Induction of (1-->3,1-->4)-beta-D-glucan hydrolases in leaves of dark-incubated barley seedlings.
When seedlings of barley (Hordeum vulgare L.) were transferred from a natural light/dark cycle into darkness, (1-->3,1-->4)-beta- D-glucan endohydrolase (EC 3.2.1.73) activity in leaf extracts increased 3- to 4-fold after 2 days. Activity decreased to normal levels within a day if the light/dark cycle was restored. Although there are two (1-->3,1-->4)-beta-D-glucan endohydrolase isoenzymes in barley, the increased enzyme activity in dark-grown seedlings was attributable entirely to increases in isoenzyme EI. Northern hybridization analyses confirmed that mRNA transcripts encoding (1-->3,1-->4)-beta-D-glucan endohydrolase isoenzyme EI accumulated in the leaves of dark-incubated seedlings; no isoenzyme EII mRNA was detected. Activity of beta-D-glucan glucohydrolases also increased 10-fold after 2 days of dark treatment. The latter, broad-specificity enzymes release glucose from (1-->3,1-->4)-beta-D-glucans and from beta-D-oligoglucosides released by (1-->3,1-->4)-beta-D-glucan endohydrolases. Consistent with the activity patterns of these enzymes, the (1-->3,1-->4)-beta-D-glucan content of leaf cell walls decreased by about 30% when barley seedlings were transferred into darkness. Soluble sugars in the leaves decreased by about 60% during the same period. Because no measurable leaf elongation was detected during the various light/dark treatments, the enzymes were unlikely to be participating in wall loosening and cell elongation. Instead, the results suggest that cell wall (1-->3,1-->4)-beta-D-glucans can be re-mobilized in the non-elongating, dark-incubated leaves and the glucose so generated could serve as an energy source under conditions of sugar depletion.
Roulin
Samuel
S
Institute of Plant Sciences, University of Bern, Altenbergrain 21, 3013 Bern, Switzerland. samuel.roulin@bag.admin.ch
Buchala
Antony J
AJ
Fincher
Geoffrey B
GB
D001483:Base Sequence; D015152:Blotting, Northern; D015153:Blotting, Western; D002473:Cell Wall / Q000378:metabolism; D002734:Chlorophyll / Q000378:metabolism; D003624:Darkness; D004790:Enzyme Induction / Q000528:radiation effects; D005959:Glucosidases / Q000096:biosynthesis / Q000235:genetics; D006026:Glycoside Hydrolases / Q000096:biosynthesis* / Q000235:genetics; D001467:Hordeum / Q000201:enzymology / Q000235:genetics*; D007527:Isoenzymes / Q000096:biosynthesis / Q000235:genetics; D008027:Light; D008969:Molecular Sequence Data; D018515:Plant Leaves / Q000201:enzymology / Q000235:genetics*; D012689:Sequence Homology, Nucleic Acid
D007527:Isoenzymes; D002734:Chlorophyll; D005959:Glucosidases; D006026:Glycoside Hydrolases; C446280:beta-D-glucan glucohydrolase; C028020:licheninase