Control of seed development in Arabidopsis thaliana by atmospheric oxygen

Seed development is known to be inhibited completely when plants are grown in oxygen concentrations below 5.1 kPa, but apart from reports of decreased seed weight little is known about embryogenesis at subambient oxygen concentrations above this critical level. Arabidopsis thaliana (L.) Heynh. plants were grown full term under continuous light in premixed atmospheres with oxygen partial pressures of 2.5, 5.1, 10.1, 16.2 and 21.3 kPa O2, 0.035 kPa CO2 and the balance nitrogen. Seeds were harvested for germination tests and microscopy when siliques had yellowed. Seed germination was depressed in O2 treatments below 16.2 kPa, and seeds from plants grown in 2.5 kPa O2 did not germinate at all. Fewer than 25% of the seeds from plants grown in 5.1 kPa oxygen germinated and most of the seedlings appeared abnormal. Light and scanning electron microscopic observation of non-germinated seeds showed that these embryos had stopped growing at different developmental stages depending upon the prevailing oxygen level. Embryos stopped growing at the heart-shaped to linear cotyledon stage in 5.1 kPa O2, at around the curled cotyledon stage in 10.1 kPa O2, and at the premature stage in 16.2 kPa O2. Globular and heart-shaped embryos were observed in sectioned seeds from plants grown in 2.5 kPa O2. Tissue degeneration caused by cell autolysis and changes in cell structure were observed in cotyledons and radicles. Transmission electron microscopy of mature seeds showed that storage substances, such as protein bodies, were reduced in subambient oxygen treatments. The results demonstrate control of embryo development by oxygen in Arabidopsis.