Bicarbonate Fixation and Malate Compartmentation in Relation to Salt-induced Stoichiometric Synthesis of Organic Acid

212

Five-or six-day old seedlings of corn (Zea inays L.) were exposed to 0.25 mM Ca (NO3) reveal that prior exposure to ammonium ion did not eliminate the initial lag phase. The same pattern also has been observed with cotton and tobacco depleted of nitrate prior to the experimental period and with 10-day-old corn grown exclusively with ammonium ion as the nitrogen source (11). The latter observation contrasted to the nitrate uptake pattern of similar corn plants grown with nitrate in which case nitrate uptake did not exhibit the initial lag phase. A similar increase in nitrate uptake rate has been reported recently for tobacco cell suspension cultures (8) which had been grown previously with urea and in cultures which were nitrogen starved.The present investigation with dark-grown corn seedlings was initiated to characterize more fully this nitrate uptake pattern by root tissue of higher plants. It is shown that a substance from the endosperm was essential for full development of the accelerated rate of nitrate uptake, and that this development was restricted by certain inhibitors of RNA and protein synthesis. MATERIALS AND METHODSAll experiments were conducted with corn hybrid (Zea mays L.) OH43 x B14, except that DeKalb XL45 hybrid was used to obtain part of the data of Figure 5. DeKalb XL45 and OH43 X B14 show similar patterns of nitrate uptake. Corn seeds were rinsed for 1 min in 5.25% NaOCl, placed in running tap water for 3 hr, rinsed again for 1 min in the NaOCl solution, rinsed thoroughly in deionized water, and about 150 seeds were spread on stainless steel screens (14 X 14 cm) over 4-liter containers of vigorously aerated 0.1 mM CaSO4. The seeds were covered with one layer of filter paper kept moist by wick action. Germination took place at 25 C in darkness. Four days later the seedlings were removed, and all roots except the primary root were excised. The primary roots of approximately 100 seedlings were then threaded through a stainless steel screen into a 4-liter beaker containing 0.5 mM (NH,),SO4, 0.6 mM KSO4, 0.4 mM KH,PO4, 1.6 mM MgSO,, 0.8 mm CaSO,, and 0.125 g of CaCO,/l. Iron was supplied at 4 mg/l as monosodium hydrogen ferric diethylenetriaminepentaacetate, and the remaining trace elements were at one-fifth the concentration of Hoagland's solution (9). The containers were again placed in the dark germinator at 25 C, and the solutions were aerated vigorously.After a further 24 or 48 hr, the etiolated seedlings were removed, and the roots were rinsed thoroughly in deionized water. The seedlings were then floated in trays on 0.5 mM CaSO, while unifrom seedlings were selected, assembled in groups of seven, and supported in plastic frames. Roots of each group were then blotted lightly with paper towels and placed 120 www.plantphysiol.org on May 12, 2018 -Published by Downloaded from

Section: Discussionmentioning

confidence: 67%

Nitrate Uptake by Dark-grown Corn Seedlings

Jackson¹,

Flesher²,

Hageman³

1973

212

“…17). In barley roots, for example, the operation of a H+ -K+ exchange pump which extrudes protons appears to stimulate the accumulation of malate in the cells (16,17). In pea internode segments, evidence has been found for a coupling of proton extrusion to K+ uptake during FC-stimulated acidification and growth (23).…”

Section: Methodsmentioning

confidence: 99%

Enhancement of CO₂ Uptake in Avena Coleoptiles by Fusicoccin

Johnson¹,

Rayle²

1976

When Avena coleoptile segments are immersed in a solution containing H'4C03-, the appearance of label in the tissue is stimulated approximately 3-fold by fusicoccin application. This effect is rapid (1-2 minutes lag time), dependent upon respiratory energy, inhibited by carbonyl cyanide m-chlorophenylhydrazone, but not appreciably altered by cycloheximide treatment. A large percentage of the cellular radioactivity is found in the form of malate. Preliminary experiments indicate that C02, as opposed to HC03-, is the favored species of "CO2" taken up by the segments. These results are consistent with the notion that C02, presumably by virtue of its fixation and conversion to malic acid, participates in the early events associated with fusicoccin-enhanced acidification of the cell wall region leading to the stimulation of cell extension growth.It is well established that auxin and the phytotoxin, fusicoccin, can cause Avena coleoptile segments to acidify the surrounding medium (4,6,7,27). It is argued that the decrease in cell wall pH resulting from IAA-or FC2-enhanced acid extrusion would lead to an increased rate of wall loosening, and hence, an increased rate of cell elongation (4,14,21,28,29). While precise knowledge of the mechanism responsible for this facilitated acidification is not yet available, there is at hand some pertinent information which any model for acidification must take into account. First, hydrogen ions per se (not organic acids) accumulate in the medium, as titration of the acidified medium shows it to lack any buffering capacity (21; Rayle and Johnson, unpublished results). Second, observed properties of the IAA and FC acidification responses are similar to reported characteristics of apparent H+ pumps in lower plant cells (18,31); that is, the acidification phenomenon requires energy metabolism (22,27), is stimulated by cations (8; Cleland, personal communication), and leads to a hyperpolarization of the plasma membrane (20). These observations could be used to argue that the acidification response is controlled by a hypothetical growth-regulatoractivated proton pump, possibly electrogenic in nature. However, the known parameters of IAA-or FC-induced acidification are equally consistent with a mechanism involving the active uptake of bicarbonate ions which have been generated extracytoplasmically by the hydration of CO2 within the cell wall region. The subsequent uptake and fixation of HC03-would result in net wall acidification indistinguishable in the final result from the outward pumping of H+ (cf. 30). This paper describes experiments carried out to assess the possibility that HC03-reabsorption could contribute to active acidification in A vena coleoptiles., 1 This work was supported by National Science Foundation Grant BMS73-07110 A01.2 Abbreviations: FC: fusicoccin; DNP: 2,4-dinitrophenol; RQ: respiratory quotient; CCCP: carbonyl cyanide m-chlorophenylhydrazone. MATERIALS AND METHODSPlant Material. Seeds of Avena sativa L. cv. Victory were surface-sterilized in 20% Clorox for...

“…If NO3-absorption or accumulation or both are controlled by total ion and charge balance, it is possible that the negative relation between malate and NO:; observed, while valid under these experimental conditions, may not hold under all experimental conditions. These data in no way invalidate the well established principle that synthesis and catabolism of malate maintain pH and charge balance with respect to ion flux including NO:-assimilation (3,6,14,27 53,1974 partmentalized (30). Second, the oxidation of 3-P-glyceraldehyde has also been proposed as a source of NADH for NO:-reduction (16 …”

mentioning

confidence: 96%

Effect of Carbon Dioxide on Nitrate Accumulation and Nitrate Reductase Induction in Corn Seedlings

Purvis

Peters²,

Hageman³

1974

Exposure of the leaf canopy of corn seedlings (Zea mays L.) to atmospheric C02 levels ranging from 100 to 800 ,ul/l decreased nitrate accumulation and nitrate reductase activity. Plants pretreated with C02 in the dark and maintained in an atmosphere containing 100 ,ul/l C02 accumulated 7-fold more nitrate and had 2-fold more nitrate reductase activity than plants exposed to 600 ,l/l C02, after 5 hours of illumination.