Oxygen dependency of germinating Brassica seeds

Establishing plants in space, Moon or Mars requires adaptation to altered conditions, including reduced pressure and composition of atmospheres. To determine the oxygen requirements for seed germination, we imbibed Brassica rapa seeds under varying oxygen concentrations and profiled the transcription patterns of genes related to early metabolism such as starch degradation, glycolysis, and fermentation. We also analyzed the activity of lactate dehydrogenase (LDH) and alcohol dehydrogenase (ADH), and measured starch degradation. Partial oxygen pressure (pO2) greater than 10% resulted in normal germination (i.e., protrusion of radicle about 18 hours after imbibition) but lower pO2 delayed and reduced germination. Imbibition in an oxygen-free atmosphere for three days resulted in no germination but subsequent transfer to air initiated germination in 75% of the seeds and the root growth rate was transiently greater than in roots germinated under ambient pO2. In hypoxic seeds soluble sugars degraded faster but the content of starch after 24 h was higher than at ambient oxygen. Transcription of genes related to starch degradation, α-amylase (AMY) and Sucrose Synthase (SUS), was higher under ambient O2 than under hypoxia. Glycolysis and fermentation pathway-related genes, glucose phosphate isomerase (GPI), 6-phosphofructokinase (PFK), fructose 1,6-bisphosphate aldolase (ALD), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), pyruvate decarboxylase (PDC), LDH, and ADH, were induced by low pO2. The activity of LDH and ADH was the highest in anoxic seeds. Germination under low O2 conditions initiated ethanolic fermentation. Therefore, sufficient oxygen availability is important for germination before photosynthesis provides necessary oxygen and the determination of an oxygen carrying capacity is important for uniform growth in space conditions.