Abstract
The organization of pigment molecules in photosystems is strictly determined. The peripheral antennae have both chlorophyll a and b, but the core antennae consist of only chlorophyll a in green plants. Furthermore, according to the recent model obtained from the crystal structure of light-harvesting chlorophyll a/b-protein complexes II (LHCII), individual chlorophyll-binding sites are occupied by either chlorophyll a or chlorophyll b. In this study, we succeeded in altering these pigment organizations by introducing a prokaryotic chlorophyll b synthesis gene (chlorophyllide a oxygenase (CAO)) into Arabidopsis. In these transgenic plants (Prochlirothrix hollandica CAO plants), approximately 40% of chlorophyll a of the core antenna complexes was replaced by chlorophyll b in both photosystems. Chlorophyll a/b ratios of LHCII also decreased from 1.3 to 0.8 in PhCAO plants. Surprisingly, these transgenic plants were capable of photosynthetic growth similar to wild type under low light conditions. These results indicate that chlorophyll organizations are not solely determined by the binding affinities, but they are also controlled by CAO. These data also suggest that strict organizations of chlorophyll molecules are not essential for photosynthesis under low light conditions.
Highlights
Synthesize other pigments such as Chl b or xanthophylls, these pigments are not incorporated into the core antenna complexes
Rescue of the Chl b-less Phenotype by Prochlorothrix chlorophyllide a oxygenase (CAO)— Chl b is synthesized by CAO in both green plants and prochlorophytes, the structures of CAO proteins are quite different (Fig. 1A)
0.81 and light-harvesting chlorophyll a/b-protein complexes II (LHCII) did not change in PhCAO plants, the absorbance corresponding to Chl a (676 nm) had decreased with the concomitant increase in the absorbance that corresponded to Chl b (650 nm) (Fig. 2, B and C)
Summary
Because of the observation that Prochlorothrix lacks the regulatory domain, it might be reasonable to speculate that it is involved in LHC formation. If AtCAO is involved in the control of preferential incorporation of Chl b into LHC apoproteins, it would be reasonable to consider that the distribution of Chl b will not properly proceed when AtCAO is replaced by PhCAO in Arabidopsis. As a result of the PhCAO introduction into the mutant background, the Chl b-less phenotype was rescued, and Chl b was incorporated into the core complexes in the transgenic plants (PhCAO plants). We discuss the control mechanism of Chl b distribution and the photosynthetic performance of these transgenic plants that contain Chl b in their core antenna complexes
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