Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2006
Publication Date: 3/1/2007
Citation: Qiu, Q., Hardin, S., Mace, J., Brutnell, T.P., Huber, S.C. 2007. Metabolic signals control the selective degradation of sucrose synthase in maize leaves during de-etiolation. Plant Physiology. 144(1):468-478. Interpretive Summary: Light has a profound impact on plant growth. When grown completely in the dark, etiolated seedlings develop that will rapidly green and become photosynthetically active in a process known as de-etiolation. In order to produce the photosynthetic enzymes and machinery necessary to become autotrophic, plants undergoing de-etiolation must selectively degrade enzymes and proteins that are no longer required such as enzymes of sucrose breakdown. One such enzyme is sucrose synthase, which is an abundant protein in etiolated seedlings but is rapidly degraded in response to light. Proteasomes, which function in the selective degradation of cellular proteins, seem not to be involved in the degradation of sucrose synthase. The degradation process seems to be controlled by availability of amino acids, in particular glutamate. The control of protein degradation by free amino acids is novel and may function to regulate protein content in response to other physiological conditions in addition to de-etiolation.
Technical Abstract: The content and activity of sucrose (Suc) synthase (SUS) protein is high in sink organs but low in source organs. In the present report, we examined light and metabolic signals regulating SUS protein degradation in maize (Zea mays L.) leaves during de-etiolation. We found that SUS protein accumulated in etiolated leaves of the dark-grown seedlings but was rapidly degraded upon exposure to white, blue, or red light. This occurred concurrent with the accumulation of photosynthetic enzymes, such as Rubisco and Rubisco activase, and enzymes of Suc biosynthesis such as Suc-phosphate synthase. De-etiolation-induced SUS degradation was not inhibited by the proteasome inhibitor MG132. Moreover, neither full-length nor truncated SUS phosphorylated at the Ser-170 site was found in the crude 26S proteasome fraction (150,000g post-microsomal pellet) isolated in the presence of MG132. However, SUS degradation was strongly inhibited by feeding cycloheximide or amino acids to detached leaves, while Suc feeding had no effect. Of the amino acids tested, exogenous glutamate had the greatest effect. Collectively, these results demonstrate that SUS protein degradation during de-etiolation: (1) is selective; (2) can be triggered by either blue- or red-light mediated signaling pathways; (3) does not involve the 26S proteasome; and (4) is inhibited by free amino acids. These findings suggest that SUS degradation is important to supply residues for the synthesis of other proteins required for autotrophic metabolism.