| Tobacco wildfire disease is an epidemic bacterial disease causing by a pathogen named Pseudomonas syringae pv. tabaci (Pst). This disease has a significant influence on the yield and quality of tobacco. The tolerance and defense mechanisms of plants against Pst infection has received considerable attention in recent years. Photosynthesis and respiration are the main targets of pathogen since they are the source of material and energy. They also play an important role in plant defense responses. However, little information is available on the effect of Pst on the photosynthetic and respiratory performance in tobacco.Pst suspensions (106) were hand-infiltrated into mesophyll with a needleless syringe on the abaxial side of the leaves. By analyzing chlorophyll a fluorescence transient and performing Western blot of thylakoid membrane, the effect of Pst on photosystem II (PSII) and photosystem I (PSI) in tobacco leaves were studied under light (200 μmol m-2 s-1) or dark conditions. Besides, the underlying mechanisms of the reduced photosystems damage in tobacco leaves induced by Pst under light conditions was elucidated. In the same time, to avoid the influence of chloroplast and Pst respiration, tobacco BY-2 cells (which do not have functional chloroplasts) and the extracellular metabolites from Pst (Pst ext, which contains tabtoxin) were used to study the effect of Pst on tobacco respiration. Exploring the effect of Pst on photosystems and respiratory electron transport chain will not only help in clarify tobacco-Pst interaction mechanisms, but also deepen the understanding of tobacco wildfire disease from a physiological perspective. Besides, it will also provide theoretical help in tobacco resistant cultivars breeding and genetic improvement works.The results are as follows:(1) A significant chlorosis and decrease of chlorophyll content of the infiltrated zone was observed at 3 days post Pst infection (dpi) under light and dark conditions. The infiltrated zone of tobacco leaves exhibiting a visible and overt wildfire symptom after Pst infection.(2) The normalized relative variable fluorescence at the K step (Wk) and the relative variable fluorescence at the J step (VJ) markedly increased in tobacco leaves at 3 dpi under light and dark conditions. The maximal quantum yield of PSII (Fv/Fm), the density of QA-reducing PSII reaction centers per cross section (RC/CSm) and the activity of PSI significantly decreased in tobacco leaves at 3 dpi under light and dark conditions. Moreover, inhibition of the K and J steps was more pronounced in the dark, as indicated by the greater increase of Wk and VJ under darkness compared with the light conditions during Pst inoculation. Besides, a dramatically (net) degradation of core proteins such as Dl protein, PsaO and PsaA at 3 dpi were observed in tobacco leaves and the decline was more exacerbated under dark than light conditions.(3) The population of Pst was 17-fold more abundant at 3 dpi under dark compared to light conditions in tobacco leaves. The H2O2 content increased at 3 dpi under light and dark conditions, which implies that a significant ROS accumulation was induced by Pst infection in tobacco leaves under light and, to a lesser extent, dark conditions.(4) Pretreatment with H2O2 alleviated chlorotic lesions and decreased Pst abundance in tobacco leaves at 3 dpi under dark conditions. Methyl viologen (MV) pretreatment had the same effects under light conditions, whereas 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) pretreatment aggravated chlorotic lesions and increased the Pst population under light conditions.(5) Pst ext treatment induced reactive oxygen species (ROS) accumulation, decline of total respiratory rate and ATP content, increase of alternative respiration pathway/total respiratory rate in tobacco BY-2 cells.The results indicate that the electron transport from QAto QB of photosynthesis electron transport chain was severely blocked, OEC was damaged and both the donor and acceptor sides, the reaction center of PSII were all severely damaged by Pst infection in tobacco leaves under light and dark conditions. Photoinhibition and photoinhibition-like damage was observed and the damage to photosynthetic apparatus induced by Pst infection under dark condition was much more severe compared to that under light condition in tobacco leaves. Chlorotic symptoms and the bacterial population are each negatively correlated with H2O2 accumulation at 3 dpi in tobacco leaves. Light appears to suppress the Pst population in tobacco leaves through the accumulation of H2O2 during infection. This maybe one of the reason for the reduced photosystems damage induced by Pst in tobacco leaves under light conditions. |
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