The ABC Transporter PXA1 and Peroxisomal β-Oxidation Are Vital for Metabolism in Mature Leaves ofArabidopsisduring Extended Darkness    

Abstract

Fatty acid b-oxidation is essential for seedling establishment of oilseed plants, but little is known about its role in leaf metabolism of adult plants. Arabidopsis thaliana plants with loss-of-function mutations in the peroxisomal ABC-transporter1 (PXA1) or the core b-oxidation enzyme keto-acyl-thiolase 2 (KAT2) have impaired peroxisomal b-oxidation. pxa1 and kat2 plants developed severe leaf necrosis, bleached rapidly when returned to light, and died after extended dark treatment, whereas the wild type was unaffected. Dark-treated pxa1 plants showed a decrease in photosystem II efficiency early on and accumulation of free fatty acids, mostly a-linolenic acid [18:3(n-3)] and pheophorbide a, a phototoxic chlorophyll catabolite causing the rapid bleaching. Isolated wild-type and pxa1 chloroplasts challenged with comparable a-linolenic acid concentrations both showed an 80% reduction in photosynthetic electron transport, whereas intact pxa1 plants were more susceptible to the toxic effects of a-linolenic acid than the wild type. Furthermore, starch-free mutants with impaired PXA1 function showed the phenotype more quickly, indicating a link between energy metabolism and b-oxidation. We conclude that the accumulation of free polyunsaturated fatty acids causes membrane damage in pxa1 and kat2 plants and propose a model in which fatty acid respiration via peroxisomal b-oxidation plays a major role in dark-treated plants after depletion of starch reserves.

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

“…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.…”

“…Previous studies have shown that the disruption of several other genes associated with lipid catabolism can also lead to TAG accumulation in vegetative tissues, although heterotrophic tissues have not generally been investigated (Kunz et al, 2009;Slocombe et al, 2009;James et al, 2010). PXA1 is a peroxisomal ATPbinding cassette transporter that is required for fatty acid import for b-oxidation (Zolman et al, 2001), and CGI58 is an enzyme that has been reported to have lipase, phospholipase, and lysophosphatidic acid acyltransferase activities (Ghosh et al, 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). In addition, several mutants have been identified that exhibit ectopic oil accumulation (Ogas et al, 1997;Xu et al, 2005;Kunz et al, 2009;Slocombe et al, 2009;James et al, 2010). Among these are pxa1 (peroxisomal ABC transporter1) and cgi58 (comparative gene identification-58), which are associated with lipid catabolism.…”

ABI4 Activates DGAT1 Expression in Arabidopsis Seedlings during Nitrogen Deficiency

“…Here, we suggest that there is also a homeostasis between TAG biosynthesis and degradation in higher plants. Experimental evidence has indicated that extended darkness treatment of fatty acid breakdown mutants, such as pxa1, cts2, and acx1acx2, led to the ectopic accumulation of TAG in the leaf (Kunz et al, 2009;Slocombe et al, 2009). The TAG content in naturally senescing leaves of fatty acid breakdown mutants was also increased (Slocombe et al, 2009).…”

Peroxisome Function, Biogenesis, and Dynamics in Plants

“…The transcripts encoding thiolase and ACX4 accumulate during carbon-starvation (Charlton et al, 2005;Contento and Bassham, 2010), implying that fatty acid b-oxidation increases upon nutrient deprivation. Indeed, several b-oxidation mutants, including pxa1 and mutants with reduced thiolase activity, die prematurely in extended darkness (Dong et al, 2009;Kunz et al, 2009). Both pxa1 and sdp1 mutants catabolize triacylglycerol inefficiently in extended darkness, but sdp1 lowers free fatty acid accumulation and ameliorates plant death in dark-treated pxa1 mutants (Fan et al, 2017), implying that free fatty acid toxicity rather than carbon starvation contributes to pxa1 death.…”