Important Role In Photoprotection Research Articles

Photoprotective role of rhodoxanthin during cold acclimation in Cryptomeria japonica

ABSTRACTTo examine the role of rhodoxanthin in long‐term acclimation to low temperatures, we monitored seasonal changes in pigment composition, photosynthesis, chlorophyll fluorescence and the level of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) in needles of wild‐type and mutant forms of Cryptomeria japonica. In winter, rhodoxanthin accumulated in sun‐exposed needles of wild‐type plants, but not in those of the mutant. The level of chlorophyll decreased in both types of plant in winter. In contrast, the level of the xanthophyll cycle pool increased in both cases. The level of the pool in the mutant was twice that in the wild type in winter, on a Chl basis, even though the levels in both were similar in summer. The synthesis of rhodoxanthin might be triggered by photo‐inhibitory conditions, as suggested by the sustained elevated levels of zeaxanthin (Z) and antheraxanthin (A). In the wild type and the mutant, the quantum yield of CO2 fixation (φ), the photosynthetic capacity, the photochemical efficiency of photosystem II (PSII), the photochemical quenching and the level of Rubisco in summer were similar. However, all these values for the wild type were higher than those for the mutant in winter. The non‐photochemical quenching (NPQ) in the mutant in winter increased rapidly even under low light conditions due to the high sustained levels of Z and A. In contrast, in the wild type, the conversion of Z via A to rhodoxanthin prevented the rapid increase in NPQ to maintain the relatively high level of φ. These findings suggest that rhodoxanthin might play an important photoprotective role in long‐term acclimation to cold. The dynamic regulation of the amount of rhodoxanthin relative to the level of the xanthophyll cycle pool might act to maintain an appropriate balance between light absorption, photosynthesis and the thermal dissipation of energy due to excess absorbed light in winter.

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

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 s-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°.

Photoprotection and acclimation of mutants with defects in nonphotochemical quenching and antioxidant synthesis

Algae and plants have evolved numerous mechanisms to cope with the potential for photo-oxidative damage that accompanies oxygenic photosynthesis. Photosynthetic light harvesting is regulated by processes such as nonphotochemical quenching (NPQ) that help to minimize production of reactive oxygen species, and various antioxidant molecules and enzymes are involved in scavenging or quenching of reactive molecules that are inevitably generated. NPQ and antioxidants are being investigated using mutants of Arabidopsis thaliana and Chlamydomonas reinhardtii. Characterization of an Arabidopsis mutant revealed an essential role for the photosystem II protein PsbS in NPQ. Based on the results of site-directed mutagenesis experiments, we hypothesize that PsbS is involved in sensing light intensity through changes in thylakoid lumen pH. NPQ has an important photoprotective role during short-term fluctuations in light intensity, because NPQ-deficient mutants were more photoinhibited than the wild type during a short treatment with high light, but in the long term the mutants were able to acclimate to high light conditions. In contrast to its Arabidopsis counterpart, the npq1 lor1 double mutant of Chlamydomonas, which is deficient in zeaxanthin, lutein, and NPQ, was unable to acclimate to high light and exhibited photo-oxidative bleaching. Extragenic suppressors of the light sensitivity of npq1 lor1 identified new mutations that affect antioxidant synthesis, providing insights into the overlapping functions of chloroplast antioxidants. By studying acclimation of npq and antioxidant mutants to high light, we also hope to uncover other processes involved in photoprotection

Effects of drought on photoprotective mechanisms in European beech (Fagus sylvatica L.) seedlings from different provenances

The effects of drought on photoprotection mechanisms of three beech provenances from three contrasting climatic origins have been studied. Morphology differed among provenances, with a lower leaf area/fine roots ratio in the xeric populations (Moncayo). This characteristic allowed this provenance to respond later than the others to a drought-stress treatment. However, in desiccating leaves the stability of PSII was higher in the plants from the stressful climate of the Pyrenees (Belagoa). Drought resulted in a reduction in pigment content (except in Belagoa) with simultaneous increases in antheraxanthin, zeaxanthin and tocopherol pools. These molecules play an important photoprotective role and reached similar values in stressed leaves from the three provenances. These antioxidants also protected the leaves from paraquat-induced oxidation. It is concluded that drought tolerance in the studied xeric ecotype is based to a greater extent on morphological characteristics than on a better photoprotective system.