Expression and organisation of antenna proteins in the light-and temperature-sensitive barley mutant chlorina-104

Abstract

The nuclear gene mutant chlorina-(104) of barley (Hordeum vulgare L.) is chlorophyll-deficient when grown under high irradiance, particularly at low temperatures. Chlorina- (104) chloroplasts had fewer thylakoids than the wild type, and fewer appressed lamellae relative to non-appressed lamellae. The freeze-fracture ultrastructure showed a loss of particles from the protoplasmic fracture face of the stacked thylakoid region (PFs), consistent with the loss of most of the light-harvesting complex (LHC) II, and a loss of some of the large particles from the same face of the unstacked thylakoid region (PFu), indicating a loss of photosystem-I particles. The mutant is remarkable for the high density of particles on the exoplasmic fracture face of the unstacked thylakoid region (EFu), levels of which fell to normal after transfer to low light. The chlorophyll deficiency was shown to be primarily caused by the loss of LHCII and LHCI-680, with the consequent loss of much of the chlorophyll (Chl) b and the xanthophylls neoxanthin and lutein. The use of a monoclonal antibody which recognises the 23-kDa polypeptide of LHCI-680, confirmed that it was severely depleted in chloroplasts from chlorina-(104) grown under restrictive conditions. The 77 K fluorescence emission spectrum was characterised by a pronounced shoulder at 720 nm, arising from the photosystem-I reaction centre (CPI). Since fluorescence from CPI is normally quenched by LHCI-730, this indicates that LHCI-680 mediates excitation energy transfer between LHCI-730 and the reaction centre. After moving seedlings to permissive conditions, LHCII and LHCI-680 began to accumulate in the chlorotic leaves and the fluorescence emission spectrum resembled that of wild-type leaves. Measurement of the steady-state mRNA levels with specific Cab probes, showed no difference between wild type and mutant, indicating that control of LHCII and LHCI-680 accumulation was at a post-transcriptional level.