Quantitative genetic analysis of thermal dissipation in Arabidopsis

Feedback deexcitation is a photosynthetic regulatory mechanism that can protect plants from high light stress by harmlessly dissipating excess absorbed light energy as heat. To understand the genetic basis for intraspecies differences in thermal dissipation capacity, we investigated natural variation in Arabidopsis (Arabidopsis thaliana). We determined the variation in the amount of thermal dissipation by measuring nonphotochemical quenching (NPQ) of chlorophyll fluorescence in Arabidopsis accessions of diverse origins. Ll-1 and Sf-2 were selected as high NPQ Arabidopsis accessions, and Columbia-0 (Col-0) and Wassilewskija-2 were selected as relatively low NPQ accessions. In spite of significant differences in NPQ, previously identified NPQ factors were indistinguishable between the high and the low NPQ accessions. Intermediate levels of NPQ in Ll-1 x Col-0 F1 and Sf-2 x Col-0 F1 compared to NPQ levels in their parental lines and continuous distribution of NPQ in F2 indicated that the variation in NPQ is under the control of multiple nuclear factors. To identify genetic factors responsible for the NPQ variation, we developed a polymorphic molecular marker set for Sf-2 x Col-0 at approximately 10-centimorgan intervals. From quantitative trait locus (QTL) mapping with undistorted genotype data and NPQ measurements in an F2 mapping population, we identified two high NPQ QTLs, HQE1 (high qE 1, for high energy-dependent quenching 1) and HQE2, on chromosomes 1 and 2, and the phenotype of HQE2 was validated by analysis of near isogenic lines. Neither QTL maps to a gene that had been identified previously in extensive forward genetics screens using induced mutants, suggesting that quantitative genetics can be used to find new genes affecting thermal dissipation.