Callose Deposition Is Responsible for Apoplastic Semipermeability
of the Endosperm Envelope of
Muskmelon Seeds1
Kyu-Ock Yim and
Kent J. Bradford*
Department of Vegetable Crops, University of California, Davis,
California 95616-8631
Semipermeable cell walls or
apoplastic "membranes" have been hypothesized to be present in
various plant tissues. Although often associated with suberized or
lignified walls, the wall component that confers osmotic
semipermeability is not known. In muskmelon (Cucumis
melo L.) seeds, a thin, membranous endosperm completely encloses the embryo, creating a semipermeable apoplastic envelope. When
dead muskmelon seeds are allowed to imbibe, solutes leaking from the
embryo are retained within the envelope, resulting in osmotic water
uptake and swelling called osmotic distention (OD). The endosperm
envelope of muskmelon seeds stained with aniline blue, which is
specific for callose (
-1,3-glucan). Outside of the
aniline-blue-stained layer was a Sudan III- and IV-staining (lipid-containing) layer. In young developing seeds 25 d after anthesis
(DAA) that did not exhibit OD, the lipid layer was already present but
callose had not been deposited. At 35 DAA, callose was detected as
distinct vesicles or globules in the endosperm envelope. A thick
callose layer was evident at 40 DAA, coinciding with development of the
capacity for OD. Removal of the outer lipid layer by brief chloroform
treatment resulted in more rapid water uptake by both viable and
nonviable (boiled) seeds, but did not affect semipermeability of the
endosperm envelope. The aniline-blue-staining layer was digested by
-1,3-glucanase, and these envelopes lost OD. Thus, apoplastic
semipermeability of the muskmelon endosperm envelope is dependent on
the deposition of a thick callose-containing layer outside of the
endosperm cell walls.
1
This work was supported by the U.S. Department
of Agriculture Binational Agricultural Research and Development Fund
(grant no. US-2422-94).
*
Corresponding author; e-mail kjbradford{at}ucdavis.edu; fax
1-530-752-4554.
Plant Physiol. (1998) 118: 83-90
Copyright Clearance Center: 0032-0889/98/118//08
© 1998 American Society of Plant Physiologists
