PARENT SESSION

Symposium S5C Biosynthesis and assembly: Pigments
Wednesday September 1st, 2004 10:20 AM-12:20 PM Room 511D
Chair: Sabeeha Merchant
Co-Chair: Sam Beale

Chlorotic Arabidopsis plants deficient in the putative aerobic oxidative cyclase CHL27 balances the two photosystems. Andreas Hansson^*,1, Poul Erik Jensen¹, ¹ Plant Biochemistry Laboratory, Department of Plant Biology, Frederiksberg C, Denmark

ABSTRACT- A SALK line (chl27) carrying a T-DNA insert in the promoter region of CHL27, the Arabidopsis homologue to Chlamydomonas Crd1, has a chlorotic phenotype. Disrupting the promoter region results in reduced expression of CHL27. Upon feeding with delta-aminolevulinic acid in darkness, chl27 accumulate Mg-protoporphyrin IX monomethyl ester, indicating that CHL27 is required for protochlorophyllide synthesis. Compared with wild-type the chlorophyll level in chl27 plants is reduced by 70% and the chlorophyll a/b ratio is increased twofold. This altered chlorophyll a/b ratio suggests that antenna proteins are especially reduced in the chl27. Blue-native PAGE and western blot analysis demonstrated an almost complete loss of LHCII protein and a significant reduction of PSI in chl27. Other features observed were that on a protein basis chl27 contains more of the NDH and cytochrome b₆f complexes. The PSI remaining in chl27 is fully functional with a quantum yield higher than for WT. Photo-oxidation of PSI by far-red light gave different response in WT and chl27, indicating an altered antenna composition. To test if the two photosystems were in balance state transition and photoinhibition was measured. No major differences were found regarding state transition, PSII-quantum yield, qN and 1-qp whereas NPQ was decreased by a factor two in chl27. The PSII-quantum yield for dark adapted plants and plants given 10 min recovery after high light treatment were similar for both WT and chl27 showing that chl27 is not more susceptible to photoinhibition than WT. Taken together it seems that the plant manage to balance the two photosystems well even when it is severely limited in chlorophyll. The way to achieve this differs for the two photosystems: regarding PSI a general reduction of core- and antenna subunits occurs with only minor change in the antenna, whereas for PSII there is a preferential loss of antenna proteins.

KEY WORDS: adaptation, chlorophyll deficiency, pigment-protein complexes