**5. The roles of ROS: cell signaling**

former. During germination, respiratory activity increased and production of ROS enhanced [6, 14, 46, 47]. Another source of ROS is peroxisomes. Peroxisomes divided into: glyoxysomes (oily seeds), peroxisomes of photosynthetic tissues, nodule peroxisomes (*Fabaceae* nodules) and gerontosomes (senescing tissues) [14, 27, 46–50]. In glyoxysomes, lipid reserves of oily seeds are converted into sugars during the first stages of seedling development [49–51].

also oxidized into uric acid by xanthine oxidase resulting with the production of superoxide

of nitric oxide (NO), (a free radical and also an important cellular signaling compound in plants) also takes place in peroxisomes [48, 51–54]. NADPH oxidases of the cell membrane are another sources of ROS in plants, these enzymes transfer electrons from cytoplasmic NADPH

dases are increased during plant infections [28, 29], in plant growth processes [55], and under severe abiotic stress conditions [56]. Enhanced activity of NADPH oxidase is reported in ABA induced generation of ROS under water stress [57, 58]. During biotic stress cell wall peroxi-

As a result, mitochondria and peroxisomes are the major sources of ROS in nonquiescent seeds, during seed development and germination. Aquaporins and peroxiporins (transmem-

but the mobility of ROS in seeds has not yet been documented. Finally, lipid oxidation can

The oxidative stress may cause damage to DNA resulting in cancer and aging [62], and the presence of reactive oxygen also may initiate a chain reaction at the cellular level resulting in damage to critical cell bio-molecules [63–65]. The uncontrolled accumulation of ROS, particularly of OH• is highly toxic for the cell. These radicals are highly toxic and thus generate oxidative stress in plants. ROS can react with the majority of biomolecules, thus resulting in oxidative stress that can become irreversible and cause cellular damage [1–5]. Many harmful effects of ROS on cellular macromolecules have been identified [1–5]. All are capable of reacting with membrane lipids, nucleic acids, proteins and enzymes, and other small molecules, resulting in cellular damage [1–5]. Lipid peroxidation, which is a free-radical chain process leading to the deterioration of polyunsaturated fatty acids (PUFAs), is the best known cellular hazard among these, and has been studied intensively in food science [66]. Lipid peroxidation is initiated by free-radical attack upon a lipid, that gives starting to a chain reaction, removing a hydrogen atom from another fatty acid chain to form a lipid hydroperoxide (LOOH) in a propagation step [67]. This process is likely to degrade PUFAs present in membranes or in reserve lipids of oily seeds. Beside membranes, nucleic acids and proteins are also potential targets of ROS [67]. The hydroxyl radical, OH•, can damage both nuclear and organelle DNA directly, by having ability to attack deoxyribose, purines and pyrimidines [67, 68]. Enzymes

is produced. In peroxisomal matrix, xanthine is

O2

in vegetative tissues [56, 60],

O2

O2

. NADPH oxi-

in the apoplast [59].

eliminating enzyme) is localized in peroxisomes [49–52]. Production

O2

to oxygen, producing superoxide radical and its dismutating product H2

dases and amine oxidases are induced leading to the formation of H2

brane proteins) are shown to play roles in the transport of H2

generate ROS that could be trapped in seed tissues [16, 61].

**4. The dual effect of ROS: from toxicity to signaling**

During this lipid oxidation process H2

O2

[49–51]. Catalase (H2

170 Advances in Seed Biology

**4.1. Toxicity of ROS**

Cellular antioxidant mechanisms control ROS concentrations, rather than to eliminate them completely, suggesting that some ROS may act as signaling molecules [5, 34–37, 46]. Although ROS have been considered as detrimental to seeds, advances in plant physiology evaluated them as messengers of various signal transduction pathways in plants. ROS are suggested as being beneficial for seed germination, seedling growth, protection against pathogens and controlling the cell redox status [28–40]. H2 O2 is shown to be involved in the tolerance to various abiotic stresses acting as a secondary messenger [71], in cellular defense mechanisms against pathogens [72]. H2 O2 has also been identified in many processes in plants, including programmed cell death (PCD) [8, 73], somatic embryogenesis [17], root gravitropism [19], and ABA-mediated stomatal closure [20, 21], response to wounding [74]. Superoxide (O2 −•) found to have roles in cell death and plant defense [24]. H2 O2 also proved to have roles in protein phosphorylation through mitogen-activated protein kinase (MAP kinase) cascades [75, 76], calcium mobilization [77, 78], and regulation of gene expression [79, 80].
