Shiguo Chen

Reactive oxygen species from chloroplasts contribute to 3-acetyl-5- isopropyltetramic acid-induced leaf necrosis of Arabidopsis thaliana

3-Acetyl-5-isopropyltetramic acid (3-AIPTA), a derivate of tetramic acid, is responsible for brown leaf-spot disease in many plants and often kills seedlings of both mono- and dicotyledonous plants. To further elucidate the mode of action of 3-AIPTA, during 3-AIPTA-induced cell necrosis, a series of experiments were performed to assess the role of reactive oxygen species (ROS) in this process. When Arabidopsis thaliana leaves were incubated with 3-AIPTA, photosystem II (PSII) electron transport beyond Q(A) (the primary plastoquinone acceptor of PSII) and the reduction of the end acceptors at the PSI acceptor side were inhibited; this was followed by increase in charge recombination and electron leakage to O(2), resulting in chloroplast-derived oxidative burst. Furthermore, the main antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) lost their activity. Excess ROS molecules directly attacked a variety of cellular components and subsequently caused electrolyte leakage, lipid peroxidation and cell membrane disruption. Finally, this led to cell destruction and leaf tissue necrosis. Thus, 3-AIPTA-triggered leaf necrosis of Arabidopsis was found to be a result of direct oxidative injury from the chloroplast-originated ROS burst initiated by the inhibition of normal photosynthetic electron transport.

Chloroplastic oxidative burst induced by tenuazonic acid, a natural photosynthesis inhibitor, triggers cell necrosis in Eupatorium adenophorum Spreng

Tenuazonic acid (TeA), a nonhost-specific phytotoxin produced by Alternaria alternata, was determined to be a novel natural photosynthesis inhibitor owning several action sites in chloroplasts. To further elucidate the mode of its action, studies were conducted to assess the production and involvement of reactive oxygen species (ROS) in the toxic activity of TeA. A series of experiments indicated that TeA treatment can induce chloroplast-derived ROS generation including not only (1)O(2) but also superoxide radical, H(2)O(2) and hydroxyl radicals in Eupatorium adenophorum mesophyll cells, resulting from electron leakage and charge recombination in PSII as well as thylakoid overenergization due to inhibition of the PSII electron transport beyond Q(A) and the reduction of end acceptors on the PSI acceptor side and chloroplast ATPase activity. The initial production of TeA-induced ROS was restricted to chloroplasts and accompanied with a certain degree of chloroplast damage. Subsequently, abundant ROS were quickly dispersed throughout whole cell and cellular compartments, causing a series of irreversible cellular harm such as chlorophyll breakdown, lipid peroxidation, plasma membrane rupture, chromatin condensation, DNA cleavage, and organelle disintegration, and finally resulting in rapid cell destruction and leaf necrosis. These results show that TeA causing cell necrosis of host-plants is a result of direct oxidative damage from chloroplast-mediated ROS eruption.

Effect of vulculic acid produced by Nimbya alternantherae on the photosynthetic apparatus of Alternanthera philoxeroides

The effect of the toxin vulculic acid produced by Nimbya alternantherae, on the photosynthetic apparatus of Alternanthera philoxeroides, was investigated via the photochemical activity and SDS-PAGE of protein on thylakoid membranes, fast chlorophyll a fluorescence transient measurements and the JIP-test. The electron transport rate of photosystem II (PSII), non-cyclic photophosphorylation activity, as well as the activity of chloroplast ATPase and Rubisco reduced significantly after vulculic acid treatment. Vulculic acid affected the O-J-I-P fluorescence induction kinetics, showing an increase of the parameters FV/FO, VK and VJ and a decrease of FO, FM, PIABS, φPo, ψEo, φEo, φRo, δRo and PItotal. In addition, it significantly decreased the amounts of major photosystem I (PSI) and PSII proteins. It is concluded that vulculic acid is a photosynthetic inhibitor with multiple action sites. The main targets are the light harvesting complex (LHC) and the oxygen evolving complex (OEC) on the PSII donor side. Vulculic acid blocks electron transport beyond QA and on the PSI acceptor side by digesting major PSI and PSII proteins.

Bioassay of the Herbicidal Activity of AAC-Toxin Produced by Alternaria alternata Isolated from Ageratina adenophora

Abstract Tenuazonic acid (TeA), a naturally occurring product of Alternaria alternata, a pathogen to croftonweed, was discovered to be a novel natural photosystem II (PSII) inhibitor. However, herbicidal activity of AAC-toxin, a metabolite of this fungus containing TeA as the main active ingredient, has not been evaluated systematically. In this study, we conducted activity-evaluation experiments in the laboratory, greenhouse, and field trials to assess the herbicidal potential of this fungal metabolite. AAC-toxin had high herbicidal activity on all species tested: croftonweed, large crabgrass, barnyardgrass, redroot pigweed, and eclipta. The AAC-toxin caused brown, leaf spot symptoms and leaf necrosis, subsequently killing the seedlings. When AAC-toxin was applied POST at 83 ml ai/ha, more than 95% of large crabgrass, barnyardgrass, and redroot pigweed plants were controlled 2 d after treatment in field trials. It can be concluded that AAC-toxin has broad-spectrum, rapid, and high herbicidal activity similar to that of paraquat and may have the potential to be developed as a microbe-based herbicide.

Analysis of Dark Drops, Dark-Induced Changes in Chlorophyll Fluorescence during the Recording of the OJIP Transient

In a search for a powerful and easy-to-handle instrument for investigation of photosynthetic energy and electron transfer reactions, the mPEA (Hansatech Instrument Ltd., King’s Lynn, Norfolk, PE30 4NE, U.K.) was developed. The instrument can record simultaneously, in vivo, the photo-induced changes in prompt (PF) and delayed (DF) chlorophyll fluorescence and modulated infrared light reflection at 820 nm (MR820) using a protocol of alternating periods of illumination and darkness. In this way the standard OJIP induction transient of PF is modified by the dark intervals causing a decrease of the PF intensity during the dark periods (so called dark fluorescence drops, DD). The dependence of the relative DD on the redox level of QA was analyzed for different initial states of bean leaves. A strong linear correlation was found between the relative dark drops and the fraction of oxidized QA for the phase starting before the J level and going up to the P level of the PF induction transient. We propose that the experimentally measurable DDs offer a tool for in vivo quantification of the redox reactions of QA and QB during the fluorescence rise from F0 to FM.

Comparative phytotoxicity of usnic acid, salicylic acid, cinnamic acid and benzoic acid on photosynthetic apparatus of Chlamydomonas reinhardtii

The effects of four phytotoxins usnic acid (UA), salicylic acid (SA), cinnamic acid (CA) and benzoic acid (BA) on photosynthesis of Chlamydomonas reinhardtii were studied in vivo to identify and localise their initial action sites on two photosystems. Our experimental evidence shows that the four phytotoxins have multiple targets in chloroplasts, which mainly lie in photosystem II (PSII), not photosystem I (PSI). They share an original action site by blocking electron transport beyond QA (primary plastoquinone acceptor) at PSII acceptor side since a fast increase of the J-step level is the greatest change in chlorophyll a fluorescence induction kinetics OJIP in C. reinhardtii cells treated with the phytotoxins. UA decreases photosynthetic activity by reducing O2 evolution rate, interrupting PSII electron transport at both the donor and acceptor sides, inactivating the PSII reaction centers (RCs), reducing the content of chlorophylls and carotenoids, destroying the conformation of antenna pigment assemblies, and casuing the degradation of D1/D2 proteins. SA damage to photosynthetic machinery is mainly attributed to inhibition of PSII electron transport beyond QA at the acceptor side, inactivation of the PSII RCs, reduction of chlorophyll content, digestion of thylakoid ploypeptides and destabilization of thylakoid membranes. Both CA and BA affect the photosynthetic process by decreasing PSII electron transport efficiency at the acceptor side and the amount of active PSII RCs. Besides, the initial cause of BA-inhibiting photosynthesis is also assocaited with the O2 evolution rate and the disconnection of some antenna molecules from PSII RCs.

Tenuazonic Acid, a Novel Natural PSII Inhibitor, Impacts on Photosynthetic Activity by Occupying the QB-Binding Site and Inhibiting Forward Electron Flow

Tenuazonic acid (TeA), a member of representative natural tetramic acids, is a phytotoxin produced by the fungus Alternaria alternata isolated from diseased Croftonweed (Eupatorium adenophorum). TeA strongly inhibits photosynthesis, especially photosystem II (PSII) activity. Evidence from fast chlorophyll fluorescence induction transients of host plant shows that the most important action site of TeA is that it interrupts electron transport beyond QA, on the acceptor side of PSII, and this is due to its binding at the QB-site. On the basis of competition experiments with [14C]atrazine, it is further confirmed that TeA does not share the same binding environment as atrazine, despite their common action target: the QB-site.