Poster: Photosynthesis

Abs # 313: Overexpressing PsbS Enhances Violaxanthin De-epoxidase Activity

Presenter:	Yamamoto, Harry Y., yamamoto@hawaii.edu
Authors	Yamamoto, Harry Y. (A)   Hieber, Andrew D. (A)   Kawabata, Osamu  (A)
Affiliations:	(A): University of Hawaii at Manoa

Zeaxanthin + antheraxanthin (Z+A), formed by the xanthophyll cycle, PsbS, the CP22 protein of Photosystem II, and low intrathylakoid pH are necessary for non-photochemical quenching (NPQ) of excess energy that protects plants from the damaging effects of high light. We show in tobacco plants transformed to overexpress, VDE, PsbS and PsbS+VDE that high initial rates of de-epoxidation and high final de-epoxidation states can be induced under moderate light intensity (450 mmol photons m^-2 sec^-1) without a correspondingly high NPQ. Under moderate light, relative to wild type, overexpressing VDE, PsbS and PsbS+VDE increased de-epoxidation by 33%, 12% and 28%, respectively. The enhancement of VDE activity by PsbS was unexpected. The enhancement by PsbS is hypothesized to be due to a more optimal pH environment for VDE activity and implies that VDE is localized sufficiently close to sense the protons bound by PsbS. Under high light, NPQ and de-epoxidation were both high, the de-epoxidation state being nearly equal in all four plant types and NPQ doubling in PsbS plants. Based on these and other results, a new hypothesis for the organization and function of the xanthophyll cycle, specifically VDE, is proposed wherein Z is a multi-target secondary messenger, formed under moderate light, for signal-transduction of protective systems against impending excess light.