Biosynthesis of Seed Storage Products during Embryogenesis in Rapeseed, Brassica napus

Abstract

Summary The timing of the synthesis of storage oils and storage proteins during embryogenesis in field-grown rapeseed, Brassica napus L. has been examined. For the first two weeks after anthesis the embryos grew by cell division until they were about 1 mm in length. Storage oil accumulation was apparent even in very young embryos but only became significant when cell division ceased and cell expansion commenced at 2-3 weeks after anthesis. The most rapid phase of oil deposition was between 4 and 6 weeks after anthesis. The bulk of the storage oil, which accumulated initially as large oil-bodies of 2-4pm diameter, was deposited before the onset of storage protein synthesis. The major seed storage proteins, cruciferin and napin, were synthesised from week 5 and the maximal rate of synthesis was between weeks 5 and 7 after anthesis. The hydrophobic protein, oleosin, which accounted for 20% of total protein in mature seeds, was synthesised between 7 and 10 weeks after anthesis. It has recently been shown that this protein is the major component of the osmiophilic membrane which develops around the storage oil-bodies at this stage of embryogenesis [Murphy et al. (1989) Biochem. J. 258,285-293]. The acquisition of a proteinaceous membrane by the oil-bodies coincided with a reduction in their average diameter from 2-4Im to 0.3 -1 p,m. It is proposed that there are three major phases in storage product formation in rapeseed embryos, (i) oil deposition into large storage bodies, (ii) polar protein deposition into vacuoles, (iii) oleosin synthesis and assembly onto oil-bodies causing their reduction in size. The significance of these results for the regulation of storage product synthesis in oilseeds is discussed.