Evidence Against the Involvement of Galactosidase or Glucosidase in Auxin- or Acid-stimulated Growth

Abstract

Research on the mode of action of auxin in the promotion of growth has shown that auxin treatment leads to hydrogen ion secretion and wall acidification. It has recently been reported that auxin stimulates cell wail 8-galactosidase activity in Avena coleoptiles, presumably by causing cell wall acidification, since (2,3,5,6,12). This has led to the suggestion that the promotion of growth by auxin may be acidmediated, i.e. auxin may stimulate hydrogen ion secretion causing a reduction in pH within the cell wall to a growth-promoting level (6). This suggestion has received strong support by recent reports that auxin does stimulate hydrogen ion secretion to an extent sufficient to lower cell wall pH to about 5.0, a level capable of causing maximal stimulation of elongation (1,10,11 by examining the properties and pH dependence of a number of cell wall bound glycosidases in Avena, especially /-galactosidase. They found that the enzyme is inhibited by substances that inhibit the promotion of elongation by acid, and they reported that auxin enhances S-galactosidase activity in coleoptile segments by 36% after a 60-min treatment period. These data suggest the possibility that auxin stimulates growth by stimulating hydrogen ion leakage which activates cell wall glycosidases leading to wall loosening.If this model of auxin and acid action is correct, it should be possible to greatly reduce the growth response to auxin or acid using specific inhibitors of glycosidases. It is known that the aldonolactones of simple sugars are specific and potent inhibitors of glycosidases active on substrates composed of those sugars (9, and references cited therein). In the work reported here, the effect of galactonolactone and gluconolactone on in vivo /2-galactosidase and /3-glucosidase activity was measured. I found that these aldonolactones strongly inhibit ,B-galactosidase and /3-glucosidase activity but have no effect on growth promotion by auxin or acid.MATERIALS AND METHODS Plant Material. Oats (A vena sativa var. Victory) were planted in an arrangement similar to that of Wiegand and Schrank (14). They were placed under dim red light for 24 hr and then allowed to grow at room temperature in darkness for 48 hr. Coleoptiles about 3 cm long were selected for harvesting, and segments 10 mm long were cut from them beginning 3 mm from the tip. The leaf was removed from each segment, and the segments were used either for growth tests or Enzyme Assays. Enzyme assays were done as described by Johnson et al. (7). Twenty coleoptile segments were rinsed with distilled water and placed in a 25-ml Erlenmeyer flask containing 1.5 ml of 13 mM phosphate-citrate buffer with or without 0.1 mm IAA and allowed to incubate at 30 C for 30 min. After 30 min, the assay was initiated by adding 0.5 ml of 40 mM p-N-gal2 or p-N-gluc. To

Hyphal Tip Bursting in Mucor rouxii: Antagonistic Effects of Calcium Ions and Acid

“…Similar models have been suggested for the extension of higher plant cells but in this case, there is evidence against the participation of enzymes which hydrolyse either the cellulose fibrils (Ruesink, 1969) or the matrix polysaccharides of the cell wall (Evans, 1974).…”

Correlative Studies of Cell Wall Enzymes and Growth

“…There is also evidence that glucanases play a role in the case of auxin-induced extension growth of excised Avena coleoptile sections (5,11). A recent study, utilizing glycosidase inhibitors, presents evidence against the involvement of f-glucosidase or /3-galactosidase in the short term auxin-or acid-promoted growth of excised Av'ena coleoptile sections (9). However, this work deals with short-term growth which may not serve as a complete model for the long-term and very large increase in cell size studied in this report.…”

Comparison of Auxin-induced and Acid-induced Elongation in Soybean Hypocotyl

“…Other data which do not support the hypothesis are not yet entirely explained: (a) Penny et al (21), assuming that the xylem vessels are part of the free space that includes the cell wall (27) major differences in auxin-and acid-stimulated cell enlargement exist, e.g. auxin, but not low pH, stimulates both radial and longitudinal cell growth in lupin hypocotyl segments (23), and auxin-promoted elongation is not transient, as is acid-stimulated elongation, in Avena coleoptile segments (25,26) and soybean hypocotyl segments (29); (c) after acid-stimulated elongation has subsided, auxin is still capable of increasing the rate of elongation in Avena coleoptile (10,25,16) and soybean hypocotyl segments (29); (d) wall-bound glycosidases which have low pH optima, described in support of the hypothesis (16), can be inhibited without inhibiting auxin-promoted elongation in lupin (22) and Avena (9); and (e) auxin does not affect proton secretion in lupin (21) and soybean (28).…”