Light-harvesting complex B7 shifts the irradiance response of photosynthetic light-harvesting regulation in leaves of Arabidopsis thaliana

Abstract

The nuclear LHCB7 gene is common in higher plants, encodes a transcript that is well expressed in a subset of leaf mesophyll cells, and is associated with a protein product that is homologous to pigment-binding components of the photosystem (PS) II peripheral antenna complex. We compared the physiological properties of wild type and LHCB7-deficient leaves [DNA insertion, Arabidopsis thaliana (At) ecotype Columbia] in terms of pigment content, CO2 exchange, in vivo transmittance at 810nm, and chlorophyll fluorescence. The latter two techniques are functional indicators for PSI and PSII, respectively. Key features of the mutant phenotype were confirmed using antisense technology and a hemizygote of two independent AtLHCB7 DNA insertion lines. Growth, leaf pigment composition, white light absorptance, and levels of AtLHCB1-6 were not significantly different in the mutant compared to wild type. Likewise, neither intrinsic PSII light capture efficiency nor partitioning of absorbed radiation to PSII was affected by the mutation. The absence of AtLHCB7 is associated with lower rates of light-saturated photosynthesis and a diminished irradiance threshold for induction of photoprotective non-photochemical quenching. Overall, the pattern of change in light utilization parameters and plastoquinol level indicated that loss of AtLHCB7 expression led to slower Rubisco turnover characterized by pH-dependent balancing of electron transport to reduced carbon assimilation capacity (photosynthetic control). No effect of AtLHCB7 genotype on xanthophyll de-epoxidation state was detected suggesting that factors in addition to lumenal pH influence zeaxanthin accumulation.