Development of leucine aminopeptidase activity during daylily flower growth and senescence |
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Authors: | M G P Mahagamasekera David W M Leung |
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Institution: | (1) Department of Plant and Microbial Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand |
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Abstract: | The possibility that exopeptidases, i.e. aminopeptidases and carboxypeptidases, in addition to the previously studied endopeptidase
might also be developmentally regulated in daylily petals was examined. The level of leucine aminopeptidase and endopeptidase
activities changed after the flower was fully open while that of carboxypeptidase activity remained relatively unchanged throughout
senescence. Leucine aminopeptidase activity seemed to increase after the flower was fully open and peaked several hours earlier
than endopeptidase did. Taken together, it is postulated that leucine aminopeptidase might play a role in protein turnover
during flower opening and in the initiation of protein hydrolysis associated with petal senescence while the endopeptidase
could be responsible for the breakdown of the bulk of proteins at the later stages. The drop in leucine aminopeptidase activity
associated with the onset of daylily petal senescence was effectively halted by a cycloheximide treatment of cut daylily flowers
for 24 h which was previously shown to prolong the vase life of the flowers and prevent protein loss from the petals. Apart
from both being developmentally regulated in daylily petals, the leucine aminopeptidase activity and the previously studied
endopeptidase are different in several aspects. They appear to have different pH optima, 8 for leucine aminopeptidase and
6.2 for endopeptidase. Unlike the endopeptidase activity, no new leucine aminopeptidase isozymes appeared during petal senescence,
and the leucine aminopeptidase did not appear to belong to the cysteine class of proteolytic enzymes. |
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Keywords: | Daylily (Hemerocallis) aminopeptidase carboxypeptidase ethylene-insensitive flower senescence proteinase (endopeptidase) |
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