Non-symbiotic hemoglobin-2 leads to an elevated energy state and to a combined increase in polyunsaturated fatty acids and total oil content when over-expressed in developing seeds of transgenic Arabidopsis plants.

Non-symbiotic hemoglobins are ubiquitously expressed in plants and divided into two different classes based on gene-expression pattern and oxygen binding-properties. Most of the published research has been on the function of class-1 hemoglobins. To investigate the role of class-2 hemoglobins, transgenic Arabidopsis plants were generated over-expressing Arabidopsis hemoglobin-2 (AHb2) under the control of a seed-specific promoter. Over-expression of AHb2 led to a 40% increase in the total fatty acid content of developing and mature seeds in three subsequent generations. This was mainly due to an increase in the poly-unsaturated C18:2 (omega-6) linoleic and C18:3 (omega-3) alpha-linolenic acids. Moreover, AHb2 over-expression led to an increase in the C18:2/C18:1 and C18:3/C18:2 ratios as well as in the C18:3 content in mol% of total fatty acids and in the unsaturation/saturation index of total seed lipids. The increase in fatty-acid content was mainly due to a stimulation of the rate of triacylglycerol synthesis which was attributable to a 3-fold higher energy state and a 2-fold higher sucrose content of the seeds. Under low external oxygen, AHb2 over-expression maintained an up to 5-fold higher energy state and prevented fermentation. This is consistent with AHb2 over-expression results in improved oxygen availability within developing seeds. In contrast to this, over-expression of class-1 hemoglobin did not lead to any significant increase in the metabolic performance of the seeds. Results provide evidence for a specific function of class-2 hemoglobin in seed oil production and in promoting the accumulation of poly-unsaturated fatty acids by facilitating oxygen supply in developing seeds.