Regulation of Photosynthetic Light Harvesting by Nonphotochemical Quenching

Chlorophyll Plate

 

A major process involved in controlling photosynthesis in excessive light is the downregulation of photosystem II activity by pH- and xanthophyll-dependent de-excitation of singlet chlorophyll and dissipation of excess absorbed light energy as heat, measured as nonphotochemical quenching of chlorophyll fluorescence (NPQ). We are providing insights into the molecular mechanism of NPQ by isolating npq mutants of the laboratory weed, Arabidopsis thaliana. Characterization of a subset of these mutants has contributed to our knowledge of the role of xanthophyll pigments in NPQ. Other npq mutants identify components besides the xanthophylls that are critical for NPQ. For example, characterization of the npq4-1 mutant uncovered an essential role of the PsbS subunit of photosystem II in NPQ. Experiments with npq4-1 have shown that NPQ protects photosystem II from photoinhibition, especially in short-term high light.

Understanding the specific function of PsbS has become a major focus of our research on NPQ. Although PsbS does not appear to be involved in light harvesting, it is a member of the light-harvesting complex (LHC) protein superfamily, so it might provide the binding site for the xanthophylls that are necessary for NPQ in vivo. The presence of acidic amino acid residues on the thylakoid lumen side of the protein suggests also a role in proton binding and sensing of lumen pH in excessive light. We are using site-directed mutagenesis and suppressor analysis to test if PsbS is binding pigments and/or protons during NPQ. We are also trying to determine the location of PsbS within photosystem II.

Petri Dish - PC: Lisa Audish

 

Another major question about NPQ concerns its actual biophysical mechanism. De-excitation of singlet chlorophyll during NPQ could theoretically occur by internal conversion of chlorophyll to the ground state or by energy transfer from chlorophyll to a xanthophyll such as zeaxanthin. In collaboration with the group of Prof. Graham Fleming (Dept. of Chemistry, UC-Berkeley and Physical Biosciences Division, LBNL), we are using a combination of genetics and ultrafast spectroscopy to investigate the mechanism of NPQ.

Ecophysiological studies by several labs have demonstrated considerable species-dependent and environmental variation in the maximum extent of NPQ. 

In general, plants growing in full sunlight have a greater maximum extent of NPQ than shade plants, suggesting that NPQ capacity is an important factor for acclimation of plants to high light. To investigate the genetic basis for variation in NPQ capacity, we are analyzing natural variation of this trait in various ecotypes of Arabidopsis.