Storage Product Metabolism in Microspore-Derived Cultures of Brassicaceae

Abstract

Major storage products that accumulate in the seeds of oleaginous crops and cereals include starch, lipid and protein. In terms of haploid systems, extensive investigations of storage product metabolism have been conducted with microspore-derived cultures of Brassicaceae. In particular, microsporederived embryos (MDEs) of Brassica napus and B. rapa progress through similar morphogenetic and metabolic changes as zygotic embryos (Weselake and Taylor 1999). Thus, studies of storage product accumulation using MDEs may be more representative of events in the developing seed then those based on investigations with somatic embryos. The ability to maintain a continuous supply of MDEs under carefully controlled conditions has not only provided a tool to accelerate breeding programs, but has also greatly increased our knowledge of storage product metabolism in this oilseed crop. Within the last 15 years, four reviews have been published which deal primarily with storage lipid biosynthesis in MDEs (Weber et al. 1992; Taylor and Weber 1994; Weselake and Taylor 1999; Weselake 2000a). The current chapter presents a somewhat broader picture of metabolism in MDEs by also including a discussion of carbohydrate and protein metabolism. The bulk of the chapter, however, is based on lipid metabolism, which continues to represent most of the storage product research on MDEs of Brassicaceae. Indeed, efforts to understand the details of canola oil formation and improve oil yield and fatty acid (FA) composition of the oil are directly linked to the economic importance of the crop. The discussion is extended to include storage product metabolism in an MD cell suspension culture of B. napus L. cv. Jet Neuf which was initially used in studies of freezing tolerance (Orr et al. 1986; Johnson-Flanagan and Singh 1987; Johnson-Flanagan et al. 1991). The MD cell suspension culture was produced early in 1983 in an attempt to generate MDEs (Simmonds et al. 1991). Where appropriate, the effects of abiotic stress and external carbon source on storage product metabolism in MD cultures of B. napus are also examined.