Cytosolic antioxidant machinery and resistance to biotic stress

diciembre 10, 2013 de hernandezlab 2 comentarios

José A. Hernández Cortés, Senior Researcher (CEBAS-CSIC, Biotechnology and Fruit Trees Group, Murcia)

The effect of over-expression of cytosolic Cu,Zn-superoxide dismutase (cytsod) and ascorbate peroxidase (cytapx) alone, or in combination, in tobacco plants, increased the resistance to bacterial wild fire, caused by Pseudomonas syringae pv. Tabaci.

In the non-transformed controls, the inoculated areas with P. syringae pv tabaci became initially chlorotic at 2 days post-inoculation (dpi), then subsequently water soaked and necrotic. At 6 dpi, the necrotic areas extended to the border of the leaves and finally the necrotic area appeared surrounded by a chlorotic yellow area. In transgenic lines harboring cytsod (line 17) or cytapx (line 51) necrotic areas were confined within the infiltrated sites, mimicking symptoms of hypersensitive reaction (HR) observed during an incompatible plant-pathogen interaction. In contrast, in the transgenic lines harboring both cytsod and cytapx genes (lines 35 and 39), infected leaf tissue presented only a chlorotic area and localized necrosis occurred only occasionally (Fig. 1).

Disease resistance against Pseudomonas syringae pv. tabaci in transgenic tobacco lines. Fully expanded leaves of 8-weeks-old tobacco plants were inoculated with 3 x 105 cfu/ml of P. syringae pv. tabaci. Photos were taken 10 days after infection

In addition, the symptoms observations correlated with reduced bacterial growth, mainly in lines showing fewer symptoms (Lines 35 and 39).  The reduced necrotic lesions observed in the transgenic lines are parallel with increased antioxidant enzyme levels (SOD, APX, Catalase, and Glutathione Reductase). In this sense, the balance between SOD (O₂^.--scavenger and H₂O₂-producer enzyme) and H₂O₂-scavenger enzymes (APX, Catalase and Peroxidases) is crucial to control ROS accumulation and the triggering of cell death (Fig. 2). The observed resistance of transgenic lines to bacterial wild fire seemed to be independent of tissue necrosis, as observed in other plant-pathogens interactions (Kiraly et al. 1999; 2008). These authors suggested that the induction of antioxidant enzymes might reduce plant cell necrosis after infection (Kiraly et al., 2002; 2008).

Fig. 2.- Schema showing the pathogen-induced ROS generation in the apoplastic space from tobacco leaves. Bacterial pathogens stimulated the superoxide (O2.-) generation in the apoplastic space by activating the plasma membrane NADPH oxidase (Lamb & Dixon 1997). Superoxide is subsequently dismutated to H2O2 by the apoplastic SOD. There are potential sources of apoplastic H2O2 generation including peroxidases, amine oxidases and oxalate oxidases (Bollwell & Wojtaszek 1997). Hydrogen peroxide can participate in the lignification process (Ros Barceló 1998) as well as in the direct killing of the pathogens. ROS, such as the protonated form of HO2.- (perhydroxyl radical) or H2O2, can permeate biological membranes. Therefore, an important part of the apoplastic ROS can also originate an oxidative stress in the cytosol, in addition to that produced by ROS originated from chloroplasts, mitochondria and peroxisomes (Diaz-Vivancos et al., 2008; Amirsadeghi et al., 2007; Van Breusegem & Dat, 2006) and can contribute to the induction of an oxidative stress in the cytosolic compartment. This ROS accumulation can induce the HR, manifested as cell death and tissue necrotization. Transgenic lines overexpressing both cytsod and cytapx (lines 35 and 39), can cope with ROS accumulation and avoid tissue necrotization.

The results of this work showed that the overexpression of CuZn-SOD and APX in the cytosol of tobacco plants enhances resistance against P. syringae pv tabaci, highlighting the importance of the cytosolic antioxidant machinery in the response to biotic stress, similar to the results showing the importance of this cell compartment in the response to abiotic stress, such as salinity or drought stress (Hernández et al., 2000; Faize et al., 2011).

For more information, please consult:

Faize, M., Burgos, L., Faize, L., Petri, C., Barba-Espin, G., Diaz-Vivancos, P., Clemente-Moreno, M.J., Alburquerque, N. and Hernández, J.A. (2012) Modulation of tobacco bacterial disease resistance using cytosolic ascorbate peroxidase and Cu, Zn-superoxide dismutase. Plant Pathol., 61, 858-866.

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