Chapter 13. Reactive oxygen species

SOBERÓN, J. R.; SGARIGILA, M.A.; SAMPIETRO, D. A.; VATTUONE, M.A.

Soil Allelochemicals

Studium Press LLC

Lugar: Houston; Año: 2011; p. 311 - 325

Molecular oxygen (O2) is essential for the survival of all aerobic organisms, because it acts as the final acceptor of mitochondrial electron chain. In this situation, O2 is reduced to H2O after the cytochrome-c oxidase reaction. Reactive oxygen species (ROS) are extremely reactive substances derived from the O2 that may be formed during the electron transfer reactions or through photon absorption by a sensitizer molecule and subsequent reaction either by energy or electron transfer reactions with O2 (Kohn and Nelson, 2007). The ROS not only comprise free radicals such as superoxide (O2_•), hydroperoxyl (HO2•), hydroxyl (HO•), peroxyl (RO2•) and alkoxyl (RO•) radicals, but also non-radicals such as hydrogen peroxide (H2O2), singlet oxygen (1O2), and hypochlorous acid (HOCl) (Gomes et al., 2005). Radicals are unstable with variable reactivity. Figure 1 shows the typical ROS generation pathway. Several allelochemicals can generate an uncontrolled production and accumulation of ROS on target plants (Bogatek and Gniazdowska, 2007). This oxidative stress is often referred as part of harmful effects of these substances on target plants (Oracz et al., 2007). Soil allelochemicals such as quinones or phenols may become semiquinone radicals that reduce the molecular oxygen to superoxide radical and then produce other ROS on soil matrix(Weir et al., 2004). It may be aided by catalytic metals such as aluminum or iron most abundant metals in soils (Dimkpa et al., 2009; Gapper and Dolan, 2006). ROS and direct depolarization by some allelochemicals [(which cause cellular disruption and ultimately the cell death (Weir et al., 2004)], can initiate the lipid peroxidation of plant root membranes (Thoma et al., 2003). Soil allelochemicals also act as inhibitory agents of antioxidizing enzymes, such as superoxide dismutase and catalase (Lin et al., 2000), thus leaving the plant vulnerable to ROS mediated oxidative damage. ROS production by plant roots indicates the signal of plant defence mechanism upon microbial attack (Laloi et al., 2004). ROS accumulates in plant cells in response to pathogen infections (causing damage to cells or finally the cell death). On the other hand, soil derived ROS e.g. H2O2, may contribute to the structural reinforcement of plant cell walls and coordinate the activation of defense genes and phytoalexin production (Grant and Loake, 2000). Many events cause ROS generation and allelochemicals production in soils, thus, soil microbiological, physicochemical and mineralogical studies often includes the ROS assessment (Georgiou et al., 2008), in soil matrix or plant roots. This chapter provides useful and sensitive tests to assess the ROS content in plant-soil system.