Regulation of ribulose 1, 5-bisphosphate carboxylase/oxygenase by 2-carboxyarabinitdol 1-phosphate

Includes bibliographical references (pages 88-93)2-carboxyarabinitol 1-phosphate (CA 1-P) is a naturally occurring inhibitor of the primary carbon fixing enzyme ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco) and is synthesized in numerous piant species. In the first part of this study, an extensive survey of the degree of dark inhibition of Rubisco was undertaken in the subfamily Papilionoideae to elucidate any evolutionary pattern in the occurrence of this regulatory mechanism. Members of the Fabaceae family show a wide variation in the extent of CA 1-P accumulation during the darkness. Eighty-eight species from 22 tribes were examined from this family. Dark inhibition was verified in ancestral tribes such as the Sophoreae, but was substantially reduced or absent in representative species of three more recently evolved tribes: Cicereae, Hedvsareae, and Viceae. Therefore, regulation of Rubisco by CA 1-P is neither of recent origin nor restricted in distribution among the Papilionoideae. CA 1-P is probably an early evolutionary adaptation in legumes rather than a "novel" regulatory mechanism of Rubisco. In the second part of this study, modulation of Rubisco activity by CA 1-P in developing soybeans was investigated using chamber grown plants. Three systems were used: (i) deetiolated primary leaves, (ii) expanded 3rd node trifoliolate leaves, and (iii) leaves at ten insertion levels on mature vegetative plants. Dark-sampled etiolated leaves did not contain measurable levels of CA 1-P, but they acquired the ability to synthesize and degrade CA 1-P after 24-h of illumination. The 3rd node leaflets showed appreciable CA 1-P synthesis during during early leaf expansion, with the dark inhibition of Rubisco by CA 1-P declining over the 23 day sampling period. In mature soybeans, variation in the extent of dark inhibition of Rubisco was evident in leaf nodes from the apex to the base of the plant. Young leaflets also showed the capacity to accumulate CA 1-P. These observations indicate that the interaction of CA 1-P with Rubisco occurs throughout normal leaf ontogeny, and suggests that CA 1-P synthesis /degradation may represent an integral regulatory mechanism for Rubisco in soybean.M.S. (Master of Science