Ascorbate-Deficient Mutants of Arabidopsis Grow in High Light Despite Chronic Photooxidative Stress

Abstract

Acclimation to changing environments, such as increases in light intensity, is necessary, especially for the survival of sedentary organisms like plants. To learn more about the importance of ascorbate in the acclimation of plants to high light (HL), vtc2, an ascorbate-deficient mutant of Arabidopsis, and the double mutants vtc2npq4 and vtc2npq1 were tested for growth in low light and HL and compared with the wild type. The vtc2 mutant has only 10% to 30% of wild-type levels of ascorbate, vtc2npq4 has lower ascorbate levels and lacks non-photochemical quenching of chlorophyll fluorescence (NPQ) because of the absence of the photosystem II protein PsbS, and vtc2npq1 is NPQ deficient and also lacks zeaxanthin in HL but has PsbS. All three genotypes were able to grow in HL and had wild-type levels of Lhcb1, cytochrome f, PsaF, and 2-cysteine peroxiredoxin. However, the mutants had lower electron transport and oxygen evolution rates and lower quantum efficiency of PSII compared with the wild type, implying that they experienced chronic photooxidative stress. The mutants lacking NPQ in addition to ascorbate were only slightly more affected than vtc2. All three mutants had higher glutathione levels than the wild type in HL, suggesting a possible compensation for the lower ascorbate content. These results demonstrate the importance of ascorbate for the long-term acclimation of plants to HL. During the course of their life cycle, plants are exposed to a varying light environment, such as slow seasonal changes and a sudden increase in light intensity because of an opening in the leaf canopy. Plants have been evolving to cope with this changing light environment in a way that not only enables them to harvest light optimally but also to protect themselves from excess light. Excess absorbed light is dangerous to plants because it can lead to the enhanced production of reactive oxygen species (ROS), such as hydrogen peroxide (H 2 O 2 ), superoxide, hydroxyl radicals, and singlet oxygen (Niyogi, 1999), which can damage many cellular components (Foyer, 1997), including PSII and PSI.Much is already known about how plants acclimate to high light (HL). Important responses include a reduction in the size of the light-harvesting complex and an increase in the rate of photosynthesis, which correlates with increases in ATP synthase, electron transport components, and Calvin-Benson cycle enzymes (Anderson and Osmond, 1987;Walters and Horton, 1994). Other acclimation responses include changes in morphology. Leaves that have developed in HL (sun leaves) are generally thicker because of more cell layers with a higher leaf mass per unit area than leaves that have developed in low light (LL; shade leaves; Bjö rkman, 1981; Pearcy, 1998).One very fast response to an increased light intensity is non-photochemical quenching of chlorophyll (Chl) fluorescence (NPQ), which dissipates excess energy as heat (Mü ller et al., 2001). NPQ is composed of three parts: qT, qI, and qE. qT is caused by state transitions, and in vascular plants, it is gen...

Cytosolic Ascorbate Peroxidase 1 Is a Central Component of the Reactive Oxygen Gene Network of Arabidopsis

“…Interestingly, the steady state level of most signaling and defense transcripts shown in Table 2 decreased at the 6-and 24-h time points, suggesting that the stress that was imposed on KO-Apx1 plants by the light shift treatment might have been transient. It is possible that KO-Apx1 plants were able to acclimate and decrease the level of stress by activating alternative H 2 O 2 scavenging mechanisms, by activating photoprotective mechanisms, or by decreasing the rate of H 2 O 2 production (Mittler, 2002;Muller-Moule et al, 2004). This finding might explain why KO-Apx1 plants were able to recover from the light stress treatment (data not shown).…”

Dehydroascorbate Reductase Affects Leaf Growth, Development, and Function

“…2B). The reduction in the amount of LHCII in D KD plants was not observed in vtc2 mutant plants (Muller-Moule et al, 2004). The correlation between DHAR activity and the abundance of RbcL and LHCII suggests that the level of DHAR activity may be important in maintaining these proteins during leaf aging.…”

“…vtc2 mutants that contain only 10% to 30% of the wildtype level of Asc exhibit sensitivity to high light and photobleach when transferred from low to high light (Muller-Moule et al, 2002. When grown in high light, vtc2 plants contain about 40% of the wild-type level of Asc and are able to grow despite signs of oxidative stress, such as lower electron transport, a lower rate of oxygen evolution, and lower PSII quantum efficiency (Muller-Moule et al, 2004). These mutants demonstrate that although some degree of photooxidative stress may result from the reduced levels of Asc, In contrast to vtc mutants, D KD plants showed reduced photosynthetic function and reduced growth in moderate light.…”

Impact of Oxidative Stress on Ascorbate Biosynthesis in Chlamydomonas via Regulation of the VTC2 Gene Encoding a GDP-l-galactose Phosphorylase

“…In plants, cellular L-ascorbic levels are increased in response to environmental stresses such as high light (1,51), high temperature (52), and exposure to UV radiation (53,54) or ozone (55,56). L-Ascorbic acid plays an important role in photosynthesis where it acts by scavenging superoxide and H 2 O 2 , participates in regeneration of ␣-tocopheryl radicals produced by ␣-tochopherol during reduction of lipid peroxyl radicals, and functions as cofactor for violaxanthin deepoxidase (1) and prolyl hydroxylases (2, 3).…”