Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant Pathology and Crop Physiology

First Advisor

Mary E. Musgrave


Interest in the improvement of crop productivity through repression of photorespiration led to experimental growth of plants in low O2 and the surprising finding that reproduction is inhibited under these conditions. Using Arabidopsis thaliana (L.) Heynh. cv. Columbia, studies were undertaken to understand the mechanism behind oxygen control of plant development. The hypothesis that developmental changes observed in low O 2 are due to repression of the plant hormones ethylene and brassinolide (which require O2 for biosynthesis) was tested. Arabidopsis was grown for 35 days in Sun bags in one of five altered O 2 atmospheres (210, 160, 100, 50, 25 mmol/mol) with 0.35 mmol/mol CO 2 in N2. Molecular analysis of ethylene biosynthesis was accomplished using cDNAs encoding ACC synthase and ACC oxidase in ribonuclease protection assays and in situ hybridizations. To understand if low O2 inhibition of brassinolide biosynthesis was responsible for developmental changes in low O2, brassinolide-replacement experiments were performed. Arabidopsis was grown for 10 days on nutrient agar or nutrient agar + 10-7 M brassinolide in magenta vessels coupled to gas mixtures of 210 or 25 mmol/mol O2, 0.35 mmol/mol CO2 in N2. When grown in 25 mmol/mol O2, plants exhibited dwarf morphology resembling the brassinolide-deficient mutant det2, and this line was incorporated into the brassinolide studies at low O2 for comparison. Leaf development under low O2 resulted in changes in leaf size, stomatal density and stomatal patterning. Stomatal density increased in all low O2 treatments but was not linked to changes in ethylene or brassinolide. Low O2 changes in leaf size, however, were mediated through depression of brassinolide biosynthesis. There was a loss of ethylene biosynthesis in siliques from plants grown at 50 and 25 mmol/mol O2, and silique ACC oxidase mRNA increased as O2 was lowered. As O2 decreased, tissue-specific patterning of ACC oxidase and ACC synthase gene expression shifted from the embryo to the silique wall. Loss of ethylene in the silique was tightly correlated with loss of the embryo in seeds produced in low O2. These data show that changes in development under low oxygen can be explained by repression of O2-sensitive plant hormones.