Different response of Arabidopsis mutants to high light intensity at low temperature and room temperature

Abstract

Recently, mutants have been engineered in Arabidopsis thaliana L. that alter the biosynthesis and light induced changes in xanthophyll pigments that are believed to be important for both light-harvesting-protein assembly and stability and photoprotective energy dissipation during excess light exposure. However, to date none has published a definitive study to quantify the influence of these mutations on the response to compound excess light and low temperature stresses. The wild type (WT) and three mutant plants including npq1 (lutein-replete violaxanthin deepoxidase-deficient), lut2 (lutein-deficient) and lut2-npq1 (double mutant, defective in both deepoxidation and lutein) were used to understand the role of xanthophylls in photoprotection during exposure to high light and /or low temperature. This study compares the response to high light intensity (2000µmol m-2s-1) at both low (5°) and room temperature (25°). Pigments were quantified by HPLC, room temperature PSII Chl a fluorescence was measured with a pulse-amplitude modulated fluorometer and 77k low temperature fluorescence spectra were collected. Under combined high light and low temperature stress, Fv/Fm and F685/F735 in WT and mutants decreased with the duration of treatment. The tolerance sequence for these two parameters among the four materials was WT> npq1> lut2> lut2-npq1. The lut2 plants suffered photoinhibition more than npq1 at low temperature. In contrast to the lutein-replete strains, the absence of lutein in lut2 and lut2-npq1 was compensated by the significant increase in the xanthophyll cycle pool size (A+V+Z). High light and low temperature exposure correlated with an increase in VAZ, neoxanthin and lutein in the WT and npq1 but with slight increase in VZA pool in the lutein deficient lines. In contrast to the xanthophylls there was no significant changes in ß-carotene and the Chl a/b ratio in all of the materials. The deepoxidation state (Z+0.5A).(V+A+2)-1 increased rapidly for the first 10 min of high light and low temperature treatment in lut2 and WT, but remained low in the npq1 and lut2-npq1. The changing sequence in Fv/Fm among four strains under high light at 25° showed a clear difference from high light at 5°. The tolerance sequence was altered as WT> lut2> npq1> lut2-npq1 in 2 hours of treatment, but after 3-4 hours, Fv/Fm in lut2 and npq1 reduced to similar values. The recovery of Fv/Fm at low light (20µmol m-2s-1, 2 hours) exhibited the same trend as that observed in the treatment. The results suggest that lutein, the most abundant carotenoid pigment, plays a more important role in photoprotection at 5° than at 25°.