Role of a callose synthase zymogen in regulating wall deposition in pollen tubes of Nicotiana alata Link et Otto |
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Authors: | Huijuan Li Antony Bacic Steve M Read |
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Institution: | (1) Plant Cell Biology Research Centre, School of Botany, University of Melbourne, Parkville, Victoria 3052, Australia, AU;(2) School of Forestry, University of Melbourne, Creswick, Victoria 3363, Australia, AU |
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Abstract: | The callose synthase (CalS) activity of membrane preparations from cultured Nicotiana alata Link & Otto pollen tubes is increased several-fold by treatment with trypsin in the presence of digitonin, possibly due to
activation of an inactive (zymogen) form of the enzyme. Active and inactive forms of CalS are also present in stylar-grown
tubes. Callose deposition was first detected immediately after germination of pollen grains in liquid medium, at the rim of
the germination aperture. During tube growth the 3-linked glucan backbone of callose was deposited at an increasing rate,
reaching a maximum of 65 mg h−1 in tubes grown from 1 g pollen. Callose synthase activity was first detected immediately after germination, and then also
increased substantially during tube growth. Trypsin caused activation of CalS throughout a 30-h time course of tube growth,
but the degree of activation was higher for younger pollen tubes. Over a 10-fold range of callose deposition rates, the assayed
CalS activity was sufficient to account for the rate of callose deposition without trypsin activation, implying that the form
of CalS active in isolated membranes is responsible for callose deposition in intact pollen tubes. Sucrose-density-gradient
centrifugation separated a lighter, intracellular membrane fraction containing only inactive CalS from a heavier, plasma-membrane
fraction containing both active and inactive CalS, with younger pollen tubes containing relatively more of the inactive intracellular
enzyme. The increasing rate of callose deposition during pollen-tube growth may thus be caused by the transport of inactive
forms of CalS from intracellular membranes to the plasma membrane, followed by the regulated activation of these inactive
forms in this final location.
Received: 1 December 1998 / Accepted: 21 January 1999 |
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Keywords: | : Callose Callose synthase Cell wall Nicotiana alata Pollen tube Zymogen |
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