Photosynthetic apparatus organization and function in the wild type and a chlorophyll b -less mutant of Chlamydomonas reinhardtii . Dependence on carbon source

Abstract

The assembly, organization and function of the photosynthetic apparatus was investigated in the wild type and a chlorophyll (Chl) b-less mutant of the unicellular green alga Chlamydomonas reinhardtii, generated via DNA insertional mutagenesis. Comparative analyses were undertaken with cells grown photoheterotrophically (acetate), photomixotrophically (acetate and HCO3-) or photoautotrophically (HCO3-). It is shown that lack of Chl b diminished the photosystem-II (PSII) functional Chl antenna size from 320 Chl (a and b) to about 95 Chl a molecules. However, the functional Chl antenna size of PSI remained fairly constant at about 290 Chl molecules, independent of the presence of Chl b. Western blot and kinetic analyses suggested the presence of inner subunits of the Chl a-b light-harvesting complex of PSII (LHCII) and the entire complement of the Chl a-b light-harvesting complex of PSI (LHCI) in the mutant. It is concluded that Chl a can replace Chl b in the inner subunits of the LHCII and in the entire complement of the LHCI. Growth of cells on acetate as the sole carbon source imposes limitations in the photon-use efficiency and capacity of photosynthesis. These are manifested as a lower quantum yield and lower light-saturated rate of photosynthesis, and as lower variable to maximal (Fv/Fmax) chlorophyll fluorescence yield ratios. This adverse effect probably originates because acetate shifts the oxidation-reduction state of the plastoquinone pool, and also because it causes a decrease in the amount and/or activity of Rubisco in the chloroplast. Such limitations are fully alleviated upon inclusion of an inorganic carbon source (e.g. bicarbonate) in the cell growth medium. Further, the work provides evidence to show that transformation of green algae can be used as a tool by which to generate mutants exhibiting a permanently truncated Chl antenna size and a higher (per Chl) photosynthetic productivity of the cells.

Efficient Assembly of Photosystem II in Chlamydomonas reinhardtii Requires Alb3.1p, a Homolog of Arabidopsis ALBINO3

“…This was already shown earlier (Polle et al, 2000), and similar results were obtained for another chlorophyll b-less mutant of C. reinhardtii, cbn1-113 (Park and Hoober, 1997). However, these data differ from those of higher plants, in which disruption of the chlorophyll a oxygenase gene in Arabidopsis does not eliminate the expression of the LHCP genes but leads to a complete loss of LHCPs (Espineda et al, 1999;J.…”

“…Because we detected a higher expression rate of LHCPs in Arabidopsis than in C. reinhardtii, the difference might result from a faster degradation of nonfunctional LHCPs in higher plants (data not shown). Spectroscopic studies of cbs3 however showed that the PSII antenna is functional (Polle et al, 2000). The presence of antenna chlorophyll molecules remains to be clarified, but from our data, it is unlikely that PSII contains LHCPs in cbs3.…”

Modulation of the light-harvesting chlorophyll antenna size in Chlamydomonas reinhardtii by TLA1 gene over-expression and RNA interference

“…(a) Growth of the algae Chlamydomonas reinhardtii strains CC503, TLA1 overexpressing lines OE1, OE2 and OE3, TLA1 RNAi transformants Ri6, Ri8 and Ri9 and the chlorophylldeficient mutant tla1 were grown to the mid-exponential growth phase either in Tris-acetate-phosphate (TAP), pH 7.4 [29,30], or in modified minimal media containing 40 mM Tris-HCl, pH 7.4, supplemented with 25 mM sodium bicarbonate (TBP medium [31]) in culture flasks at 258C under continuous illumination of 150-300 mmol photons m 22 s 21 provided by coolwhite fluorescent lamps. The cultures were stirred continuously to ensure a uniform illumination of the cells and to prevent settling.…”

Simultaneous regulation of antenna size and photosystem I/II stoichiometry in Arabidopsis thaliana

“…Chlorophylls a and b are indispensable for the stabilization of LHC proteins in land plants (Plumley and Schmidt 1995) and Chlamydomonas reinhardtii (Polle et al 2000). Indeed, the LHCII trimer is absent in the Arabidopsis ch1 mutant, which does not accumulate chlorophyll b due to mutation of the chlorophyll a oxygenase (CAO) gene (Espineda et al 1999;Tanaka et al 1998).…”