ABSTRACT
Hypoxic pretreatment of tomato (Lycopersicon esculentum M.) roots induced an acclimation to anoxia. Survival in the absence of oxygen was improved from 10 h to more than 36 h if external sucrose was present. The energy charge value of anoxic tissues increased during the course of hypoxic acclimation, indicating an improvement of energy metabolism. In acclimated roots ethanol was produced immediately after transfer to anoxia and little lactic acid accumulated in the tissues. In nonacclimated roots significant ethanol synthesis occurred after a 1-h lag period, during which time large amounts of lactic acid accumulated in the tissues. Several enzyme activities, including that of alcohol dehydrogenase, lactate dehydrogenase, pyruvate decarboxylase, and sucrose synthase, increased during the hypoxic pretreatment. In contrast to maize, hexokinase activities did not increase and phosphorylation of hexoses was strongly inhibited during anoxia in both kinds of tomato roots. Sucrose, but not glucose or fructose, was able to sustain glycolytic flux via the sucrose synthase pathway and allowed anoxic tolerance of acclimated roots. These results are discussed in relation to cytosolic acidosis and the ability of tomato roots to survive anoxia.
ABSTRACT
Several enzyme activities were measured in extracts from acclimated and nonacclimated maize (Zea mays) root tips at pH 6.5 and 7.5, corresponding to cytoplasmic pH in anaerobiosis or aerobiosis, respectively, to determine what causes the decline of the glycolytic flux observed in anoxia in nonacclimated tips. We found that phosphorylation of hexoses by kinases was a major limiting step of glycolysis in anoxia. When fructose was substituted for glucose, glycolysis was slightly enhanced and survival improved, but neither matched that of acclimated tips. Decrease of kinase activities was not the result of proteolytic degradation but was more likely the result of inhibition by internal factors (low pH and low ATP). There was no evidence of induction during the hypoxic pretreatment of isoenzymes better adapted to the anoxic cellular environment. Maintenance of the glycolytic flux in acclimated tissues is explained by a combination of a rise in kinase activities and decreased inhibition resulting from a higher cytoplasmic pH and ATP content. The behavior of intact root tips is discussed in comparison with the behavior of excised root tips.
ABSTRACT
Hypoxic pretreatment (3 kPa oxygen) of maize (Zea mays L.) root tips improved their survival time in a subsequent anoxic incubation from 10 h to more than 3 d, provided that glucose was added to the medium to sustain metabolism. The glycolytic flux (lactate + ethanol) was the same in both pretreated and untreated root tips during the 1st h after transfer to anoxia. It was only after 2 h that it declined sharply in untreated tips, but was sustained in pretreated ones. Right after the transition from normoxia to anoxia of untreated root tips, the only fermentative product detected was lactic acid, which accumulated in a 7:1 proportion after 30 min in tissue and medium, respectively. It took 10 min before ethanol could be detected and 20 min for it to be produced at its maximum rate at the expense of lactate production, which slowed down. In contrast, in hypoxically pretreated root tips, ethanol was produced at a maximum rate right after the transfer to anoxia. Concurrently, low amounts of lactic acid were produced that accumulated in a 1:1 proportion after 30 min in tissue and medium, respectively. This large efflux of lactic acid could account for the higher cytoplasmic pH values always found in pretreated tissues. The presence of cycloheximide during pretreatment abolished this difference, suggesting that the greater efficiency of lactate efflux was linked to protein synthesis. The role of lactate in cytosolic pH regulation and in sensitivity to anoxia is discussed.
ABSTRACT
Sugar-depleted excised maize (Zea mays L.) root tips were used to study the kinetics and the specificity of hexose uptake. It was found that difficulties induced by bulk diffusion and penetration barriers did not exist with root tips. Several lines of evidence indicate the existence of a complex set of uptake systems for hexoses showing an overall biphasic dependence on external sugar concentrations. The results suggest that the high and the low affinity components might be located on the same carrier. One uptake system was specific for fructose, but the high affinity component was repressed by high concentrations of external glucose. A second system was specific for glucose and its analogs (2-deoxy-d-glucose and 3-O-methyl-d-glucose), and a third one, more complex, had a high affinity for glucose and its analogs but could transport fructose when glucose was not present in the external solution. A simple method is proposed to determine the inhibitor constants in competition experiments.
ABSTRACT
Young intact plants of maize (Zea mays L. cv INRA 508) were exposed to 2 to 4 kilopascals partial pressure oxygen (hypoxic pretreatment) for 18 hours before excision of the 5 millimeter root apex and treatment with strictly anaerobic conditions (anoxia). Hypoxic acclimation gave rise to larger amounts of ATP, to larger ATP/ADP and adenylate energy charge ratios, and to higher rates of ethanol production when excised root tips were subsequently made anaerobic, compared with root tips transferred directly from aerobic to anaerobic media. Improved energy metabolism following hypoxic pretreatment was associated with increased activity of alcohol dehydrogenase (ADH), and induction of ADH-2 isozymes. Roots of Adh1(-) mutant plants lacked constitutive ADH and only slowly produced ethanol when made anaerobic. Those that were hypoxically pretreated acclimated to anoxia with induction of ADH2 and a higher energy metabolism, and a rate of ethanol production comparable to that of nonmutants. All these responses were insensitive to the presence or absence of NO(3) (-). Additionally, the rate of ethanol production was about 50 times greater than the rate of reduction of NO(3) (-) to NO(2) (-). These results indicate that nitrate reductase does not compete effectively with ADH for NADH, or contribute to energy metabolism during anaerobic respiration in this tissue through nitrate reduction. Unacclimated root tips of wild type and Adhl(-) mutants appeared not to survive more than 8 to 9 hours in strict anoxia; when hypoxically pretreated they tolerated periods under anoxia in excess of 22 hours.
ABSTRACT
The growth of scions and rootstocks of compatible (Prunus persica L. Batsch cv. Springtime/Prunus cerasifera L. Ehrh. cv. myrobolan P2032) and incompatible (Prunus persica L. Batsch cv. Springtime/Prunus cerasifera L. Ehrh. cv. myrobolan P18) peach/plum grafts were compared. The composition of soluble carbohydrates in phloem and cortical tissues of both peach/plum grafts and ungrafted plums and the translocation of these compounds across the union of grafted plants were examined. Sorbitol and sucrose were the dominant sugars in the phloem and cortical tissues of plum. A cyanogenic glycoside, prunasin, was present in peach tissues in amounts equivalent to those of sorbitol or sucrose, whereas only small amounts of prunasin were detected in plum tissues. The concentration of prunasin was significantly higher in the phloem of the P18 rootstock of the incompatible graft. Sorbitol was the only sugar significantly depleted in rootstock tissues of the incompatible graft when the first foliar symptoms of graft incompatibility became evident. Translocation studies with 1-(14)C-deoxyglucose showed that the relative distribution of radioactivity across the graft union was similar in both compatible and incompatible grafts. However, the total amount of radioactivity translocated across the incompatible graft was less than one-third of that translocated across the compatible graft. The results are consistent with the hypothesis of a progressive poisoning of the root system in the incompatible graft by a compound synthesized in peach foliage. The role of prunasin as a possible candidate is discussed.
ABSTRACT
After feeding the scutellum of young maize seedlings with a labeled analog of glucose, 2-deoxy-d-glucose, the progress of radioactivity along the root was followed when either 70% of the path or the whole root were in strict anoxic conditions, and was compared with the translocation pattern of aerobic seedlings. Special care was taken to suppress the internal O(2) transport and to control its occurrence.In air, the radioactive compounds accumulated from 30 minutes in the root tip mainly as an analog of sucrose. When the whole root was anoxic, the progress of the radioactivity was very slow and never reached the tip which did not survive more than 8 hours. When 70% of the path was in strict anoxia and the sink (root tip) in air, the translocation was not impaired and the radioactivity accumulated in the tips as fast as in aerobic controls. The addition of 3 millimolar NaF, which inhibits the fermentative energy production, did not modify these results. It is concluded that long distance transport in maize sieve tubes has no special energy requirements and is controlled by source-sink relationships. The inhibition of sugar supply in anoxic root tips is attributed to an effect on unloading processes rather than on sink metabolism.
ABSTRACT
Internal transport of O2 from the aerial tissues along the adventitious roots of intact maize plants was estimated by measuring the concentrations of adenine nucleotides in various zones along the root under an oxygen-free atmosphere. Young maize plants were grown in nutrient solution under conditions that either stimulated or prevented the formation of a lysigenous aerenchyma, and the roots (up to 210 mm long) were then exposed to an anaerobic (oxygen-free) nutrient solution. Aerenchymatous roots showed higher values than non-aerenchymatous ones for ATP content, adenylate energy charge and ATP/ADP ratios. We conclude that the lysigenous cortical gas spaces help maintain a high respiration rate in the tissues along the root, and in the apical zone, by improving internal transport of oxygen over distances of at least 210 mm. This contrasted sharply with the low energy status (poor O2 transport) in non-aerenchymatous roots.
ABSTRACT
A method based on the measurement of ATP/ADP ratios is described. It permits the determination of the critical respiratory oxygen pressure of any organ, or part of any organ, of an intact plant. The data obtained by this method with intact maize (Zea mays L. INRA 508) root tips are compared with polarographic determinations on similar excised tissues.When internal O(2) transport from the aerial part was prevented, the critical oxygen pressure found for the respiration of intact tips was similar to that found with excised tips. It was close to 10 kilopascals in a humid atmosphere and about 30 kilopascals in a liquid medium. Flooding of the gas spaces by vacuum infiltration did not modify these results. When internal O(2) transport from the aerial parts of the plant occurred, significantly lower values were obtained in liquid medium for the critical oxygen pressure, which shifted from more than 21 to 6 kilopascals. The higher values observed with excised root tips, compared to those obtained with intact tissues, can be explained by the lack of internal O(2) transport, rather than by the flooding of gas spaces.Data are presented which show that root growth started to be limited at a significantly higher pressure than the respiration. These results are attributed to nonrespiratory oxidative processes with a low affinity for O(2) involved in root elongation.
ABSTRACT
Oxygen uptake and ATP/ADP ratio were simultaneously monitored during incubation of excised maize (Zea mays L. INRA 508) root tips under varying O(2) partial pressure. Both variables were independent of O(2) tension until a critical O(2) pressure was reached. Below this pressure, ATP/ADP ratio and respiratory rate declined. However, in tissues having a high glycolytic capacity, the correlation between the ATP/ADP ratio and the respiratory rate breaks down as O(2) tension decreases, due to the increasing contribution of fermentative processes.In presence of 2 millimolar NaF, the ATP/ADP ratio varied solely as a function of the O(2) tension, without interference by fermentative activity, and a close correlation links the ATP/ADP ratio and the respiratory rate of excised maize root tips over the whole range of O(2) tensions tested.Using this correlation, a method is proposed for the quantitative determination of the relative cellular respiratory rate permitted by O(2) transport from the aerial part of young maize seedlings along the seminal root placed in an anoxic environment.Data are presented which demonstrate the preeminent part played by the cortical air spaces in O(2) transport. Their contribution to respiration was high in the first few centimeters nearest the seed and decreased rapidly as the distance from the aerated source increased. It is concluded that O(2) transport might contribute to the survival or to adaptive responses of root tissues in flooded soils but that the ventilation of the apical growing zone was inadequate to sustain the growth.
ABSTRACT
Energy charge and fermentative metabolism under anoxia were monitored in excised maize root tips after various times of aging in air and were related to their soluble sugar content. The energy charge value, which was 0.9 in air irrespective of the time of aging, dropped to a lower value within minutes of transfer to a nitrogen atmosphere. This value was dependent upon sugar content of the tissues which was itself a function of aging. The energy charge value after transfer to nitrogen was 0.6 in freshly excised tissue but only 0.2 in tissue aged for 4 hours. When aged tissues supplied with 0.2 molar glucose were transferred to nitrogen, the energy charge was 0.6, irrespective of the time of aging. When 0.2 molar glucose was added under nitrogen, energy charge rose to 0.6. This rise was faster in root tips aged for 8 hours than those aged for 24 hours.The rate of ethanol plus lactate production (representing 60 and 10%, respectively, of the total sugar consumption in anoxia) was closely correlated to the level of energy charge. It is concluded that, in anoxia, there is a quantitative relationship between the energy charge value and the level of metabolic activity via fermentative pathways.
ABSTRACT
Oxygen uptake and energy charge were monitored during aging of excised maize root tips and related to the soluble sugar content and exogenous sugar supply.Oxygen uptake declined immediately after excision to 50 to 30% of its initial value after 8 and 24 hours of aging at 25 C. There was also a sharp decline of the total sugar content (glucose, fructose, and sucrose). Starch content was very low at the time of excision and almost negligible 5 hours later. During the same period, the respiratory quotient declined from 1 to 0.75 and then remained stable.The addition of exogenous sugars induced a rapid rise of the respiratory rate which stabilized at a level correlated to the external sugar concentration. Addition of 0.2 molar glucose was necessary to restore the respiratory rate to the initial, also the maximum, level. These results indicate that metabolic activity of root tips is highly reliant on sugar import and carbohydrate reserves at the time of excision cannot compensate for the cessation of import. The control of respiration by substrate supply is in good agreement with the failure for dinitrophenol to stimulate oxygen uptake in aged sugar-depleted root tips.The energy charge remained constant at about 0.9, irrespective of the presence or absence of glucose and in spite of a large decline of respiratory activity in aged, sugar-depleted tissues.