Postgerminative growth and lipid catabolism in oilseeds lacking the glyoxylate cycle

Abstract

The glyoxylate cycle is regarded as essential for postgerminative growth and seedling establishment in oilseed plants. We have identified two allelic Arabidopsis mutants, icl-1 and icl-2, which lack the glyoxylate cycle because of the absence of the key enzyme isocitrate lyase. These mutants demonstrate that the glyoxylate cycle is not essential for germination. Furthermore, photosynthesis can compensate for the absence of the glyoxylate cycle during postgerminative growth, and only when light intensity or day length is decreased does seedling establishment become compromised. The provision of exogenous sugars can overcome this growth deficiency. The icl mutants also demonstrate that the glyoxylate cycle is important for seedling survival and recovery after prolonged dark conditions that approximate growth in nature. Surprisingly, despite their inability to catalyze the net conversion of acetate to carbohydrate, mutant seedlings are able to break down storage lipids. Results suggest that lipids can be used as a source of carbon for respiration in germinating oilseeds and that products of fatty acid catabolism can pass from the peroxisome to the mitochondrion independently of the glyoxylate cycle. However, an additional anaplerotic source of carbon is required for lipid breakdown and seedling establishment. This source can be provided by the glyoxylate cycle or, in its absence, by exogenous sucrose or photosynthesis.

On the role of a Lipid-Transfer Protein. Arabidopsis ltp3 mutant is compromised in germination and seedling growth.

“…[30][31] So, Atltp3 was further evaluated for its seedling phenotype. To that aim, mutant and wild type seeds were grown in darkness without any carbon supply addition to assess TAG dependent postgerminative growth.…”

“…Germination was evaluated under magnifying glass and defined as radicle protrusion. To analyze postgerminative growth etiolated seedlings 5 d after stratification were photographed and digitalized using a Nikon DS-FI 1 camera and hypocotyl length was determined from the original images using IMAGE J software (NIH, Bethesda, Md), according to Eastmond et al 30 . Seed viability was determined using 1% TTC as vital dye.…”

“…In oilseeds such as A. thaliana, seedling growth is accomplished by the catabolism of fatty acids. 30 Briefly, oils stored in the cotyledons are converted to fatty acids and glycerol and the former are next broken down by b-oxidation in the peroxisomes to produce succinate through the glyoxylate cycle. The resulting succinate is further converted to carbohydrates and used to sustain seedling growth.…”

The COMATOSE ATP-Binding Cassette Transporter Is Required for Full Fertility in Arabidopsis

“…In this scenario, acetyl-CoA produced by b-oxidation is converted to citrate, which is exported from the peroxisome and participates in the citric acid cycle (Pracharoenwattana et al, 2005). Thus, products of fatty acid catabolism can pass from the peroxisome to the mitochondrion independently of the glyoxylate cycle, as is the case in Arabidopsis mutants, which lack isocitrate lyase (Eastmond et al, 2000).…”

Plant respiration under low oxygen

“…For example, in Arabidopsis seeds, lipids are stored in cotyledons that show marked icl and mls gene expression coding for isocitrate lyase and malate synthase, respectively, two key enzymes in the glyoxylate cycle (Eastmond and Graham, 2001). Light plays an important role in post germination growth, it being observed in mutants of Arabidopsis (icl) that a reduction in the activity of isocitrate lyase inhibits lipid breakdown and strongly reduces the growth of hypocotyl (Eastmond et al, 2000). In seedlings, the glyoxylate cycle plays a key role in growth because it is involved in lipid metabolism through the ß-oxidation of fatty acids (Finkelstein and Gibson, 2002).…”