Ghouziel Benhassaine-Kesri

With rare exceptions, dicot plastids have been reported to contain only a multisubunit (prokaryotic) form of acetyl-coA carboxylase (ACCase), the first committed step of lipid biosynthesis. The sensitivity of most monocots to cyclohexanediones (CHDs) such as sethoxydim, has been shown to be associated with the presence in their plastids of a multifunctional (eukaryotic) form of ACCase. Little is known about the effects of sethoxydim on lipid metabolism and ACCase activity in dicots. Here it is shown that foliar lipid biosynthesis is differentially affected by the herbicide treatment in two dicot species, Nicotiana sylvestris (wild tobacco) and Glycine max (soybean). In N. sylvestris, the total lipid content of neoformed leaves harvested 2 weeks after the sethoxydim treatment was unaffected by doses of up to 10⁻³ M sethoxydim. In soybean, lipid content decreased by 45% when 10⁻⁵ M sethoxydim was used, and this was associated with a 30% reduction in fatty acid synthesis activity. ACCase activity of soybean plastidial preparations was 60% reduced in the presence of sethoxydim, whereas that of N. sylvestris was unaffected. Finally, the presence of a biotinylated 220 kDa polypeptide, corresponding in size to multifunctional ACCase, was observed in soybean plastids. Possible relationships between sensitivity of plastidial soybean ACCase towards sethoxydim, plastidial protein content, and altered de novo lipid biosynthesis in herbicide-treated plants are discussed.

Rape (Brassica napus L. var. Bienvenue) is a 16:3 plant which contains predominantly prokaryotic species of monogalactosyldiacylglycerol i.e. sn‐1 C18, sn‐2 C16 (C18/C16 MGDG). Rape plants were exposed to a restricted water supply for 12 days. Under drought conditions, considerable changes in lipid metabolism were observed. Drought stress provoked a decline in leaf polar lipids, which is mainly due to a decrease in MGDG content. Determination of molecular species in phosphatidylcholine (PC) and MGDG indicated that the prokaryotic molecular species of MGDG (C18/C16) decreased after drought stress while the eukaryotic molecular species (C18/C18) remained stable. Drought stress had different effects on two key enzymes of PC and MGDG synthesis. The in vitro activity of MGDG synthase (EC. 2.4.1.46) was reduced in drought stressed plants whereas cholinephosphotransferase (EC. 2.7.8.2) activity was not affected. Altogether these results suggest that the prokaryotic pathway leading to MGDG synthesis was strongly affected by drought stress while the eukaryotic pathway was not. It was also observed that the molecular species of leaf PC became more saturated in drought stressed plants. This could be due to a specific decrease in oleate desaturase activity.