Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/18/2006
Publication Date: 12/1/2006
Citation: Volk, G.M., J. Crane, A.M. Caspersen, L.M. Hill, C.A. Gardner and C. Walters. 2006. Massive cellular disruption occurs during early imbibition of Cuphea seeds containing crystallized triacylglycerols. Planta 224:1415-1426.
Interpretive Summary: This work investigates the basis for poor storage behavior of seeds produced by several species originating from tropical and subtropical regions. We use the genus Cuphea as a test system because seeds of congeners vary in storage behavior, and this can be predicted reliably from the lipid composition within the seeds. Composition of lipids affects their phase behavior and lipids with longer or more saturated fatty acids will crystallize and melt at higher temperatures than lipids with shorter or more unsaturated fatty acids. In earlier studies, we demonstrated a cause-effect relationship between imbibition when storage lipids were crystalline and seed death. In this paper we examine cellular changes using transmission electron microscopy in order to localize the site and mechanism of damage when seeds were given combinations of temperature and imbibition treatments. We show that cells disintegrate during early imbibition if the lipids were crystallized. Interaction between water and storage lipids in seeds is often perceived as insignificant because ‘oil and water don’t mix.’ Our work suggests that the hydrophilic and hydrophobic forces that ordinarily keep water and lipids separated, and amphiphilic molecules at the interface, change when the lipids crystallize and may result in destabilized cell structures when water is added to dry cells. If this is the case, then water-lipid interactions are profoundly important to the overall integrity of the cell.
Technical Abstract: The transition from anhydrobiotic to hydrated state occurs during early imbibition of seeds and is lethal if lipid reserves in seeds are crystalline. Lipids are crystallized by low temperatures used during seed storage. Here, we examine the nature of cellular damage observed in seeds of Cuphea wrightii and C. lanceolata that differ in triacylglycerol composition and phase behavior. Intracellular structure, observed using transmission electron microscopy, is profoundly and irreversibly perturbed if seeds with crystalline triacylglycerols are imbibed briefly. A brief heat treatment that melts triacylglycerols before imbibition prevents the loss of cell integrity; however, residual effects of cold treatments in C. wrightii cells is reflected by the apparent coalescence of protein and oil bodies. The timing and temperature dependence of cellular changes suggests that damage arises via a physical mechanism, perhaps as a result of shifts in hydrophobic and hydrophilic interactions when triacylglycerols undergo phase changes. Stabilizers of oil body structure such as oleosins, that rely on a balance of physical forces, may become ineffective when triacylglycerols crystallize. Recent observation linking poor oil body stability and poor seed storage behavior are potentially explained by the phase behavior of the storage lipids.