The molecular control of oil biosynthesis in Arabidopsis

Abstract

The enzyme acetyl-CoA carboxylase exists in Arabidopsis as one plastidic heteromeric form and two homomeric isoforms, ACC1 and ACC2; with different subcellular localisations, ACC1 is cytosolic, while ACC2 is plastidial. Plants that are mutant for ACC1 show that this protein is essential for the synthesis of very long chain fatty acids in the developing seed of Arabidopsis, and is also required for correct patterning of cell division in the developing embryo. While the ACC1 gene is transcribed in both seed and leaves, the enzyme is only active in the seed at high levels, leading to the accumulation of storage triacyl glycerols (TAGs). In investigating the possible post-transcriptional control of ACC1 activity, we found that the ACC1 transcript is alternatively spliced. Analysis of ACC1 splice isoforms in both wildtype seedlings and in seedlings of the mdf1 loss-of-function mutant, which is defective in splicing control, shows that the ACC1 transcript has an alternative donor site in the 5' UTR, upstream of the translation start site. Leaf and seed show different proportions of spliced and non-spliced versions of the ACC1 transcript, with the non-spliced isoform being abundant in leaf, consistent with a role for splicing in regulating ACC1 activity. Mis-splicing of ACC1 in the mdf mutant or in transgenic MDF overexpressors is associated with aberrant cell division in the root meristem and ectopic lipid accumulation. Genetic complementation of the mdf mutant with a genomic sequence of the ACC1 gene under the control of the MDF gene promoter leads to a partial rescue of the meristematic activity in the root and shoot systems, enhancing the development of lateral roots and true leaves, respectively. These results show that correct splicing of ACC1 is required for both correct cell division and tissue development in Arabidopsis.