestizo patients in comparison with ethnically similar controls.

METHODS

Seventy nine patients with RA and 65 ethnically matched healthy controls were genotyped for HLA-DRB1, HLA-DQA1, HLA-DQB1, and TNFalpha and TNFbeta alleles using PCR amplification. Clinical severity was assessed as mild, moderate, or severe in 35 of the patients.

RESULTS

TNFalpha6 showed the strongest association with disease susceptibility. The TNFalpha6 allele was more common in patients than in controls (p<0.0076) and the proportion of patients with at least one copy of this allele was greater (p<0.015, relative risk 2.35). Among the HLA-DRB1* alleles with the shared epitope sequence, only the DRB1*1402 allele was significantly increased in patients compared with controls (p<0.0311), as was the proportion of patients with at least one copy of this allele (p<0.0232, relative risk 2.74). In contrast, the overall frequency of alleles with the shared epitope was not different in patients and controls. The haplotype HLA-DRB1*1402-DQB1*0301-DQA1*0401 was significantly more common in patients. TNFalpha6 was more common in patients whether or not they had this haplotype. None of the 11 patients lacking the TNFalpha6 allele had severe disease.

CONCLUSIONS

This study shows for the first time that TNF gene polymorphism is associated with susceptibility to RA in a non-white population. TNFalpha6 and HLA-DRB1*1402 independently conferred significantly increased risk in Peruvian mestizo patients.

Tolerability profiles of rofecoxib (Vioxx) and Arthrotec. A comparison of six weeks treatment in patients with osteoarthritis

OBJECTIVE

To compare the incidence of selected spontaneously reported adverse events (AEs) in patients with osteoarthritis (OA) treated with rofecoxib (VIOXX, 12.5 mg qd) or Arthrotec (diclofenac 50 mg/misoprostol 200 mcg bid).

METHODS

Double-blind, parallel-group, 6-week study of patients aged > or = 40 years with a clinical diagnosis of OA treated with rofecoxib or Arthrotec. Primary endpoint: self-reported diarrhea; secondary endpoints: abdominal pain, discontinuations due to AEs, GI AEs and NSAID-type GI AEs (ie., acid reflux, dyspepsia, epigastric discomfort, heartburn, nausea, vomiting).

RESULTS

Among 483 patients (80.3% females, mean age 62.1), the rofecoxib group vs the Arthrotec group respectively reported diarrhea 6.2% vs 16.2% (p<0.001); drug-related diarrhea 3.7% vs 16.2% (p<0.001); one or more clinical AEs 52.9% vs 73.0% (p<0.001); GI AEs 28.9% vs 48.5% (p<0.001); NSAID-type GI AEs 18.6% vs 29.9% (p=0.004); discontinuations due to abdominal pain 0.4% vs 3.7% (p<0.05); and discontinuations due to any AE 4.1% vs 9.1% (p=0.029). No significant differences were observed in efficacy.

CONCLUSION

Rofecoxib 12.5 mg qd has improved GI tolerability and similar efficacy compared to Arthrotec (diclofenac 50 mg/misoprostol 200 mcg bid).

Comparison of the effect of rofecoxib (a cyclooxygenase 2 inhibitor), ibuprofen, and placebo on the gastroduodenal mucosa of patients with osteoarthritis: a randomized, double-blind, placebo-controlled trial. The Rofecoxib Osteoarthritis Endoscopy Multinational Study Group

OBJECTIVE

This randomized, double-blind study tested the hypothesis that rofecoxib, a drug that specifically inhibits cyclooxygenase 2, would cause fewer gastroduodenal ulcers than ibuprofen (in a multicenter trial), and its side effects would be equivalent to those of placebo (in a prespecified analysis combining the results with another trial of identical design).

METHODS

Seven hundred seventy-five patients with osteoarthritis were randomized to receive rofecoxib at a dosage of 25 mg or 50 mg once daily, ibuprofen 800 mg 3 times daily, or placebo. Gastroduodenal ulceration was assessed by endoscopy at 6, 12, and (for active treatment) 24 weeks. The primary and secondary end points were the incidence of gastroduodenal ulcers at 12 and 24 weeks, respectively.

RESULTS

Ulcers were significantly less common (P < 0.001) following treatment with rofecoxib (25 mg or 50 mg) than with ibuprofen after 12 weeks (5.3% and 8.8% versus 29.2%, respectively) or 24 weeks (9.9% and 12.4% versus 46.8%, respectively). In the combined analysis, the 12-week ulcer incidence with 25 mg rofecoxib (4.7%) and with placebo (7.3%) satisfied prespecified criteria for equivalence.

CONCLUSION

At 2-4 times the therapeutically effective dose, rofecoxib caused fewer endoscopically detected ulcers than did ibuprofen. Rofecoxib at a dose of 25 mg (the highest dose recommended for osteoarthritis) satisfied prespecified criteria for equivalence to placebo.

Comparison of the effect of rofecoxib (a cyclooxygenase 2 inhibitor), ibuprofen, and placebo on the gastroduodenal mucosa of patients with osteoarthritis: a randomized, double-blind, placebo-controlled trial. The Rofecoxib Osteoarthritis Endoscopy Multinational Study Group

OBJECTIVE

This randomized, double-blind study tested the hypothesis that rofecoxib, a drug that specifically inhibits cyclooxygenase 2, would cause fewer gastroduodenal ulcers than ibuprofen (in a multicenter trial), and its side effects would be equivalent to those of placebo (in a prespecified analysis combining the results with another trial of identical design).

METHODS

Seven hundred seventy-five patients with osteoarthritis were randomized to receive rofecoxib at a dosage of 25 mg or 50 mg once daily, ibuprofen 800 mg 3 times daily, or placebo. Gastroduodenal ulceration was assessed by endoscopy at 6, 12, and (for active treatment) 24 weeks. The primary and secondary end points were the incidence of gastroduodenal ulcers at 12 and 24 weeks, respectively.

RESULTS

Ulcers were significantly less common (P < 0.001) following treatment with rofecoxib (25 mg or 50 mg) than with ibuprofen after 12 weeks (5.3% and 8.8% versus 29.2%, respectively) or 24 weeks (9.9% and 12.4% versus 46.8%, respectively). In the combined analysis, the 12-week ulcer incidence with 25 mg rofecoxib (4.7%) and with placebo (7.3%) satisfied prespecified criteria for equivalence.

CONCLUSION

At 2-4 times the therapeutically effective dose, rofecoxib caused fewer endoscopically detected ulcers than did ibuprofen. Rofecoxib at a dose of 25 mg (the highest dose recommended for osteoarthritis) satisfied prespecified criteria for equivalence to placebo.

[Standardization of ELISA IgM and IgA for immunodiagnosis of human trichinosis]

An ELISA test for trichinosis using as antigen a larvae soluble fraction from Trichinella spiralis was carried out for the detection of IgM and IgA specific antibodies in 45 serum samples from patients confirmed or suspected to have trichinosis by strong clinical and epidemiological evidences. All the patients had positive serology detected by precipitin test, bentonite floculation test, indirect hemagglutination test and ELISA IgG test. The cut-off value was determined using two criteria. Criterion A was determined in each plate, using three positive controls and two negative ones; the average of the negative controls and the weakest positive control, multiplied by a 1.2 factor was, considered the cut-off value. Criterion B was determined using the average plus three standard deviations from 64 apparently healthy persons serum samples. In both cases, three serum dilutions (1:10, 1:100 and 1:500) were used. The sensitivity of ELISA IgM was 100.0, 93.3 and 82.2% using serum dilutions of 1:10, 1:100 and 1:500 respectively (criterion A) and 100.0, 97.8 and 95.6% for the same dilutions (criterion B), whereas the values for ELISA IgA were: 100.0, 91.1 and 86.7% (criterion A) and 100.0, 100.0 and 91.1% (criterion B). In order to find out the specificity of ELISA IgM and ELISA IgA, additional 118 serum samples from individuals with other parasitoses, such as cysticercosis (18) hydatidosis (39), fascioliasis (12), toxocariasis (30), Chagas' disease (12) and individuals with non-specific eosinophilia (7), were also tested. ELISA IgM presented a specificity of 92.3, 93.4 and 97.3% (criterion A) and 96.2, 97.8 and 97.8% (criterion B) whereas the results for ELISA IgA were 97.8, 98.9 and 99.4% (criterion A) and 98.4% for the 1:10 and 1:100 dilutions and 100.0% for the 1:500 dilution (criterion B). The positive predictive values of ELISA IgM were 76.3, 77.8 and 88.1% (criterion A) and 86.5, 91.7 and 91.5% (criterion B) whereas the negative ones were 100.0, 98.3 and 95.7% (criterion A) and 100.0, 99.4 and 98.9% (criterion B). The positive predictive values of ELISA IgA were 91.8, 95.3 and 97.5% (criterion A) and 93.8, 93.8 and 100.0% (criterion B) whereas the negatives ones were: 100.0, 97.8 and 96.8% (criterion A) and 100.0, 100.0 and 97.8% (criterion B). The use of ELISA IgM and ELISA IgA in the immunodiagnosis of trichinosis is discussed.

Effects of antihypertensive treatment on cardiac IGF-1 during prevention of ventricular hypertrophy in the rat

There is some evidence that cardiac rather than circulating insulin-like growth factor-1 (IGF-1) levels contribute to the development of renovascular hypertensive left ventricular hypertrophy (LVH), remaining unknown the effects of antihypertensive drugs on IGF-1 levels. We have assessed here the preventive effects of enalapril, losartan, propanolol and alpha-methyldopa on left ventricle (LV) and circulating IGF-1 levels in a rat model of hypertension and LVH (Goldblatt, GB). Our results show that relative LV mass and the LV content of IGF-1 were significantly lower with all antihypertensive drugs in GB rats (p<0.001). Serum concentrations of IGF-1 were lower in GB rats treated with enalapril, alpha-methyldopa and propanolol (p<0.01), but not in those treated with losartan. These results support the hypothesis that local rather than seric IGF-1 contributes to the development of left ventricular hypertrophy induced by pressure overload in the rat.

Selective increase in cardiac IGF-1 in a rat model of ventricular hypertrophy

There is evidence that insulin-like growth factor-1 (IGF-1) plays a role in the development of left ventricular hypertrophy, but it is uncertain whether cardiac IGF-1 changes before or after hypertension is established, and whether circulating IGF-1 are involved in cardiac hypertrophy. We have investigated changes in circulating and left ventricular IGF-1 and in the expression of the IGF-1 gene in the left ventricles of rats during the development of hypertensive left ventricular hypertrophy (Goldblatt model; 2 kidney-1 clamped). Our results show that the left ventricular contents of IGF-1 and its mRNA were increased at one and four weeks of hypertension and hypertrophy, and that both returned to control values after nine weeks. These changes were unrelated to the seric concentration of IGF-1 in the blood. These results show that local rather than circulating IGF-1 levels contributed to the development of renovascular hypertensive left ventricular hypertrophy.

Changes in dietary fiber content and its composition as affected by processing of black beans (Phaseolus vulgaris, Tamazulapa variety)

This paper presents the effect that the traditional cooking process of black beans (Phaseolus vulgaris, Tamazulapa variety) has on the quantity and composition of soluble (SDF) and insoluble (IDF) dietary fiber of beans, as well as on its protein digestibility and protein quality. There was an increase of IDF from 18.1% in cooked beans to 22.4% in fried beans, and a decrease in SDF from 8.4% to 6.6%, respectively. Starch content decreased from 34.5% to 31.3%. No change was found in lignin. The xylose content was higher in IDF than in SDF and decreased to some extent from cooked to fried beans. Arabinose content was similar in IDF and SDF with no change caused by processing. The fraction containing glucose, mannose and galactose in IDF was higher than in SDF, the content increasing in IDF and decreasing in SDF, with processing. Protein content in IDF was higher than in SDF, with no major change when processing. About 29.5% of the total protein of beans was bound in DF. Protein digestibility and protein quality decreased from cooked to fried beans and was positively related to IDF.

[Effect of reheating on iron availability and the protein nutritive value of cooked black bean (Phaseolus vulgaris)]

In Guatemala, at the rural level, beans are prepared for family consumption every two days. Beans are cooked again every 12 hours for 15 minutes prior to their consumption. Due to the adverse effect that the process causes on nutritive value, the present study evaluated the method and preparation practice of beans on protein value; tannin and soluble and ionizable iron content. As to the effect of re-cooking on protein level, findings revealed that there were no significant statistical effects on net protein ratio (NPR) or in protein efficiency ratio (PER) and number of bean cooking. Nevertheless, a constant effect in quality in the first and second cooking procedure was detected. The second cooking gave a similar value as the first. Also, the first and second heating reduced protein digestibility, but the third gave a value similar to the initial one. No changes were observed in the digestibility of dry matter. Relatively high amounts of protein were found in the cooking broth, which was not affected by the number of heating. It was also found that the bean broth contained high levels of tannins in the initial heating, decreasing later significantly. The same was observed in the cooked beans alone and in beans with their broth. With respect to iron, findings showed a relatively high transference of beans to broth, in total iron as well as in soluble, ionizable and insoluble iron. No changes were observed in whole beans without broth caused by number of heatings, on total, soluble or ionizable iron. In beans with their broth, a similar effect was observed, although a slight increase in insoluble iron was detected at the end of heating. A decrease in soluble and insoluble iron was observed in broth with number of heatings. Correlations were calculated among the parameters studied, which suggested an effect of tannins on the bioavailability of iron caused by tannins.

[Dietary fiber content and nitrogen digestibility in Central American foods: Guatemala]

The study herein reported presents information on the dietary fiber content of four food groups consumed in Central America. These are: cereals, grains and products; raw and processed beans; raw and processed vegetables, and starchy foods such as potatoes, cassava and plantain. Besides data on soluble and insoluble fiber, data on in vitro protein digestibility are included. The total dietary fiber content of the wheat flour products varied from 1.62 to 2.83% on a fresh basis, with the exception of whole-wheat bread, which showed a 7.57% content. The maize tortilla presented values ranging from 3.96 to 5.21% in respect to beans, and the values for cooked and raw beans fluctuated between 6.36 and 7.00%, independent of the color; however, fried beans reported values from 15.28 to 17.58%. Vegetables contained total dietary fiber values of 1.51 to 4.34, and the tubers, from 1.31 to 2.86%.

[Intake of dietary fiber in the Central American isthmus: nutritional implications]

Mean intakes of dietary fiber (DF) were estimated in the population of Central America and Panama, using the results of dietary surveys conducted in 1969 and 1986 both in rural and urban areas, as well as data on the DF content of foods as consumed in the region. Data on preschool children were also estimated. The results indicated that DF intake in urban areas is lower than that of rural areas, particularly in Costa Rica and Panama. In 1969, intake varied from 32g in El Salvador to 15g in Panama in urban areas, while in rural areas intake was from 45g in El Salvador to 13g in Panama. The foods which contributed most to the total intake in Guatemala, El Salvador and Honduras were tortillas and beans, while in Costa Rica and Panama, beans provided the largest intake. In preeschool children, intake was 12.5g in El Salvador and 5.4g in Costa Rica in 1969, which is the same tendency as that found for adults. From more recent data on food intake, it was found that DF intake had decreased in the rural areas of El Salvador, Honduras and Costa Rica, being between 4 and 9% in the first two countries and 12% in Costa Rica. In the urban area of the latter, from 1969 to 1986 a decrease in DF intake of around 20% has taken place.

Water Relations, Diurnal Acidity Changes, and Productivity of a Cultivated Cactus, Opuntia ficus-indica

Physiological responses of the Crassulacean acid metabolism (CAM) plant Opuntia ficus-indica (Cactaceae) were studied on a commercial plantation in central Chile. Young cladodes (flattened stems) and flower buds exhibited daytime stomatal opening, whereas mature cladodes and fruit exhibited the nocturnal stomatal opening characteristic of CAM plants. Severe water stress suppressed the nocturnal stomatal opening by mature cladodes, but their high water vapor conductance occurring near dawn was not affected. Nocturnal acidity increases were not as sensitive to water stress as was the nocturnal stomatal opening. The magnitude of the nocturnal acidity increases depended on the total daily photosynthetically active radiation (PAR), being 90% PAR-saturated at 27 moles per square meter per day for a mean nighttime air temperature of 5 degrees C and at 20 moles per square meter per day for 18 degrees C. Inasmuch as the PAR received on unshaded vertical surfaces averaged about 21 moles per square meter per day, nocturnal acidity increases by the cladodes were on the verge of being PAR-limited in the field. The net assimilation rate, which was positive throughout the year, annually averaged 3.4 grams per square meter per day for 1.0- and 2.0-year-old plants. Plants that were 5.4 years old had 7.2 square meters of cladode surface area (both sides) and an annual dry weight productivity of 13 megagrams (metric tons) per hectare per year when their ground cover was 32%. This substantial productivity for a CAM plant was accompanied by the highest nocturnal acidity increase so far observed in the field, 0.78 mole H(+) per square meter.

Model for the interaction of membrane-bound substrates and enzymes. Hydrolysis of ganglioside GD1a by sialidase of neuronal membranes isolated from calf brain

Microsomes and synaptosomes isolated from calf brain contain a sialidase which cleaves ganglioside substrates. The hydrolysis of [3H]ganglioside GD1a by the membrane-bound enzyme has been studied under various conditions. The reaction rate decreased with increasing ionic strength in the incubation mixture, and was progressively enhanced by increasing concentrations of the primary alcohols n-pentanol to n-octanol. This stimulation correlates quantitatively with an increase in membrane 'fluidity' caused by these alcohols as measured by fluorescence depolarization employing 1,6-diphenyl-1,3,5-hexatriene as probe. The dependence of the reaction rate on the amount of enzyme in the incubation mixture was linear only with water-soluble substrates but not with the lipophilic ganglioside substrate. Evidence is presented that lipophilic substrate and enzyme interact mainly within the plane of the membrane presumably by lateral diffusion. Taking this into consideration Michaelis-Menten theory was modified accordingly. As predicted, apparent Km values increased linearly with the amount of membrane-bound enzyme added and decreased with the concentration of n-hexanol in the incubation mixture. In the presence of varying n-hexanol concentrations the apparent Km-value decreased with increasing membrane 'fluidity', as measured by fluorescence depolarization of 1,6-diphenyl-1,3,5-hexatriene. On the other hand, as expected, V values were not affected by membrane 'fluidity' and increased linearly with the amount of membrane protein.

Diurnal growth trends, water potential, and osmotic adjustment of maize and sorghum leaves in the field

The daily cycle of leaf elongation rate, water potential, and solute potential of maize and sorghum, as well as temperature, were monitored in the field. Major climatic features were high radiation and a minimum air temperature of about 12 C. Leaf elongation of both crops was slowest at night, presumably because of low temperature. Peak elongation rates were in daytime when leaf water potential (Psi) was low. Solute potential also decreased during daylight, thus permitting the maintenance of appreciable turgor pressure, a critical parameter for cell expansion.Leaf Psi versus relative water content (RWC) curves were developed by sampling detached leaves at intervals as they dried quickly in the laboratory. At a given RWC, Psi was lower in leaves at midday than early in the morning, which is evidence that the decrease in psi. at midday was caused by an increase in the amount of solute in the tissue. Estimates of psi. at 100% RWC were 4 bars lower at midday than early in the morning in both crops. Soluble sugars, mainly nonreducing, accounted for most of the observed psi. decrease in sorghum leaves. Shading the leaves from light eliminated most of the solute buildup.

Immediate and subsequent growth responses of maize leaves to changes in water status

Elongation of intact young leaves of maize was found to be dynamically dependent on soil water supply. With adequate water, elongation was remarkably constant but slowed when the water potential of the soil in pots dropped from -0.1 to -0.2 bar and stopped when it dropped to -2.5 bars. The corresponding range of leaf water potential was -2.8 to -7 bars. Elongation resumed in less than a few seconds after a mildly water-stressed plant was rewatered.The effects on leaf elongation of step-wise changes in water potential of the root solution were determined. When the water potential of the root medium suddenly decreased below 0 bar, growth stopped initially and then resumed at a lower rate. When the water potential was suddenly increased back to 0 bar, growth accelerated transitorily to a high rate before slowing to the steady state rate. These results suggest an increase in cell extensibility during water stress.Leaves stressed for 1 or more days attained after rewatering almost the length of the control leaves. Growth rate after rewatering did not exceed that of the control at the corresponding developmental stage except during the short transitory rapid phase lasting only a fraction of an hour.As stress developed, growth stopped before carbon dioxide assimilation decreased noticeably. Upon the release of mild and short stress, the transitory rapid growth completely made up for the reduced elongation during stress, suggesting that metabolic processes for cell expansion might have proceeded unchecked during the stress period.The sensitivity and rapidity of response to changes in water status all point to the direct role of water in growth; its uptake provides the physical force for cell enlargement.