Differences in the Anaerobic Lactate-Succinate Production and in the Changes of Cell Sap pH for Plants with High and Low Resistance to Anoxia

Abstract

Plants are frequently subjected to ROA,' yet the mechanisms that permit some species to tolerate oxygen deprivation are poorly understood (7,8). Most studies have focused on plant roots (7,12), since these organs experience oxygen deprivation more frequently, as a result of waterlogged soil, for example. Seed germination and early growth of seed plants inhabiting particular ecological niches, such as rice fields and natural wetlands, are also affected by ROA. The same may be true of seedlings of crop plants, due to temporary soil waterlogging. It has been reported (15) that shoots resist ROA better than roots. In the case of rice and Echinochloa (3), shoot but not root growth occurred in the absence of oxygen. Therefore, biochemical mechanisms relevant to ROA resistance are likely to be better expressed in shoot rather than in root tissues (3,8,9).In the absence of oxygen the main pathway for energy production in plants is alcoholic fermentation. Lactic fermentation, however, was maintained in plants for purposes not completely understood. In maize root tips, transient lactate production was the induction signal for alcoholic fermentation (12). This model, however, seems inadequate to explain the prolonged lactate production in barley roots (7) and the limited cytoplasmic acidosis in maize root tips (1 1). Lactate, as opposed to ethanol, was a negligible source of ATP in maize and pea root tips, but appreciable in the roots of 'Abbreviations: ROA, restricted oxygen availability; GABA, yaminobutyric acid.Ranunculus sceleratus and Senecius aquaticus (13 and references therein).In animals with marked resistance to anoxia, lactic fermentation is frequently replaced by pathways leading to succinate (6). Advantages of this substitution are an increased amount of ATP generated per mole of substrate consumed and a reduced proton production per mole of ATP cycled (6).Succinate accumulation in higher plants under oxygen deficiency has been reported in only three cases: Fagopyrum aesculentum seedlings (4), Rhododendron ponticum leaves (2), and rice seedlings (9). In the last work, succinate accumulated in shoots, but not in roots. The succinate-lactate ratio was 5 to 1, suggesting that, in plants with high resistance to ROA, as in animals, succinate production may replace that of lactate. Here we report marked differences of anaerobic succinate-lactate production and of concomitant changes in cell sap pH between seedling shoots ofROA resistant and sensitive plants. MATERIALS AND METHODS

Ammonium Assimilation and the Role of [gamma]-Aminobutyric Acid in pH Homeostasis in Carrot Cell Suspensions

“…In vivo 14N NMR experiments showed that the cells contained a substantial ammonium pool at the start of the labeling period and that the size of this pool varied throughout the 18-h time course (data not shown). Increases in GABA, ranging from 5-fold in 24 h in rice shoots (Reggiani et al, 1988) to 100-fold (with a 7-fold increase in Ala) in 4 h in radish leaves (Streeter and Thompson, 1972a) have been observed previously in a wide range of plants under hypoxia (Streeter and Thompson, 1972a;Tsushida and Murai, 1987;Reggiani et al, 1988;Menegus et al, 1989;Fan et al, 1992;Roberts et al, 1992). However, in these studies oxygen-free nitrogen was supplied to ensure complete anaerobiosis, whereas in the experiments reported here the oxygen supply was only mildly restricted.…”

“…GABA is present in virtually all plant tissues and often accumulates under conditions of environmental stress, including hypoxia (Streeter and Thompson, 1972a;Tsushida and Murai, 1987;Reggiani et al, 1988;Menegus et al, 1989;Fan et al, 1992;Roberts et al, 1992), low temperature (Wallace et al, 1984), heat shock (Mayer et al, 1990), and low pH (Lane and Stiller, 1970). It is mainly produced by the action of GDC on glutamate and it can be further metabolized to succinic semialdehyde and succinate via GAB-T and SSADH (Streeter and Thompson, 1972b;Satya Narayan and Nair, 1986).…”

“…It has also been suggested that the high accumulation of GABA in many tissues could be the result of a change in the relative activities of GDC and GAB-T following a decrease in pH,, (Streeter and Thompson, 1972b;Wallace et al, 1984). Because GDC activity increases as pH decreases and because glutamate decarboxylation is a protonconsuming reaction, it follows that GABA production could act as a sink for excess protons in the cytoplasm and thus contribute to pH regulation (Murphy et ai., 1983;Reid et al, 1985;Menegus et al, 1989;Roberts et al, 1992;Snedden et al, 1992).…”

Long-Term Anaerobic Metabolism in Root Tissue (Metabolic Products of Pyruvate Metabolism)

“…Although it has been argued that succinate accumulation plays a role in resistance to hypoxia (Menegus et al, 1988(Menegus et al, , 1989, Roberts et al (1992) found that succinate does not accumulate in corn root tips. We found a similar pattern, with succinate showing a short-term increase in levels followed by a long-term decline.…”

Limit Dextrinase - Does Its Malt Activity Relate to Its Activity During Brewing?

“…In anoxic sycamore (Acer pseudopantanus) cells the release of H + ions accompanying the Pi liberating hydrolysis of NTP was the principle cause of cytoplasmic pH drop and the H + pumping ATPase located in the plasma membrane was also blocked 10 . In the barley endosperm, acidification occurs at the late stages of grain germination and is caused by the accumulation of malic acid 22,27 , at this stage of grain germination the activity of debranching enzymes such as limit dextrinase would be of utmost importance to degrade dextrins produced following starch hydrolysis by amylases.…”