**3. ROS production in seeds**

In plants, transport chain of electrons toward oxygen can potentially generate ROS. Seeds represent a particular case in this regard. During germination, the seed metabolic activity vary dramatically, meanwhile the sources of ROS in seeds also vary considerably [13, 39–45]. The mitochondrial respiratory chain is one of the major sources of ROS; electron leakage from the transport chain generates superoxide, and subsequently H<sup>2</sup> O2 , by dismutation of the 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]. During this lipid oxidation process H2 O2 is produced. In peroxisomal matrix, xanthine is also oxidized into uric acid by xanthine oxidase resulting with the production of superoxide [49–51]. Catalase (H2 O2 eliminating enzyme) is localized in peroxisomes [49–52]. Production 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 to oxygen, producing superoxide radical and its dismutating product H2 O2 . NADPH oxidases 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 peroxidases and amine oxidases are induced leading to the formation of H2 O2 in the apoplast [59]. As a result, mitochondria and peroxisomes are the major sources of ROS in nonquiescent seeds, during seed development and germination. Aquaporins and peroxiporins (transmembrane proteins) are shown to play roles in the transport of H2 O2 in vegetative tissues [56, 60], but the mobility of ROS in seeds has not yet been documented. Finally, lipid oxidation can generate ROS that could be trapped in seed tissues [16, 61].
