Enhancement of CO₂ Uptake in Avena Coleoptiles by Fusicoccin

Abstract

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in the medium, as titration of the acidified medium shows it to lack any buffering capacity (21; Rayle and Johnson, unpublished results).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...

Response to osmotic medium and fusicoccin by seeds of radish (Raphanus sativus) in the early phase of germination

“…3) was not increased by mannitol either in the absence or presence of FC, indicating the lack of a specific activation of the synthesis of osmoticaliy active molecules. The slight increase in malic acid per seed after 42 h of incubation in mannitol in the absence of FC might be linked also in this case to the metabolicresponse to the activation of H* extrusion (Johnson and Rayle 1976, Haschke and Liittge 1977, Stout et al 1978, Cocucci and Morgutti 1986; the higher increase in malic acid per seed in the presence of FC alone confirms this hypothesis. The slight increase in sucrose per seed after 42 h of incubation in the presence of mannitol, both in the absence and presence of FC, might be the consequence of its lower utilization due to the inhibition of seed metabolism.…”

Abscisic acid effects on intracellular pH and ion fluxes in barley leaf segments

“…This parameter, according to the biochemical pH-stat theory of Davies (1973), should reflect the changes in the rate of synthesis or decarboxylation of malate, due to cytosolic alkalinization or acidification, respectively. The results of experiments carried out in the presence of KCl or FC plus KCl, indicated that while FC markedly increased COj fixation, as already shown in other materials (Johnson and Rayle, 1976;Romani et al 1983), ABA induced a clear inhibition of CO2 fixation rate in both cases (Fig. 4).…”

Nonhormonal induction of H + efflux from plant tissues and its correlation with growth

“…Because a smaller amount of lateral shrinkage is observed during auxin-induced growth (21), it seems likely that auxin induces osmoregulation, thus allowing a sustained growth response. Malate levels are reported to increase in coleoptile segments treated with indoleacetic acid (22)(23)(24).…”