Molecular Characterization of Pseudomonas syringae pv. coriandricola and Biochemical Changes Attributable to the Pathological Response on Its Hosts Carrot, Parsley, and Parsnip

Bacterial leaf spot caused by the plant pathogenic bacterium Pseudomonas syringae pv. coriandricola (Psc) was observed on carrot, parsnip, and parsley grown on a vegetable farm in the Vojvodina Province of Serbia. Nonfluorescent bacterial colonies were isolated from diseased leaves and characterized using different molecular techniques. Repetitive element PCR fingerprinting with five oligonucleotide primers (BOX, ERIC, GTG5, REP, and SERE) and the randomly amplified polymorphic DNA-PCR with the M13 primer revealed identical fingerprint patterns for all tested strains. Multilocus sequence analysis of four housekeeping genes (gapA, gltA, gyrB, and rpoD) showed a high degree (99.8 to 100%) of homology with sequences of Psc strains deposited in the Plant-Associated Microbes Database and NCBI database. The tested strains caused bacterial leaf spot symptoms on all three host plants. Host-strain specificity was not found in cross-pathogenicity tests, but the plant response (peroxidase induction and chlorophyll bleaching) was more pronounced in carrot and parsley than in parsnip.

Interactive effects of high irradiance and moderate heat on photosynthesis, pigments, and tocopherol in the tree-fern Dicksonia antarctica

Effects of high irradiance and moderate heat on photosynthesis of the tree-fern Dicksonia antarctica (Labill., Dicksoniaceae) were examined in a climate chamber under two contrasting irradiance regimes (900 and 170 µmol photons m–2 s–1) and three sequential temperature treatments (15°C; 35°C; back to 15°C). High irradiance led to decline in predawn quantum yield of photochemistry, Fv/Fm (0.73), maximal Rubisco activity (Vcmax; from 37 to 29 µmol m–2s–1), and electron transport capacity (Jmax; from 115 to 67 µmol m–2 s–1). Temperature increase to 35°C resulted in further decreases in Fv/Fm (0.45) and in chlorophyll bleaching of high irradiance plants, while Vcmax and Jmax were not affected. Critical temperature for thylakoid stability (Tc) of D. antarctica was comparable with other higher plants (c. 47°C), and increases of Tc with air temperature were greater in high irradiance plants. Increased Tc was not associated with accumulation of osmotica or zeaxanthin formation. High irradiance increased the xanthophyll cycle pigment pool (V+A+Z, 91 v. 48 mmol mol–1 chlorophyll–1), de-epoxidation state (56% v. 4%), and α-tocopherol. Temperature increase to 35°C had no effect on V+A+Z and de-epoxidation state in both light regimes, while lutein, β-carotene and α-tocopherols increased, potentially contributing to increased membrane stability under high irradiance.

Anthocyanins in leaves: light attenuators or antioxidants?

Anthocyanins have the potential to mitigate photooxidative injury in leaves, both by shielding chloroplasts from excess high-energy quanta, and by scavenging reactive oxygen species. To distinguish between the impacts of these two putative mechanisms, superoxide (O2•–) concentration and chlorophyll oxidation were measured for Lactuca sativa L. chloroplast suspensions under various light and antioxidant-supplemented environments. A red cellulose filter, the optical properties of which approximated that of anthocyanin, effected a 33% decline in rate of O2•– generation and 37% reduction in chlorophyll bleaching, when used to shield irradiated chloroplasts. Colourless and blue tautomers of cyanidin 3-(6-malonyl)glucoside at pH 7 removed up to 17% of O2•– generated by chloroplasts, indicating that cytosolic anthocyanins can serve as effective antioxidants. Red tautomers, typical of vacuolar anthocyanins, also showed strong reducing potentials as indicated by cyclic voltammetry. These potentials declined by 40% after 15 min exposure to O2•–. Maximum quantum efficiencies of photosynthesis were similar for red and green portions of intact L. sativa leaves, but the red regions were less photoinhibited, and recovered more extensively after exposures to strong light. Anthocyanins evidently offer effective and versatile protection to leaves without significantly compromising photosynthesis.

Herbicidal Activity of UCC-C4243 and Acifluorfen is Due to Inhibition of Protoporphyrinogen Oxidase

Laboratory and greenhouse studies were conducted to determine the mode of action of soil- and foliar-applied UCC-C4243. Experiments demonstrated that UCC-C4243 required light for phytotoxicity, phytotoxic symptoms were similar to inhibitors of porphyrin synthesis such as acifluorfen, and UCC-C4243 potently inhibited protoporphyrinogen oxidase. Germination and emergence of field pennycress and lentil in the dark were not affected by soil-incorporated UCC-C4243 at rates more than 10 times greater than like treatments that killed all plants in the light. Soil-incorporated UCC-C4243 required light for activity and killed seedlings within 1 d after emergence; sublethal doses caused desiccation, veinal necrosis, and leaf deformation. Field pennycress and lentil were susceptible to soil-incorporated UCC-C4243 and acifluorfen in the light, but were 5 to 93 times less sensitive to the herbicides in the dark. Wheat was not affected by either herbicide in the light or dark. Injury symptoms from UCC-C4243 applied POST to redroot pigweed were similar to symptoms from diphenyl ether and bipyridinium herbicides: rapid, light-dependent chlorophyll bleaching, desiccation, and necrosis. UCC-C4243, acifluorfen-methyl, and acifluorfen acid caused light- and concentration-dependent chlorophyll bleaching and electrolyte leakage from cucumber leaf disks (I50= 1.0, 1.8, and 4.3 μM, respectively). An inhibitor of the porphyrin synthesis pathway, 4,6-dioxoheptanoic acid, almost completely inhibited herbicide-induced electrolyte leakage. δ-Aminolevulinic acid, a tetrapyrrole precursor and stimulator of the porphyrin synthesis pathway, caused synergistic effects with each herbicide. Protoporphyrinogen oxidase from barley etioplast preparations was inhibited 50% by 40 nM UCC-C4243. Barley leaf sections treated with 100 μM UCC-C4243 accumulated protoporphyrin IXin vivoto levels > 75 times non-treated controls. These data indicate the light-requiring herbicide activity of UCC-C4243, like acifluorfen, is due to inhibition of protoporphyrinogen oxidase.

Lack of Cross-Resistance of Paraquat-Resistant Hairy Fleabane (Conyza bonariensis) to Other Toxic Oxygen Generators Indicates Enzymatic Protection is Not the Resistance Mechanism

Cross-resistance of the paraquat-resistant (R) hairy fleabane to other compounds that accept electrons from photosystem I (PSI) or produce toxic oxygen species was determined by chlorophyll loss, electron microscopy, and chlorophyll fluorescence suppression. Although the R bioype is approximately 100 x more resistant to paraquat than the susceptible (S) biotype based upon the assays for tissue damage, little or no cross-resistance was observed to a number of other PSI electron acceptors, including the bipyridilium herbicide morfamquat. A low level of resistance (approximately 10-fold) was noted to diquat and the singlet oxygen generator rose bengal. As measured by chlorophyll fluorescence suppression, the R biotype was about 100-fold resistant to paraquat, but only 10-fold resistant to diquat, and exhibited no resistance to morfamquat. Because differences observed with this protocol are direct measures of the ability of the herbicide to reach the active site and the results correlate with the level of resistance observed by chlorophyll bleaching or electron microscopy, these data suggest that compartmentalization is the major factor in paraquat resistance in hairy fleabane.