According to their descriptions, a cyanobacterial vipp1 mutant strain showed an almost complete lack of thylakoid membrane, as well as a loss of photosynthetic activity; in plants, the depletion of VIPP1 by T-DNA insertion led to a loss of vesicle formation from chloroplasts and thylakoid formation. This concept became widely accepted in the study of photosynthetic membrane formation in the next 8 years.

Dr. GAO Hong from the Research Group of Algal Genetics and Biotechnology at Institute of Hydrobiology, Chinese Academy of Sciences (IHB) found in his doctoral research that vipp1 is an essential gene in cyanobacterium whose inserted mutant could not be segregated. However, if a vipp1 gene with a copper-regulated promoter (PpetE-vipp1) was integrated into a neutral platform in the genome of the merodiploid mutant, a complete segregation of the vipp1 mutant could be easily obtained. By use of this strain expressing VIPP1 under the control of a copper-responsive promoter, Gao found that the depletion of VIPP1 under copper-deficient condition first reduced PSII activity, then PSI activity; afterwards, cell viability and thylakoids were lost in consequence.

In particular, PpetE-vipp1 cells grown in medium with 0.025 mM Cu2+ showed no reduction of thylakoid membranes, but greatly reduced photosynthetic activity and viability. These results clearly suggested that VIPP1 affected photosystem activities more directly. In addition, the absence of vipp1 homologue in another prokaryotic alga, Prochlorococcus marinus, also suggested that VIPP1 is not essential for thylakoid formation. This research explicitly questioned the role of VIPP1 in thylakoid formation, but was not approved by peer reviewers immediately. After some setback, it was finally published in FEMS Microbiology Letters (292: 63-70, 2009).

In the meantime, a European group published a paper in Plant Physiology (149: 735-744, 2009), reporting the essentiality of vipp1 in cyanobacterium. However, they did not construct a strain with artificially controlled vipp1. Instead, based on the studies of a partially segregated mutant, they came to a conclusion that VIPP1 affects PSI biogenesis and stability.

The research completed by Dr. GAO Hong and his adviser Prof. XU Xudong, gradually drew the attention of international colleagues. Scientists from 7 European institutions including Max Planck Institute for Molecular Plant Physiology decided to systematically study the function of VIPP1 with a eukaryotic alga, Chlamydomonas reinhardtii. Their results, recently published online in Plant Cell (http://www.plantcell.org/content/early/2012/02/01/tpc.111.092692, open access article), supported the conclusion by GAO and XU.

The C. reinhardtii mutant strains in which VIPP1 was depleted by RNA interference, showed no obvious change of thylakoids, yet under the stress of high light or high temperature, photosystem II activity reduced more rapidly than photosystem I activity, with declined levels of some photosynthetic protein complexes. The Plant Cell article pointed out that, VIPP1 should be involved in the biogenesis/assembly of thylakoid membrane protein complexes as suggested by Gao and Xu rather than thylakoid membrane formation as proposed earlier. Furthermore, they proposed a new hypothesis that VIPP1 may serve to provide structural lipids for core complex assembly in thylakoid membrane.

The study of thylakoid formation at the IHB was supported by key project of the National Natural Science Foundation of China and the Key Project of Knowledge Innovation Program of Chinese Academy of Sciences.

Contact:
Prof. XU Xudong
Research Group of Algal Genetics and Biotechnology
Institute of Hydrobiology, Chinese Academy of Sciences