Regulation of Photosynthetic Light Harvesting

In nature, algae and plants experience a large dynamic range of light intensities, and they need to regulate photosynthesis to balance the absorption and utilization of light energy. In saturating light, photosynthetic light harvesting is regulated by a suite of non-photochemical quenching (NPQ) mechanisms that safely dissipate excess absorbed light energy as heat. The major goal of our NPQ research is to understand the mechanisms of different NPQ components in diverse photosynthetic organisms.

There are at least four different NPQ components (qE, qZ, qH, and qI), which differ in their induction and relaxation kinetics and molecular mechanisms. We have identified and characterized plant and algal mutants that are defective in NPQ, and we collaborate closely with the lab of Graham Fleming (UC-Berkeley and LBNL) to apply ultrafast spectroscopy and modeling to understand how NPQ works. The mutant analysis helped to define the carotenoid pigments (zeaxanthin and lutein) that are involved in NPQ and showed that a photosystem II protein of previously unknown function, PsbS, is necessary for the rapidly reversible component of NPQ (qE) in plants such as Arabidopsis. PsbS appears to function as a sensor of thylakoid lumen pH that is essential for catalyzing the rapid induction and relaxation of qE in vivo. Using a mutant that lacks PsbS, we recently discovered a novel type of slowly reversible NPQ (qH) that occurs in the light-harvesting antenna of photosystem II, and we have identified several proteins that are involved in qH.

We are also actively investigating the diversity of NPQ mechanisms that exist in nature. In the model green alga Chlamydomonas, we discovered that the stress-related antenna protein called LHCSR is essential for qE. LHCSR-like proteins have been shown to function in qE in diverse algae and mosses, but they were lost during evolution of vascular plants. LHCSR is involved in sensing thylakoid lumen pH (like PsbS), but it also appears to be a site for quenching of excitation energy. LHCSR expression and qE capacity in Chlamydomonas are inducible by exposure to high light, UV light, and blue light, and we are dissecting the signal transduction pathway(s) involved in this light regulation. We also found that Chlamydomonas and related green algae have a completely different type of de-epoxidase enzyme that is involved in making zeaxanthin from violaxanthin.