PMID:12237386

Citation	Niyogi, KK, Bjorkman, O and Grossman, AR (1997) Chlamydomonas Xanthophyll Cycle Mutants Identified by Video Imaging of Chlorophyll Fluorescence Quenching. Plant Cell 9:1369-1380
Abstract	The photosynthetic apparatus in plants is protected against oxidative damage by processes that dissipate excess absorbed light energy as heat within the light-harvesting complexes. This dissipation of excitation energy is measured as nonphotochemical quenching of chlorophyll fluorescence. Nonphotochemical quenching depends primarily on the [delta]pH that is generated by photosynthetic electron transport, and it is also correlated with the amounts of zeaxanthin and antheraxanthin that are formed from violaxanthin by the operation of the xanthophyll cycle. To perform a genetic dissection of nonphotochemical quenching, we have isolated npq mutants of Chlamydomonas by using a digital video-imaging system. In excessive light, the npq1 mutant is unable to convert violaxanthin to antheraxanthin and zeaxanthin; this reaction is catalyzed by violaxanthin de-epoxidase. The npq2 mutant appears to be defective in zeaxanthin epoxidase activity, because it accumulates zeaxanthin and completely lacks antheraxanthin and violaxanthin under all light conditions. Characterization of these mutants demonstrates that a component of nonphotochemical quenching that develops in vivo in Chlamydomonas depends on the accumulation of zeaxanthin and antheraxanthin via the xanthophyll cycle. However, observation of substantial, rapid, [delta]pH-dependent nonphotochemical quenching in the npq1 mutant demonstrates that the formation of zeaxanthin and antheraxanthin via violaxanthin de-epoxidase activity is not required for all [delta]pH-dependent nonphotochemical quenching in this alga. Furthermore, the xanthophyll cycle is not required for survival of Chlamydomonas in excessive light.
Links	PubMed PMC157004 Online version:10.1105/tpc.9.8.1369
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Annotations

Gene product	Qualifier	GO Term	Evidence Code	with/from	Aspect	Extension	Notes	Status
ARATH:ZEP		GO:0010028: xanthophyll cycle	ECO:0000315:		P		The assignment of this protein to an activity is an infererence from the mutant phenotype related to accumulation of different compounds "the Npq- phenotype of npq2 co- segregated with the accumulation of zeaxanthin and absence of antheraxanthin, violaxanthin, and neoxanthin (data not shown)." According to figure 6, the xanthophyll pool in wild type cells in low light is primarily violaxanthin. In high light this violaxanthin decreases, producing antheraxanthin and zeaxanthin. However, the amount of zeaxanthin in the NPQ2 mutant remains the same when under low light. There is no production of antheraxanin or violaxanthin. This is due to the lack of zeaxanthin epoxidase activity.	complete CACAO 9782

References

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