Understanding the mechanism of lipolysis in plants: an opportunity to increase oil yield in crops

Oil from seeds forms a major source of nutrition for humans and livestock. It also has many important industrial uses, among them providing an increasingly relevant source of renewable energy (bio-diesel). The rate of oil accumulation in developing seeds is governed predominantly by biosynthesis. However, a number of studies have reported that a significant amount of oil is also turned over during seed development. Blocking this turnover could potentially elevate oil levels by between 5 and 25%, depending on the species and growth conditions. Controlling oil breakdown in seeds requires knowledge of the molecular mechanism, which until recently was completely lacking. This process also occurs after seed germination where it plays a fundamentally important role in providing energy for early seedling growth. I have gained a new insight into the mechanism of oil breakdown by isolating mutants in the model oilseed plant Arabidopsis that are impaired in post-germinative growth. I have discovered that one of these mutants, called sugar-dependent1, has a defect in the enzyme triacylglycerol hydrolase, which catalyses the first step in oil breakdown. The rate of oil breakdown is dramatically slowed in this mutant and as a consequence the developing seeds accumulate significantly more oil. The goals of this proposal are (i) To study how SDP1 is regulated and establish whether oil breakdown can be inhibited during seed development and not following germination. This would allow oil yield to be enhanced with the minimum impact on seedling vigour. (ii) To identify additional structural and regulatory proteins that function with SDP1 to govern the rate of oil breakdown. Disruption of these proteins will be used to block oil breakdown completely and thereby maximize oil accumulation. (iii) To investigate the role of SDP1 in the crop species oilseed rape and determine if oil yield can also be increased by impairing oil turnover. Addressing these objectives will contribute greatly to our fundamental knowledge of the mechanism and regulation of lipolysis, which is major metabolic process that is essential for the life cycle of many plants. The work could also lead to the development of crop plants with a higher oil yield.

Theoretically, oil accumulation in developing seeds depends on a balance between synthesis and degradation. Several studies suggest that a significant turnover of oil does occur in a variety of oilseed species including the crop oilseed rape. This is evident particularly in the later stages of seed development where there is a fall in oil content. Restricting oil breakdown during seed development might therefore represent a new strategy to enhance oil accumulation and yield in oilseed crops. I have recently discovered the triacylglycerol lipase (SDP1) that is largely responsible for initiating oil breakdown in seeds. Disruption of SDP1 in Arabidopsis impairs oil breakdown following seed germination and therefore slows the rate of early seedling growth. However, it also elevates the level of oil in mature seeds. This supports the hypothesis that there is significant oil turnover during seed development. The first objective of the proposal will be to investigate the regulation of SDP1 and whether oil breakdown can be inhibited during seed development but not following germination. This will be addressed by transforming Arabidopsis with various constructs designed to tag SDP1 in vivo or to control when during development SDP1 is produced. The second objective will be to identify genes that function together with SDP1 to govern oil breakdown, and to disrupt them in order to maximize oil accumulation. These genes will be identified in Arabidopsis using a combination of experimental approaches that utilise mutants I have previously isolated. The third objective of this proposal is to determine if oil yield can also be increased in the crop oilseed rape. This will be investigated using a transgenic approach. SDP1 levels will be suppressed in developing seeds using RNAi and the effect on oil accumulation will be measured.