Reduction of phenoxyl radicals mediated by monodehydroascorbate reductase

Plant phenolics behave as radical scavengers in the context of insect (Manduca sexta) hemolymph and midgut fluid

To evaluate the prooxidant versus antioxidant properties of plant phenolics toward leaf-feeding caterpillars, quenching of the stable ABTS radical by five phenolics was measured in two physiological contexts: hemolymph and midgut fluid. Addition of tannic acid, chlorogenic acid, quercetin, or catechin to Manduca sexta (L.) gut fluid increased its total antioxidant capacity by 12-45%, with tannic acid and quercetin being the most powerful in this regard. The antioxidant contribution of the phenolics increased with longer (30-60 min) incubation time in gut fluid. Chlorogenic acid and caffeic acid exhibited the weakest antioxidant activity in gut fluid. The total antioxidant capacity of hemolymph is considerably less than that of gut fluid, and in hemolymph chlorogenic and caffeic acids sometimes acted as mild prooxidants, particularly after longer incubation periods (30-60 min), although this trend was not statistically significant. Tannic acid, catechin, and quercetin behaved as antioxidants in hemolymph. These results suggest that many phenolics have radical scavenging activity in the digestive tract, but some may have more detrimental effects after absorption into the hemolymph compartment.

Peroxisomal monodehydroascorbate reductase. Genomic clone characterization and functional analysis under environmental stress conditions

In plant cells, ascorbate is a major antioxidant that is involved in the ascorbate-glutathione cycle. Monodehydroascorbate reductase (MDAR) is the enzymatic component of this cycle involved in the regeneration of reduced ascorbate. The identification of the intron-exon organization and the promoter region of the pea (Pisum sativum) MDAR 1 gene was achieved in pea leaves using the method of walking polymerase chain reaction on genomic DNA. The nuclear gene of MDAR 1 comprises nine exons and eight introns, giving a total length of 3,770 bp. The sequence of 544 bp upstream of the initiation codon, which contains the promoter and 5' untranslated region, and 190 bp downstream of the stop codon were also determined. The presence of different regulatory motifs in the promoter region of the gene might indicate distinct responses to various conditions. The expression analysis in different plant organs by northern blots showed that fruits had the highest level of MDAR. Confocal laser scanning microscopy analysis of pea leaves transformed with Agrobacterium tumefaciens having the binary vectors pGD, which contain the autofluorescent proteins enhanced green fluorescent protein and enhanced yellow fluorescent protein with the full-length cDNA for MDAR 1 and catalase, indicated that the MDAR 1 encoded the peroxisomal isoform. The functional analysis of MDAR by activity and protein expression was studied in pea plants grown under eight stress conditions, including continuous light, high light intensity, continuous dark, mechanical wounding, low and high temperature, cadmium, and the herbicide 2,4-dichlorophenoxyacetic acid. This functional analysis is representative of all the MDAR isoforms present in the different cell compartments. Results obtained showed a significant induction by high light intensity and cadmium. On the other hand, expression studies, performed by semiquantitative reverse transcription-polymerase chain reaction demonstrated differential expression patterns of peroxisomal MDAR 1 transcripts in pea plants grown under the mentioned stress conditions. These findings show that the peroxisomal MDAR 1 has a differential regulation that could be indicative of its specific function in peroxisomes. All these biochemical and molecular data represent a significant step to understand the specific physiological role of each MDAR isoenzyme and its participation in the antioxidant mechanisms of plant cells.

Enzymatic nitration of phytophenolics: evidence for peroxynitrite-independent nitration of plant secondary metabolites

Peroxynitrite (ONOO(-)), a reactive nitrogen species, is capable of nitrating tyrosine residue of proteins. Here we show in vitro evidence that plant phenolic compounds can also be nitrated by an ONOO(-)-independent mechanism. In the presence of NaNO(2), H(2)O(2), and horseradish peroxidase (HRP), monophenolic p-coumaric acid (p-CA, 4-hydroxycinnamic acid) was nitrated to form 4-hydroxy-3-nitrocinnamic acid. The reaction was completely inhibited by KCN, an inhibitor for HRP. The antioxidant ascorbate suppressed p-CA nitration and its suppression time depended strongly on ascorbate concentration. We conclude that nitrogen dioxide radical (NO(2)(radical)), but not ONOO(-), produced by a guaiacol peroxidase is the intermediate for phytophenolic nitration.

The use of multiple transcription starts causes the dual targeting of Arabidopsis putative monodehydroascorbate reductase to both mitochondria and chloroplasts

Monodehydroascorbate reductase (MDAR) isoforms exist in mitochondria, chloroplasts, cytosol and microbodies. Two putative MDAR sequences with an extended N-terminal region are found in Arabidopsis. They differ in the length of the extension by 21 bp. We have shown that these two isoforms arise from a single gene by the use of multiple transcription starts. Green fluorescent protein was fused to each extension, revealing that the longer and shorter fusion proteins were imported into mitochondria and chloroplasts, respectively. These results demonstrate that putative MDAR is a dual-targeting protein transported into both mitochondria and chloroplasts. Although there have been several reports of dual targeting of proteins to mitochondria and chloroplasts, this is the first example in which the dual targeting of the protein to mitochondria and chloroplasts is achieved by the use of multiple transcription initiation sites.

High-susceptibility of photosynthesis to photoinhibition in the tropical plant Ficus microcarpa L. f. cv. Golden Leaves

BACKGROUND

The tropical plant Ficus microcarpa L. f. cv. Golden Leaves (GL) is a high-light sensitive tropical fig tree in which sun-leaves are yellow and shade-leaves are green. We compared the response of photosynthetic activities to strong light between GL and its wild-type (WT, Ficus microcarpa L. f.).

RESULTS

Field measurements of maximum photosystem II (PSII) efficiency (Fv/Fm) of intact sun-leaves in GL showed that photo synthetic activity was severely photoinhibited during the daytime (Fv/Fm = 0.46) and subsequently recovered in the evening (Fv/Fm = 0.76). In contrast, WT did not show any substantial changes of Fv/Fm values throughout the day (between 0.82 and 0.78). Light dependency of the CO2 assimilation rate in detached shade-leaves of GL showed a response similar to that in WT, suggesting no substantial difference in photosynthetic performance between them. Several indicators of photoinhibition, including declines in PSII reaction center protein (D1) content, Fv/Fm value, and O2 evolution and CO2 assimilation rates, all indicated that GL is much more susceptible to photoinhibition than WT. Kinetics of PAM chlorophyll a fluorescence revealed that nonphotochemical quenching (NPQ) capacity of GL was lower than that of WT.

CONCLUSION

We conclude that the photosynthetic apparatus of GL is more highly susceptible to photoinhibition than that of WT.