Turnover of Fatty Acids during Natural Senescence of Arabidopsis, Brachypodium, and Switchgrass and in Arabidopsis β-Oxidation Mutants      

Abstract

During leaf senescence, macromolecule breakdown occurs and nutrients are translocated to support growth of new vegetative tissues, seeds, or other storage organs. In this study, we determined the fatty acid levels and profiles in Arabidopsis (Arabidopsis thaliana), Brachypodium distachyon, and switchgrass (Panicum virgatum) leaves during natural senescence. In young leaves, fatty acids represent 4% to 5% of dry weight and approximately 10% of the chemical energy content of the leaf tissues. In all three species, fatty acid levels in leaves began to decline at the onset of leaf senescence and progressively decreased as senescence advanced, resulting in a greater than 80% decline in fatty acids on a dry weight basis. During senescence, Arabidopsis leaves lost 1.6% of fatty acids per day at a rate of 2.1 mg per leaf (0.6 mg mg 21 dry weight). Triacylglycerol levels remained less than 1% of total lipids at all stages. In contrast to glycerolipids, aliphatic surface waxes of Arabidopsis leaves were much more stable, showing only minor reduction during senescence. We also examined three Arabidopsis mutants, acx1acx2, lacs6lacs7, and kat2, which are blocked in enzyme activities of b-oxidation and are defective in lipid mobilization during seed germination. In each case, no major differences in the fatty acid contents of leaves were observed between these mutants and the wild type, indicating that several mutations in b-oxidation that cause reduced breakdown of reserve oil in seeds do not substantially reduce the degradation of fatty acids during leaf senescence.

The SUGAR-DEPENDENT1 Lipase Limits Triacylglycerol Accumulation in Vegetative Tissues of Arabidopsis

“…TAG was separated from total lipids by thinlayer chromatography (TLC), transmethylated, and quantified by gas chromatography of fatty acid methyl esters (Xu et al, 2005). The data showed that TAG levels are extremely low (around 0.02% of dry weight) in all three wild-type tissues, consistent with previous studies (Yang and Ohlrogge, 2009). However, in the stems and roots of sdp1-5, substantially more TAG accumulated, while the effect was much less pronounced in leaves ( Fig.…”

“…In this study, we show that disruption of the TAG lipase SDP1 (Eastmond, 2006) leads to an accumulation of oil in vegetative tissues of Arabidopsis, suggesting that TAG turnover occurs in these tissues despite their very low steady-state TAG content (Yang and Ohlrogge, 2009). Although we show that SDP1 is expressed (and the protein is present) in all tissues of a rosette plant, the accumulation of TAG is far more pronounced in heterotrophic tissues (i.e.…”

“…CGI58 is one candidate, since the protein has lipase activity (Ghosh et al, 2009) and the mutant accumulates TAG in its leaves to around 0.2% of dry weight (James et al, 2010), which is higher than we detected in sdp1. The disruption of fatty acid b-oxidation also leads to TAG accumulation in leaves (Slocombe et al, 2009;James et al, 2010), but the impact on total fatty acid content appears to be small (Yang and Ohlrogge, 2009), unless the tissue is subjected to carbohydrate starvation (Kunz et al, 2009;Slocombe et al, 2009). Therefore, TAG turnover in the cytosol may simply be less rapid in leaves than in roots and stems.…”

“…Nevertheless, the oil content of leaves, stems, and roots is extremely low in all but a very few plant species (Durrett et al, 2008). For example, oil accounts for much less than 0.1% of dry weight in Arabidopsis (Arabidopsis thaliana) leaves (Yang and Ohlrogge, 2009). However, a number of studies have established that the oil content can be boosted by the overexpression of individual oil biosynthetic enzymes such as ACYL-COENZYME A:DI-ACYLGLYCEROL ACYLTRANSFERASE1 (DGAT1; Bouvier-Navé et al, 2000) or transcriptional "master" regulators that govern the expression of multiple enzymes in the pathway, such as WRINKLED1 (WRI1), LEAFY COTYLDON1 (LEC1), and LEC2 (Cernac and Benning, 2004;Mu et al, 2008;Andrianov et al, 2010;Sanjaya et al, 2011).…”

Comprehensive Dissection of Spatiotemporal Metabolic Shifts in Primary, Secondary, and Lipid Metabolism during Developmental Senescence in Arabidopsis

“…Therefore, in order to consider the evidence of specific roles for metabolites in senescence, a more precise analysis of metabolite levels in different tissues and cell types during the process of senescence is required. Although, to date, a number of targeted metabolite profiles, particularly those focused on lipids as seed storage compounds (Yang and Ohlrogge, 2009;Seltmann et al, 2010) and sugars, amino acids, and nutrient ions (Masclaux et al, 2000;Quirino et al, 2001;Stessman et al, 2002;Diaz et al, 2005;Masclaux-Daubresse et al, 2005, 2007Pourtau et al, 2006;Wingler et al, 2006Wingler et al, , 2012, have been generated from maturing and senescing tissues, a broad overview of metabolic changes during senescence, including the interactions between various metabolic pathways, is still lacking. Therefore, we performed a metabolomics study to obtain comprehensive metabolite profiles, including primary and secondary metabolites and lipids, over the course of developmental senescence in rosette leaves before and after bolting and siliques of Arabidopsis in order to investigate the metabolic response and spatiotemporal distribution of metabolites during senescence at the whole-plant level and within single leaves.…”