Redox Sensing of Photooxidative Stress and Acclimatory Mechanisms in Plants. In: Aro E, Andersson B, editors. Regulation of Photosynthesis. Dordrecht: Springer Netherlands; 2001. pp. 469-86. [DOI: 10.1007/0-306-48148-0_27]

The rise of the photosynthetic rate when light intensity increases is delayed in ndh gene-defective tobacco at high but not at low CO2 concentrations

The 11 plastid ndh genes have hovered frequently on the edge of dispensability, being absent in the plastid DNA of many algae and certain higher plants. We have compared the photosynthetic activity of tobacco (Nicotiana tabacum, cv. Petit Havana) with five transgenic lines (ΔndhF, pr-ΔndhF, T181D, T181A, and ndhF FC) and found that photosynthetic performance is impaired in transgenic ndhF-defective tobacco plants at rapidly fluctuating light intensities and higher than ambient CO2 concentrations. In contrast to wild type and ndhF FC, which reach the maximum photosynthetic rate in less than 1 min when light intensity suddenly increases, ndh defective plants (ΔndhF and T181A) show up to a 5 min delay in reaching the maximum photosynthetic rate at CO2 concentrations higher than the ambient 360 ppm. Net photosynthesis was determined at different CO2 concentrations when sequences of 130, 870, 61, 870, and 130 μmol m(-2) s(-1) PAR sudden light changes were applied to leaves and photosynthetic efficiency and entropy production (Sg) were determined as indicators of photosynthesis performance. The two ndh-defective plants, ΔndhF and T181A, had lower photosynthetic efficiency and higher Sg than wt, ndhF FC and T181D tobacco plants, containing full functional ndh genes, at CO2 concentrations above 400 ppm. We propose that the Ndh complex improves cyclic electron transport by adjusting the redox level of transporters during the low light intensity stage. In ndhF-defective strains, the supply of electrons through the Ndh complex fails, transporters remain over-oxidized (specially at high CO2 concentrations) and the rate of cyclic electron transport is low, impairing the ATP level required to rapidly reach high CO2 fixation rates in the following high light phase. Hence, ndh genes could be dispensable at low but not at high atmospheric concentrations of CO2.

Functional roles of the pepper pathogen-induced bZIP transcription factor, CAbZIP1, in enhanced resistance to pathogen infection and environmental stresses

Transcription factors often belong to multigene families and their individual contribution in a particular regulatory network remains difficult to assess. We identify and functionally characterize the pepper bZIP transcription factor CAbZIP1 gene isolated from pepper leaves infected with Xanthomonas campestris pv. vesicatoria. Transient expression analysis of the CAbZIP1-GFP fusion protein in Arabidopsis protoplasts revealed that the CAbZIP1 protein is localized in the nucleus. The N-terminal region of CAbZIP1 fused to the GAL4 DNA-binding domain is required to activate transcription of reporter genes in yeast. The CAbZIP1 transcripts are constitutively expressed in the pepper root and flower, but not in the leaf, stem and fruit. The CAbZIP1 gene is locally or systemically induced in pepper plants infected by either X. campestris pv. vesicatoria or Pseudomonas fluorescens. The CAbZIP1 gene is also induced by abiotic elicitors and environmental stresses. The CAbZIP1 transgenic Arabidopsis exhibits a dwarf phenotype, indicating that CAbZIP1 may be involved in plant development. The CAbZIP1 overexpression in the transgenic Arabidopsis plants confers enhanced resistance to Pseudomonas syringae pv. tomato DC3000, accompanied by expression of the AtPR-4 and AtRD29A. The transgenic plants also exhibit increased drought and salt tolerance during all growth stages. Moreover, the transgenic plants are tolerant to methyl viologen-oxidative stress. Together, these data suggest that the CAbZIP1 transcription factor function as a possible regulator in enhanced disease resistance and environmental stress tolerance.

CASAR82A, a pathogen-induced pepper SAR8.2, exhibits an antifungal activity and its overexpression enhances disease resistance and stress tolerance

Pepper SAR8.2 gene (CASAR82A) was previously reported to be locally or systemically induced in pepper plants by biotic and abiotic stresses. In this study, the physiological and molecular functions of the pepper SAR8.2 protein in the plant defense responses were investigated by generating Arabidopsis transgenic lines overexpressing the CASAR82A gene. The transgenic Arabidopsis plants grew faster than the wild-type plants, indicating that the CASAR82A gene was involved in plant development. The ectopic expression of CASAR82A in Arabidopsis was accompanied by the expression of the Arabidopsis pathogenesis-related (PR)-genes including AtPR-1, AtPR-4 and AtPR-5. CASAR82A overexpression enhanced the resistance against infections by Pseudomonas syringae pv. tomato, Fusarium oxysporum f.sp. matthiolae or Botrytis cinerea. The transgenic plants also exhibited increased NaCl and drought tolerance during all growth stages. Moreover, the methyl viologen test showed that the transgenic plants were tolerant to oxidative stress. The purified recombinant CASAR82A protein and crude protein extracts of the transgenic plants exhibited antifungal activity against some phytopathogenic fungi, indicating that the enhanced resistance of the transgenic plants to fungal pathogen infection may be due to the antifungal effect of SAR8.2 protein.

Antisense reduction of thylakoidal ascorbate peroxidase in Arabidopsis enhances paraquat-induced photooxidative stress and nitric oxide-induced cell death

The production and characterization of Arabidopsis plants containing a transgene in which the Arabidopsis tAPX is inserted in antisense orientation, is described. tAPX activity in these transgenic tAPX plants is around 50% of control level. The tAPX antisense plants are phenotypically indistinguishable from control plants under normal growth conditions; they show, however, enhanced sensitivity to the O2- -generating herbicide, Paraquat. Interestingly, the tAPX antisense plants show enhanced symptoms of damage when cell death is triggered through treatment with the nitric oxide-donor, SNP. These results are in accordance with the ones recently obtained with transgenic plants overexpressing tAPX; altogether, they suggest that tAPX, besides the known ROS scavenging role, is also involved in the fine changes of H2O2 concentration during signaling events.

Arabidopsis thaliana plants overexpressing thylakoidal ascorbate peroxidase show increased resistance to Paraquat-induced photooxidative stress and to nitric oxide-induced cell death

Ascorbate peroxidases (APX), localized in the cytosol, peroxisomes, mitochondria and chloroplasts of plant cells, catalyze the reduction of H(2)O(2) to water by using ascorbic acid (ASA) as specific electron donor. The chloroplastic isoenzymes of APX are involved in the water-water cycle, which contributes to the photophosphorylation coupled to the photosynthetic electron transport. In order to better clarify the contribution of thylakoidal APX (tAPX) to the reactive oxygen species (ROS) scavenging activity, as well as to the fine modulation of ROS for signaling, we produced Arabidopsis lines overexpressing tAPX. These lines show an increased resistance to treatment with the O(2)(-) generating herbicide Paraquat (Pq). However, when challenged with photoinhibitory treatments at high light or low temperature, or with iron (Fe) or copper (Cu) overload, the tAPX-overexpressing lines show no increased resistance with respect to controls, indicating that in such experimental conditions, tAPX overexpression does not reinforce plant defenses against the oxidative stresses tested. Interestingly, the nitric oxide (NO)-donor sodium nitroprusside (SNP) represses accumulation of tAPX transcript; SNP also partially inhibits tAPX enzymatic activity. After treatment with SNP, the tAPX-overexpressing lines show reduced symptoms of damage with respect to control plants treated with SNP. These transgenic lines confirm that H(2)O(2) acts in partnership with NO in causing cell death and highlight the important role of tAPX in the fine modulation of H(2)O(2) for signaling.