Stress hardening under long-term cadmium treatment is correlated with the activation of antioxidative defence and iron acquisition of chloroplasts in Populus.

Cadmium (Cd), a highly toxic heavy metal affects growth and metabolic pathways in plants, including photosynthesis. Though Cd is a transition metal with no redox capacity, it generates reactive oxygen species (ROS) indirectly and causes oxidative stress. Nevertheless, the mechanisms involved in long-term Cd tolerance of poplar, candidate for Cd phytoremediation, are not well known. Hydroponically cultured poplar (Populus jacquemontiana var. glauca cv. 'Kopeczkii') plants were treated with 10 µM Cd for 4 weeks. Following a period of functional decline, the plants performed acclimation to the Cd induced oxidative stress as indicated by the decreased leaf malondialdehyde (MDA) content and the recovery of most photosynthetic parameters. The increased activity of peroxidases (PODs) could have a great impact on the elimination of hydrogen peroxide, and thus the recovery of photosynthesis, while the function of superoxide dismutase (SOD) isoforms seemed to be less important. Re-distribution of the iron content of leaf mesophyll cells into the chloroplasts contributed to the biosynthesis of the photosynthetic apparatus and some antioxidative enzymes. The delayed increase in photosynthetic activity in relation to the decline in the level of lipid peroxidation indicates that elimination of oxidative stress damage by acclimation mechanisms is required for the restoration of the photosynthetic apparatus during long-term Cd treatment.

Impact of iron supply on the kinetics of recovery of photosynthesis in Cd-stressed poplar (Populus glauca).

BACKGROUND AND AIMS: Cadmium (Cd) causes Fe-deficiency-like symptoms in plants, and strongly inhibits photosynthesis. To clarify the importance of Cd-induced Fe deficiency in Cd effects on photosynthesis, the recovery processes were studied by supplying excess Fe after the Cd symptoms had developed. METHODS: Fe-citrate at 10 microm or 50 microm was given with or without 10 microm Cd(NO3)2 to hydroponically cultured poplars (Populus glauca 'Kopeczkii') with characteristic Cd symptoms. Ion, chlorophyll and pigment contents, amount of photosynthetic pigment-protein complexes, chlorophyll fluorescence and carbon assimilation were measured together with the mapping of healing processes by fluorescence imaging. KEY RESULTS: In regenerated leaves, the iron content increased significantly, while the Cd content did not decrease. As a result, the structural (increase in the amount of photosynthetic pigments and pigment-protein complexes, decrease in the F690/F740 ratio) and functional (elevation of CO2 fixation activity and DeltaF/Fm') recovery of the photosynthetic machinery was detected. Cd-induced, light-stress-related changes in non-photochemical quenching, activity of the xanthophyll cycle, and the F440/F520 ratio were also normalized. Imaging the changes in chlorophyll fluorescence, the recovery started from the parts adjacent to the veins and gradually extended to the interveinal parts. Kinetically, the rate of recovery depended greatly on the extent of the Fe supply, and chlorophyll a/b ratio and DeltaF/Fm' proved to be the most-rapidly reacting parameters. CONCLUSIONS: Iron deficiency is a key factor in Cd-induced inhibition of photosynthesis.

Multicolor fluorescence imaging of leaves--a useful tool for visualizing systemic viral infections in plants.

Multicolor fluorescence induced by UV light is a sensitive and specific tool that may be used to provide information about the primary and secondary metabolism of plants by monitoring signals of the chlorophyll fluorescence (Chl-F) and blue-green fluorescence (BGF), respectively. We have followed the systemic infection of Nicotiana benthamiana plants with the Pepper mild mottle virus (PMMoV) by means of a multicolor fluorescence-imaging system, to detect differences between two strains of PMMoV during the infection process and to establish a correlation between the virulence and changes induced in the host plant. Changes in both BGF and Chl-F were monitored. BGF increased mainly in the abaxial side of the leaf during pathogenesis and the corresponding images showed a clear vein-associated pattern in leaves of infected plants. HPLC analysis of leaf extracts was carried out to identify compounds emitting BGF, and determined that chlorogenic acid was one of the main contributors. BGF imaging was able to detect viral-induced changes in asymptomatic (AS) leaves before detection of the virus itself. Chl-F images confirmed our previous results of alterations in the photosynthetic apparatus of AS leaves from infected plants that were detected with other imaging techniques. Fluorescence ratios F440/F690 and F440/F740, which increase during pathogenesis, were excellent indicators of biotic stress.

Photosynthetic activity of homoiochlorophyllous desiccation tolerant plant Haberlea rhodopensis during dehydration and rehydration.

The functional state of the photosynthetic apparatus of flowering homoiochlorophyllous desiccation tolerant plant Haberlea rhodopensis during dehydration and subsequent rehydration was investigated in order to characterize some of the mechanisms by which resurrection plants survive drought stress. The changes in the CO2 assimilation rate, chlorophyll fluorescence parameters, thermoluminescence, fluorescence imaging and electrophoretic characteristics of the chloroplast proteins were measured in control, moderately dehydrated (50% water content), desiccated (5% water content) and rehydrated plants. During the first phase of desiccation the net CO2 assimilation decline was influenced by stomatal closure. Further lowering of net CO2 assimilation was caused by both the decrease in stomatal conductance and in the photochemical activity of photosystem II. Severe dehydration caused inhibition of quantum yield of PSII electron transport, disappearance of thermoluminescence B band and mainly charge recombination related to S2QA- takes place. The blue and green fluorescence emission in desiccated leaves strongly increased. It could be suggested that unchanged chlorophyll content and amounts of chlorophyll-proteins, reversible modifications in PSII electron transport and enhanced probability for non-radiative energy dissipation as well as increased polyphenolic synthesis during desiccation of Haberlea contribute to drought resistance and fast recovery after rehydration.

Ferritin2 gene in paraquat-susceptible and resistant biotypes of horseweed Conyza canadensis (L.) Cronq.

Ferritins, the multimeric iron storage proteins, are the main regulators of the cellular level of uncomplexed iron. Ferritins are encoded by small gene families and expressed differentially under various developmental conditions depending on iron availability, effect of hormones or oxygen radical generating agents. In the present work the primary structure of the ferritin2 gene from resistant and susceptible biotypes of horseweed Conyza canadensis was determined. This gene was found to exhibit great similarity and possess all the structural characteristics of known plant ferritin2 genes. The C. canadensis ferritin2 genes had identical primary structure in the two biotypes and were upregulated by paraquat (Pq) in both susceptible and resistant plants. The enhanced expression level was probably connected with defence reactions in the plants after Pq treatment.

Presence of 'PSI free' LHCI and monomeric LHCII and subsequent effects on fluorescence characteristics in lincomycin treated maize.

The cause of the strong non-photochemical fluorescence quenching was examined in maize (Zea mays L.) plants that were treated with lincomycin during the 72 h period of greening. They were deficient in core complexes but seemed to contain the full complement of antennae. The following results were obtained: (1) High F(o) could not be attributed to the dark reduction of Q(A) but to the presence of a high amount of not properly organized antenna complexes due to the inhibited synthesis of reaction centres. (2) On illumination fluorescence intensity dropped considerably below F(o) within 20 s, and reached a steady state still below F(o). (3) Slowly relaxing part of non-photochemical quenching was significantly higher than in control plants. (4) De-epoxidation state was constant, and corresponded to the maximal value of the control. (5) Free Lhca1/4 dimers could be detected in all submembrane fractions, including the grana, obtained by digitonin fractionation. (6) Increase in the 679 and 700 nm fluorescence emissions could be attributed to the monomerisation of part of LHCII and to the presence of free Lhca2 or LHCII aggregates, respectively. (7) LHCII or PSII+LHCII and Lhca1/4 interaction may contribute to the increase of long-wavelength fluorescence in the granal fraction. We assume that the elevated fluorescence quenching of monomeric LHCII as well as the interaction between LHCII or PSII+LHCII and Lhca1/4 can be considered as an explanation for the extensive non-photochemical fluorescence quenching in lincomycin treated plants. The permanent presence of zeaxanthin may have contributed to the fast formation of quenching.

A review of physiological and biochemical aspects of resistance to atrazine and paraquat in Hungarian weeds.

The most important results in the field of atrazine and paraquat resistance research by Hungarian researchers are reviewed. Pleiotropic effects accompanying atrazine resistance were investigated in atrazine-resistant (AR) and susceptible (S) biotypes of horseweed (Conyza canadensis (L) Cronq). No significant difference in carbon dioxide assimilation rate was found between the AR and S plants. The rates of the Hill reaction of the AR and S chloroplasts exhibited different temperature dependence. The thylakoid membrane lipids contained a lower amount of polar lipid and the fatty acid content exhibited a higher degree of unsaturation in the AR biotype. Photosynthetic apparatus of the AR biotype had better adaptive ability at low temperature and showed enhanced susceptibility to high-temperature stress. AR horseweed plants had reduced activity of xanthophyll cycle, limited capacity of light-induced non-photochemical and photochemical quenching, higher photosensitivity and susceptibility to photo-inhibition. In the case of paraquat resistance, horseweed found in Hungary exhibited a resistance factor of 450; the resistance is not based on an elevated level and activity of the antioxidant enzyme system. The suggested role of polyamines in the resistance mechanisms can be excluded. The higher putrescine and total polyamine content of paraquat-treated R leaves can be regarded as a general stress response rather than as a symptom of paraquat resistance. A paraquat-inducible, nuclear-coded protein, which presumably functions by carrying paraquat to the vacuole, is supposed to play a role in resistance.