PLANT PHYSIOLOGY , Vol 110, Issue 2 599-609, Copyright © 1996 by American Society of Plant Biologists
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DEVELOPMENT AND GROWTH REGULATION |
Polyembryony in Citrus (Accumulation of Seed Storage Proteins in Seeds and in Embryos Cultured in Vitro)
A. M. Koltunow, T. Hidaka and S. P. Robinson
Commonwealth Scientific and Industrial Research Organization Division of Horticulture, G.P.O. Box 350, Adelaide 5001, South Australia
Citrus exhibits polyembryonic seed development, an apomictic process in
which many maternally derived embryos arise from the nucellus surrounding
the developing zygotic embryo. Citrus seed storage proteins were used as
markers to compare embryogenesis in developing seeds and somatic
embryogenesis in vitro. The salt-soluble, globulin protein fraction
(designated citrin) was purified from Citrus sinensis cv Valencia seeds.
Citrins separated into two subunits averaging 22 and 33 kD under denaturing
sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A cDNA clone was
isolated representing a citrin gene expressed in seeds when the majority of
embryos were at the early globular stage of embryo development. The
predicted protein sequence was most related to the globulin seed storage
proteins of pumpkin and cotton. Accumulation of 33-kD polypeptides was
first detected in polyembryonic Valencia seeds when the majority of embryos
were at the globular stage of development. Somatic Citrus embryos cultured
in vivo were observed to initiate 33-kD polypeptide accumulation later in
embryo development but accumulated these peptides at only 10 to 20% of the
level observed in polyembryonic seeds. Therefore, factors within the seed
environment must influence the higher quantitative levels of citrin
accumulation in nucellar embryos developing in vivo, even though nucellar
embryos, like somatic embryos, are not derived from fertilization events.
