Screening Stress Tolerance Traits in Arabidopsis Cell Cultures.

Screening for tolerance traits in plant cell cultures can combine the efficiency of microbial selection and plant genetics. Agrobacterium-mediated transformation can efficiently introduce cDNA library to cell suspension cultures generating population of randomly transformed microcolonies. Transformed cultures can subsequently be screened for tolerance to different stress conditions such as salinity, high osmotic, or oxidative stress conditions. cDNA inserts in tolerant cell lines can be easily identified by PCR amplification and homology search of the determined nucleotide sequences. The described methods have been tested and used to identify regulatory genes controlling salt tolerance in Arabidopsis. As cDNA libraries can be prepared from any plants, natural diversity can be explored by using extremophile plants as gene source.

Stigmatellin probes the electrostatic potential in the QB site of the photosynthetic reaction center.

The electrostatic potential in the secondary quinone (QB) binding site of the reaction center (RC) of the photosynthetic bacterium Rhodobacter sphaeroides determines the rate and free energy change (driving force) of electron transfer to QB. It is controlled by the ionization states of residues in a strongly interacting cluster around the QB site. Reduction of the QB induces change of the ionization states of residues and binding of protons from the bulk. Stigmatellin, an inhibitor of the mitochondrial and photosynthetic respiratory chain, has been proven to be a unique voltage probe of the QB binding pocket. It binds to the QB site with high affinity, and the pK value of its phenolic group monitors the local electrostatic potential with high sensitivity. Investigations with different types of detergent as a model system of isolated RC revealed that the pK of stigmatellin was controlled overwhelmingly by electrostatic and slightly by hydrophobic interactions. Measurements showed a high pK value (>11) of stigmatellin in the QB pocket of the dark-state wild-type RC, indicating substantial negative potential. When the local electrostatics of the QB site was modulated by a single mutation, L213Asp → Ala, or double mutations, L213Asp-L212Glu → Ala-Ala (AA), the pK of stigmatellin dropped to 7.5 and 7.4, respectively, which corresponds to a >210 mV increase in the electrostatic potential relative to the wild-type RC. This significant pK drop (ΔpK > 3.5) decreased dramatically to (ΔpK > 0.75) in the RC of the compensatory mutant (AA+M44Asn → AA+M44Asp). Our results indicate that the L213Asp is the most important actor in the control of the electrostatic potential in the QB site of the dark-state wild-type RC, in good accordance with conclusions of former studies using theoretical calculations or light-induced charge recombination assay.