PARENT SESSION

Symposium S1C Photosynthate transport and transporters
Monday August 30th, 2004 10:20 AM-12:20 PM Room 510A
Chair: Andreas Weber
Co-Chair: Marinus Pilon

Copper delivery for photosynthesis in plant chloroplasts. Marinus Pilon^*,1, Salah Abdel-Ghany¹, Jason Burkhead¹, ¹ Colorado State University, Fort Collins, Colorado, USA

ABSTRACT- Two major targets for Cu delivery in plant chloroplasts are the electron carrier plastocyanin in the thylakoid lumen and Cu/Zn-superoxide dismutase (Cu/ZnSOD) in the stroma. Key components involved in Cu delivery to chloroplasts include two Cu-transporters and soluble Cu-chaperones. PAA1 and PAA2 encode copper-transporting P-type ATPases. Characterization of paa1 and paa2 mutants showed that the two transporters have distinct functions; whereas both transporters are required for copper delivery to plastocyanin and efficient electron transport, copper delivery to the stroma is only inhibited in paa1 but not in paa2 mutants. Direct localization experiments showed that PAA1 and PAA2 function in chloroplasts and that the PAA1 protein is in the chloroplast periphery. A paa1 paa2 double mutant was seedling lethal, underscoring the importance of copper to photosynthesis in plants. We propose that PAA1 and PAA2 function sequentially in copper transport over the envelope and thylakoid membrane respectively. A third component of the Cu delivery system in chloroplasts is CpCCS, a stromal Cu-chaperone that most likely delivers Cu to stromal Cu/ZnSOD. Next to Cu/ZnSOD, plants have a FeSOD in the stroma of the chloroplast. Interestingly, Cu availability in the chloroplast regulates the activity of the stromal SOD isoforms. At low Cu levels, the FeSOD is active and Cu/ZnSOD expression is shut off, so Cu is preferentially targeted to plastocyanin in the thylakoid lumen. At higher Cu levels, FeSOD expression is shut off, possibly saving Fe for other uses, and Cu/Zn SOD becomes a major sink for Cu in the stroma. The effects of Cu feeding and paa1 and paa2 mutations on transcript levels of superoxide dismutase and CpCCS strongly suggest that stromal copper levels regulate expression of these nuclear genes. Thus a novel signaling pathway exists which senses Cu levels in the chloroplast and affects expression of nuclear genes.

KEY WORDS: electron transport, chloroplast biogenesis, copper proteins, copper transport and homeostasis